Training-induced are independent
skeletal muscle adaptations of systemic adaptations
JOHN R. MINOTTI, EMILY C. JOHNSON, TRACEE L. HUDSON, EIICHI FUKUSHIMA, GLEN MURATA, LOIS E. WISE, THOMAS W. CHICK, AND MILTON V. ICENOGLE
GLEN
ZUROSKE,
Department of Internal Medicine, Veterans Administration Medical Center, and Research Division, Lovelace Medical Foundation, Albuquerque, New Mexico 87108
MINOTTI, JOHN R., EMILY C. JOHNSON, TRACEE L. HUDSON, GLEN ZUROSKE, EIICHI FUKUSHIMA, GLEN MURATA, LOIS E. WISE, THOMAS W. CHICK, AND MILTON V. ICENOGLE. Training-induced skeletal muscle adaptations are independent of systemic adaptations. J. Appl. Physiol. 68(I): 289-294,
troscopy (31P-MRS) was used to determine muscle pH and the inorganic phosphate-to-phosphocreatine ratio (Pi/P(%) as a function of submaximal work load, in the dominant (D) and nondominant (ND) forearms of normal subjects (20). The D forearm demonstrated lower PJPCr and higher pH at the same submaximal work load, implying an improved metabolic response to exercise. Because these subjects were not involved in unilateral arm training, the asymmetry in Pi/PCr and pH were attributed to adaptations induced by differences in daily use of the forearms. This observation suggests that training adaptations of forearm skeletal muscle are sensitive to repetitive activity and could possibly be trained independently of systemic training. To isolate the peripheral components of the training response, a small muscle group was trained at an intensity below the threshold necessary for systemic circulatory adaptations. This study determined the effects of localized forearm training on forearm PJPCr, pH, blood flow, muscle mass, and exercise endurance independent of cardiovascular adaptations. Such training may provide a means of conditioning patients who are too debilitated to undergo systemic training.
1990.-To isolate the peripheral adaptations to training, five normal subjectsexercisedthe nondominant (ND) wrist flexors for 41 & 11 days, maintaining an exerciseintensity below the threshold required for cardiovascular adaptations. Before and after training, intracellular pH and the ratio of inorganic phosphate to phosphocreatine(PiJPCr) were measuredby 31Pmagnetic resonancespectroscopy.Also maximal O2 consumption (VO, max),musclemass,and forearm blood flow were determined by gradedsystemic exercise,magnetic resonanceimaging, and venous occlusion plethysmography, respectively. Blood flow, PJPCr, and pH were measuredin both forearms at rest and during submaximalwrist flexion at 5,23, and 46 J/min. Training did not affect voz max,exerciseblood flow, or musclemass. RestingpH, PJPCr, and blood flow were alsounchanged.After training, the ND forearm demonstratedsignificantly lower Pi/ PCr at 23 and 46 J/min. Endurance, measuredas the number of contractions to exhaustion, also was increasedsignificantly (63%) after training in the ND forearm. We conclude that I) forearm training resultsin a lower PJPCr at identical submaximal work loads;2) this improvement is independentof changes in iTop maxy muscle mass, or limb blood flow; and 3) these differences are associatedwith improved endurance and may reflect improved oxidative capacity of skeletal muscle. MATERIALS AND METHODS
Study design. The same studies were performed on the nuclear magnetic resonancespectroscopy;musclemetabolism; subjects before and after 40 days of forearm training. endurancetraining; forearm blood flow ADAPTATIONS to endurance training of large muscle groups have been well characterized. These adaptations include greater maximal exercise cardiac output, maximal oxygen consumption (VOW&, and peak exercise muscle blood flow (4). In conjunction with these central effects, peripheral skeletal muscle demonstrates increased mitochondrial density and oxidative phosphorylation enzyme activity (11, 16). Muscles with high mitochondrial density exhibit increased oxidative capacity and augmented capacity for ATP production via oxidative phosphorylation (9, 16). These peripheral adaptations have been suggested as the primary determinant of endurance during submaximal exercise (23). Little is known about the response of human skeletal muscle to training independent of systemic cardiovascular adaptations. Recently 31P magnetic resonance specTHE CARDIOVASCULAR
Measured variables were obtained over 3 days in the following order: day I, resting forearm blood flow followed by systemic exercise testing; day 2, forearm muscle cross-sectional area and maximal voluntary contraction; day 3, forearm Pi/PCr, pH, and exercise blood flow during submaximal exercise and assessment of maximal forearm endurance. Subjects. Six normal, right-handed, nonsmoking males free of cardiovascular, pulmonary, and musculoskeletal disease were recruited from the house staff, students, and faculty of the University of New Mexico School of Medicine. Subjects were 24-40 yr old, had no physical limitations to exercise, and were not involved in any unilateral arm training (Table 1). The protocol was approved by the Human Research Review Committee of the University of New Mexico School of Medicine and the Institutional Review Board of the Lovelace Medical Foundation. The risks and benefits of the study were 289
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TRAINING-INDUCED
290 TABLE
1. Physical characteristics Subject
PP BW MG LG DC Mean * There
difference
Age, Yr
Height, cm
Weight, kg
39 40 33 38 24
166.6 178.8 169.0 154.3 166.6
71.3 73.1 88.6 54.0 68.1
167.Ok8.7
between
the nondominant
Superconducting
ADAPTATIONS
Nondominant Flexor Muscle Area, * cm2
71.0zk12.3 forearm
explained to each participant, and informed written consent was obtained. IYxercise protocol. Subjects arrived at the laboratory postprandially. Before exercise, each subject rested with his forearm in the magnet for -15 min to allow field shimming. Exercise consisted of wrist flexion over an arc (linear distance 96 mm) against an exercise handle connected to a customized computer-controlled dynamometer (Kin-Corn, Chattecx). Flexion repetition was cued every 5 s with a flashing light synchronized to 31P-MRS data collection, and exercise spectrum was obtained after each wrist flexion. Metabolic changes during exercise were obtained from the forearm flexors positioned over the spectroscopy coil (Fig. 1). After an 8-min resting spectra and warm-up, the ND forearm was exercised at power levels of 5, 23, and 46 J/min. Each work load, including warm-up, lasted 8 min. The ND forearm endurance test (see Enduranceperformance) was completed before progressing to the same warm-up and submaximal exercise in the D forearm. One subject complained of mild ND forearm fatigue at 24 J/min. To facilitate completion of the protocol, his highest work load was adjusted to 34 J/min. The ND 31P-MRS spectra at this work load demonstrated PCr depletion, so these data were grouped with the 46-J/min data from the other subjects. After training, subjects were tested using the same work loads as their pretest. Endurance performance. To test ND endurance, subjects performed wrist flexion at 69 J/min in the magnet on the same apparatus used for submaximal exercise. Each subject performed wrist flexion at his own pace until unable to complete a subsequent flexion within 5 s of the previous contraction. The number of contractions performed was defined as exercise endurance. 31P-MRS. A magnetic resonance spectrometer/imager (Nalorac) and a 1.9-T superconducting magnet (Oxford) with a 3O-cm-diam, horizontal, room temperature bore 1.97
MUSCLE
of subjects
34.8k6.6
,t SD
was no significant
SKELETAL
Magnet
am
flexor
muscle
Before
After
29.8 25.5 24.3 17.7 19.2
27.2 26.5 25.8 18.8 20.3
23.3t4.9 area before
and after
23.723.8 training.
were used to collect spectra. The subject’s grounded forearm rested over a 4-cm, two-turn surface coil made of 14-gauge copper wire under a 3-mm acrylic sheet. Guides were attached to the probe on either side of the forearm to facilitate reproducible positioning. Magnetic field homogeneity was optimized by shimming with the phosphorus coil tuned to the proton frequency, initially using a phantom and then the subject’s arm, The optimum forearm proton line width for the magnitude spectrum was 60 Hz. Free induction decays (FIDs) were collected after 45-ps pulses, which were 180" pulses at the sample surface and 90” pulses 12 mm into the sample. Sweep width and filter settings were 3,000 Hz. The last 60 FIDs (5 min) at rest and each exercise load were used for analysis. FIDs were multiplied by a decaying exponential to give 7 Hz line broadening. Relative Pi, PCr, and ,&ATP spectral peak heights were measured from the base line. Normalized ATP concentration was calculated as ATP = Muscle intracellular cal shift difference scribed (2).
P-ATP Pi + PCr + P-ATP
(0
pH was calculated from the chemibetween Pi/PCr, as previously de-
Resting skeletal muscle blood flow. Venous occlusion plethysmography was used to measure skeletal muscle blood flow (5, 20, 28). This technique utilizes two inflation cuffs with the upper cuff secured as far from the
olecranon process as possible and the lower cuff secured around the wrist. When the upper cuff is briefly inflated to 40 mmHg (venous occlusive pressure), arterial blood enters the forearm but venous outflow is obstructed, resulting in an increase in the volume of the forearm. If it is assumed that the shape of the forearm approximates a cylinder, changes in circumference are directly proportional to changes in volume and, therefore, blood flow. A single-strand mercury-in-Silastic strain gauge positioned 10 cm distal to the olecranon process measured changes in forearm circumference. To ensure adequate venous drainage, the arm rested on a board and was raised 10 cm above the level of the right atrium.
FIG. 1. Magnetic resonance nous occlusion plethysmography
spectroscopy magnet, flow apparatus.
exercise,
and ve-
Resting blood flows were obtained as follows. The wrist cuff was inflated to 240 mmHg for 1 min before flow measurements to eliminate venous return from the hand. Arterial inflow tracings were then obtained by inflating the upper cuff to 40 mmHg to occlude the venous system. Three resting blood flow determinations were averaged.
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TRAINING-INDUCED
SKELETAL
Resting blood flow was calculated as milliliters per 100 ml of volume per minute. Exercise skeletal muscle blood flow. Forearm blood flow during submaximal exercise was also measured with venous occlusion plethysmography. For exercise measurements, the strain gauge was secured on the forearm 1 cm proximal to the region where the 31P-MRS signal was obtained. A rapid-inflation cuff was then secured around the upper arm, and the arm was positioned in the bore of the magnet at approximately the same height as the shoulder to ensure no obstruction to venous return. The cuff was inflated to 40 mmHg, and arterial inflow tracings were obtained within 2 s after completion of the final exercise contraction at each work load. Exercise blood flow was measured at all work loads in both forearms and was calculated and expressed similarly to resting flows. Forearm mu&e mass. Relative muscle mass in the ND forearm, before and after training, was determined by magnetic resonance imaging (20) with a 1.5 T. imaging system (Signa, General Electric). Two-dimensional transverse images were taken using a Helmholtz extremity coil. The images were obtained from 3-mm-thick slices in the same location on the forearm, approximately one-third the ulnar length distal to the olecranon process. A spin echo sequence with a pulse repetition time (TR) of 300 ms, an echo time (TE) of 20 ms, acquisition matrix of 256 x 256, and a lo-cm field of view was used without signal averaging. Images were analyzed with the GE areatracing software. Cross-sectional area of the flexor muscle group was measured before and after training. Bone and subcutaneous fat were excluded from area measurements; vascular and nerve areas were small and not separated from surrounding tissue (see Fig. 4). It was assumed, before and after training, that within the same forearm, the cross-sectional area was proportional to muscle mass. Maximal voluntary contraction (MVC). MVC strength of the wrist flexors was tested on an arm exercise apparatus identical with the one used in the magnet. With the arm positioned as it was for the magnet studies, subjects performed a maximal isometric wrist flexion contraction with the handle at the midpoint of the range of motion. Three maximal efforts were performed, separated by at least 15 s of rest. The highest force of these three contractions was defined as MVC strength. Systemic exercise testing. Upright exercise testing was performed using an electronically braked cycle ergometer (Erich Jaeger, Rockford, IL) which maintains constant work load at pedal frequencies of 40-100 rpm. Testing was performed in an air-conditioned laboratory with an ambient temperature of 22-24°C and humidity of 3040%. Before exercise subjects rested upright on the cycle ergometer for 3-5 min. Exercise was then performed at 40 W for 3 min, succeeded by increments of 40 W every 3 min. Exercise was continued until exhaustion, which was defined as the inability to maintain critical pedal frequency (>40 rpm). Standard verbal encouragement was used for all subjects. Respiratory gas exchange was measured continuously (Ergupneumotest, Erich Jaeger). VoZmax was defined as a plateau of Vop with increasing work load in association with a respiratory exchange
MUSCLE
291
ADAPTATIONS
ratio X.15. These criteria were met in all tests. Training program. The training sessions consisted of wrist flexion exercise using a hand-held weight performed with the ND forearm. The sessions were performed an average of six times per week, for a mean total of 41 -i- 11 training sessions. The weights were plastic containers measuring 5.5 x 11 cm filled with lead shot to attain the desired weight and padding to stabilize the shot. The initial work loads were the same (1.9 kg) for each subject. The work load was increased once during training (3.3 t 1.5 kg). All work loads were selected to maintain a heart rate within 5% of base line. Each exercise session was performed in an interval fashion with three sets of 8 min each separated by a 5min rest period. Exercise was performed with the dorsal aspect of the forearm stabilized on a flat surface below the level of the shoulder so that the wrist could hyperextend. The weight was lifted through the entire range of motion once every 5 s and then returned to the original position. Subjects were instructed to place the nonexercising arm in a comfortable position and to avoid any muscular contraction in that arm. Each subject completed an exercise log, recording the frequency of training and the heart rate response during training. They were also encouraged to continue their normal activities and not to initiate any additional exercise program during their training period. Statistical analysis. Data from subjects at rest and during submaximal exercise for each forearm before and after training were compared by using the Wilcoxon signed ranks test. P < 0.05 was considered indicative of a significant difference. All data are reported as means t SD. RESULTS
All subjects completed the training protocol without complications. PiJPCr and pH were measured at rest and at work loads of 5, 23, and 46 J/min. One subject was excluded from the study because of an increase in heart rate during training that was ~5% over his base line. Table 2 and Fig. 2 show the effect of forearm training on P;/PCr, blood flow, and pH in the ND forearm of our TABLE 2. Effects of training
PH Rest 5 J/min 23 J/min 46 J/min Forearm blood flow, ml 100 ml-l min-’ Rest 5 J/min 23 J/min 46 J/min Maximal endurance,* contractions MVC strength, N l
Values * Statistical 0.05).
are means difference
on nondominant
forearm
Before Training
After Training
7.06zkO.08 6.73zk0.29 6.701kO.21 6.53kO.27
6.96&O. 14 6.9OkO. 11 7.02zko.09 6.87kO.10
3.1kO.7 5.8t1.8 7.3t2.9 12.5t5.1 51.6k23.4
3.4tl.2 5.524.1 7.7tl.4 10.9t4.0 83.2k52.0
l
284.8MO
265.8&64
t SD. MVC, maximal voluntary between before- and after-training
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contraction. values (P C
292
TRAINING-INDUCED 2.5
SKELETAL
MUSCLE 2.5
epIB 3EFORE IIAINING m AFfER TRAJNING
ADAPTATIONS m -
BEFORE TRAINING AFTER TRAINING
2.0 1 $
1.5
\ cl-
1.0 0.5 0.0 RfSt
23
3
40
WORKLOAD (JMN’I)
WORKLOAD (JMN-‘) FIG, 2. Inorganic phosphate to phosphocreatine ratio (Pi/PCr) before and after training at rest and during submaximal exercise in nondominant forearm. * Statistically significant difference between before- and after-training Pi/PCr.
FIG. 3. Inorganic phosphate to phosphocreatine ratio {Pi/PCr) before and after training at rest and during submaximal exercise in dominant forearm. * Statistically significant difference between beforeand after-training Pi/PCr.
3. Effects of nondominunt training on dominant untrained forearm
TABLE
4. Relative ATP concentrations before and after training in nondominant forearm
TABLE
Before Training
PII 7.02kO.07 Rest 6.86kO.07 5 J/min 23 Jjmin 6.91kO.09 46 J/min 6.95kO.09 Forearm blood flow, ml. 100 ml-’ min-l Rest 3.1k1.2 5 J/min 3.521.0 9.2k3.3 23 J/min 12.7k3.1 46 J/min Values are means + SD. No statistical difference and after-training values.
Before Training
After Training
7.05t0.05 6.99kO.07 7.01~0.05 7.Olt,O.O5
Rest 5 J/min 23 J/min 46 J/min Values are means k SD. and after-training values.
After Training
0.13t_O.O6 0.13t,o.o3 0.15zk0.01 0.16kO.03 0.16zk0.03 0.15kO.03 0.14t0.02 0.16t0.01 No statistical difference between before-
l
3.0tO.9 3.6t2.5 7.5k1.8 8.6kl.l between before-
subjects. After training, Pi/PCr was decreased in every subject at each work load compared with before-training values. These decreases were significant at work loads of 23 and 46 J/min. The greatest changes were seen in subjects with the highest pretraining exercise values for Pi/PCr. Endurance was also increased in the ND forearm after training in our subjects (Table 2). Training had no significant effect on blood flow or relative ATP concentration at rest or at any level of submaximal exercise in the ND forearm (Tables 2 and 4). Forearm muscle cross-sectional area and MVC were unaffected by the training protocol (Tables 1 and 2). VO 2 max also remained unchanged by forearm training (before 42.9 t 4.3 vs. after 43.0 k 3.7 ml O2 gkg-’ gmin-l). Table 3 and Fig. 3 show the effect of ND forearm training on the nontrained D forearm in our subjects. A significant decrease in Pi/PCr was seen at 23 J/min. l
DISCUSSION
This study demonstrated lower Pi/PCr during submaximal wrist flexion in the ND forearm of normal subjects after 41 t 11 days of exercise training. This program, designed to train the wrist flexor muscles, did not cause changes in 602 max)limb blood flow, or forearm flexor muscle mass. Previous studies of peripheral skeletal muscle adapta-
tions to endurance exercise typically have investigated responses to training with large muscle groups (10, 11). The effects of such endurance training include systemic (cardiac output and VO 2max) and peripheral (circulatory and muscle metabolic) adaptations which enhance exercise performance. Peripheral skeletal muscle adaptations to endurance training of this type include increases in mitochondrial density and oxidative phosphorylation enzyme activity (11, 16). Because most of these training studies have been conducted at a level above 60% of VOzrnax or 70% of peak heart rate to maximize improvements in cardiovascular fitness (1, 12), it is difficult to separate peripheral and central changes. In the present study, systemic cardiovascular training effects were avoided by training a small muscle group and by maintaining the training heart rate at a level -(5% above rest values (1). The unchanged VOWmaxafter training in these subjects supports the assumption that systemic adaptations did not contribute to the changes seen in forearm metabolism and endurance. This strategy allows the peripheral component of the training response to be evaluated independent of systemic changes. The energy required for skeletal muscle contraction is supplied by ATP hydrolysis to ADP via myofibrillar adenosinetriphosphatase (ATPase). During repetitive contractions, ADP and Pi, the breakdown products of ATP and PCr (16), are elevated in proportion to work intensity. The relationship of cytosolic ADP concentration to a given absolute submaximal work load reflects the oxidative capacity of the muscle. Unfortunately, ADP concentrations cannot be measured noninvasively. However, Pi/PCr is proportional to ADP at submaximal work loads (6.8), thereby providing an index of oxidative
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TRAINING-INDUCED
SKELETAL
capacity that can be measured by 31P-MRS. During steady-state submaximal exercise the 60, required for a given work load is constant. Therefore, the available mitochondrial respiratory chains m.ust be stimulated to the extent necessary to achieve the VoZ required to produce ATP aerobically. Endurance training increases the muscle mitochondrial content and oxidative capacity without changing the O2 requirements at a given steady-state submaximal exercise. Because the number of respiratory chains increases with training, each individual chain is required to work at a lower intensity after training to produce the ATP required for work performance (22). This lower intensity results in less ADP at a given submaximal work load and would be reflected by a lower Pi/PCr. We suspect that the lower Pi/PCr demonstrated after training in this study during submaximal exercise most likely reflects an increase in the oxidative capacity of the exercising muscle. However, several other mechanisms could also explain the reduced Pi/PCr and these were examined. Increases in muscle mass or strength presumably would decrease the Pi/PCr at a given submaximal work load because the sampled tissue would be working at a lower relative intensity. In the present study, neither muscle mass, estimated by magnetic resonance imaging, nor maximal strength, measured as MVC strength, was affected by training. Therefore, neither of these mechanisms is likely to account for the observed decrease in Pi/PCr. Another mechanism that might explain the lower Pi/ PCr after training would be differences in neural recruitment patterns before and after training. Although some types of exercise training, such as weight lifting, produce alterations in motor unit recruitment patterns (14, 21), ND Forearm post-training
ND Forearm pre-training
FIG. 4. Representative of nondominant forearm
magnetic resonance cross-sectional before and after training.
images
MUSCLE
ADAPTATIONS
293
the effect of endurance training on this parameter is not well defined (22). To minimize neural contributions to changes in Pi/PCr with training, each forearm was exercised in an identical manner at the same absolute work loads before and after training. Because motor unit recruitment proceeds in an orderly fashion depending on the force and orientation of the movement (22), we believe that any possible neural contribution to the observed decrease in Pi/PCr was minimal. The MRS measurement of muscle Pi/PCr during exercise is restricted to the sample volume directly over the surface coil. Inconsistent placement of the forearm over the coil could lead to evaluation of slightly different regions of muscle before and after training, which could influence Pi/PCr. To facilitate collecting data from the same muscle sample volume, great care was taken to position the forearm identically with each test. Furthermore, the 31P-MRS probe was shaped to comfortably support, stabilize, and minimize movement of the forearm during exercise. Although the investigated sample volume probably includes both working and nonworking muscle, a comparable fraction of working muscle was evaluated in the forearms before and after training because there were no significant differences between forearms in cross-sectional area (Fig. 4). Thus metabolic comparisons of forearms should be valid. Another variable that might cause a reduction in Pi/ PCr would be an increase in blood flow. The effect of training on submaximal exercise blood flow in normal subjects is not clearly defined. Researchers have reported that submaximal limb blood flow is unchanged (23) or reduced after training (3, 27). Our data are consistent with the finding that training does not alter total limb blood flow during an absolute submaximal exercise. Nevertheless, blood flow distribution may be altered by training (19), which could result in more effective matching of perfusion to tissue requirements, thereby reducing Pi/PCr. Although venous occlusion plethysmography provides an approximate index of flow to exercising skeletal muscles (24-26), changes in inter- and intramuscular blood flow distribution would not be detected by this technique. A clearer understanding of the training response obviously requires the determination of how training affects blood flow distribution within active muscle. However, total limb perfusion in itself is an important concept when dealing with a compromised circulatory system. If submaximal muscle metabolism and endurance can be achieved without altering overall limb perfusion, and therefore without requiring an elevated cardiac output, such training may offer a clinical benefit with minimal risk. Importantly, exercise performance could be improved in patients with debilitating cardiovascular disease without the hazards associated with systemic training. An unexpected finding was the trend toward a lower Pi/PCr after training in the untrained D forearm, which reached significance only at 23 J/min. Although the physiological relevance of these small changes in the D forearm is unclear, it warrants discussion. Training of one limb may elicit improvements in performance in the untrained, as well as the trained, limb. This phenome-
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294
TRAINING-INDUCED
SKELETAL
non, termed “cross-transfer,” is well documented in strength training (14) and has been attributed to neural adaptations (22), with little or no contribution by muscle hypertrophy (21) or metabolic adaptations (17). Although less well characterized, endurance training may also produce a cross-transfer effect on the untrained limb. Hardman et al. (13) found that one-leg training resulted in improved endurance in the untrained leg although, in contrast to the trained limb, parallel improvements in skeletal muscle oxidative capacity were not found. Another possible explanation for the lower P;/PCr after training is that contractions of the nonexercising forearm occurred during training in an attempt to stabilize body position. This could have resulted in a small training effect, which would be reflected by a reduction in PJPCr in the D forearm. Because this protocol was not designed to address training effects in the untrained limb, conclusions cannot be drawn. A protocol to specifically examine this observation is ongoing. We conclude that I) forearm training results in lower PJPCr at identical submaximal work loads; 2) this improvement is independent of changes in ~~~~~~~muscle mass, or limb blood flow; and 3) these differences are associated with improved endurance and may reflect improved oxidative capacity of skeletal muscle. The authors thank Deanna Adams and Julia Martin for excellent secretarial support. This work was supported in part by the New Mexico Affiliate of the American Heart Association and General Clinical Research Program Division of Research Resources Grant 5-MOl-RR-00997-14 from the National Institutes of Health. Address for reprint requests: J. R. Minotti, Dept. of Internal Medicine, Section of Cardiology, 1 llC, Veterans Administration Medical Center, University of California, 4150 Clement, San Francisco, CA 94121. Received 28 March 1989; accepted in final form 28 August 1989. REFERENCES 1. AMERICAN COLLEGE OF SPORTS MEIXINE. Position paper on the recommended quantity and quality of exercise for developing and maintaining fitness in healthy adults. 1Med. Sci. Sports 10: VII-X, 1978. 2. ARNOLD, D. L., P. M. MATTHEWS, AND G. K. RADDA. Metabolic recovery after exercise and assessment of mitochondrial function in vivo in human skeletal muscle by means of 31P NMR. Magn. Reson. Med. 1: 307-315,1984. 3. BERGMAN, H., P. BJ~RNTORP, T.-B. CONRADSON, M. FAHLEN, J. STENBERG, AND E, VARNAUSKAS. Enzymatic and circulatory adjustments to physical training in middle-aged men. Eur. J. CLin. hvest. 3: 414-418, 1973. 4. BLOMQVIST, C. G., AND B. SALTIN. Cardiovascular adaptations to physical training. Annu. Reu. Physiol. 45: 169-189, 1983. 5. BURGER, H. C., H. W. HOREMAN, AND A. J. M. BRAKKEE. Comparison of some methods for measuring peripheral blood flow. Phys. Med. BioZ. 4: 168-175, 1959. 6. CHANCE, B., J. S. LEIGH, JR., B. J. CLARK, J. MARIS, J. KENT, S. NIOKA, AND D. SMITH. Control of oxidative metabolism and oxygen delivery in human skeletal muscle: a steady-state analysis of the work/energy cost transfer function. Proc. NC&L.Acad. Sci. USA 82: $384~8388,1985. 7. CHANCE, B., J. S. LEIGH, JR., J. KENT, AND K. MCCULLY. Metabolic control principles and 31P NMR. Federation Proc. 45: 29152920,1986. 8. CHANCE. B.. AND G. R. WILLIAMS. Respiratorv enzvmes in oxida-
MUSCLE
ADAPTATIONS
tive phosphorylation. I. Kinetics of oxygen utilization. J. Biol. Chem. 217: 383-393,1955. 9 CONSTABLE, S. H., R. J. FAVIER, J. A. MCLANE, R. D. FELL, M. * CHEN, AND J. 0. HOLLOSZY. Energy metabolism in contracting rat skeletal muscle: adaptation to exercise training. Am. J. Physiol. 253 (CeZZPhysiol. 22): C316-C322, 1987. P. D., R. B. ARMSTRONG, C. W. SAUBERT, K. PIEHL, lo* GOLLNICK, AND B. SALTIN. Enzyme activity and fiber composition in skeletal muscle of untrained and trained men. J. AppZ. Physid. 33: 312319,1972. 11. GOLLNICK, P. D,, AND B. SALTIN. Significance of skeletal muscle oxidative enzyme enhancement with endurance training. Clin. Physiol. Oxf. 2: 1-12, 1982. 12, GOSSARD, D., W. L. HASKELL, C. B. TAYLOR, J. K. MUELLER, F. ROGERS, M. CHANDLER, D. K. AHN, N. H. MILLER, AND R. F. DEBUSK. Effects of low- and high-intensity home-based exercise training on functional capacity in healthy middle-aged men. Am. CurdioZ. 57: 446-449,1986. 13 J. HARDMAN, A. E., C. WILLIAMS, AND L. H. BOOBIS. Influence of ’ single-leg training on muscle metabolism and endurance during exercise with the trained limb and the untrained limb. J. Sports Sci. 5: 105-116, 1987. HELLEBRANDT, F. A., AND S. J. HOUTZ. Influence of bimanual 14* exercise on unilateral work capacity. J. Appl. Physiol. 2: 446-452, 1950. 15. HENRITZE, J., A. WELTMAN, R. L. SCHURRER, AND K. BARLOW. Effects of training at and above the lactate threshold on the lactate threshold and maximal oxygen uptake. Eur. J. Appl. Physiol. Occup. Physiol. 54: 84-88, 1985. l6 HOLLOSZY, J. O., AND F. W. BOOTH. Biochemical adaptations to endurance exercise in muscle. Annu. Rev. Physiol. 38: 273-291, 1976. 17. HOUSTON, M. E., FROESE, E. A., VALERIOTE, ST. P., GREEN, H. J., AND D. A. RANNEY. Muscle performance, morphology and metabolic capacity during strength training and detraining: a one leg model. Eur. J. AppZ, Physiol. Occup. Physiol. 51: 25-35, 1983. 18. JACOBUS, W. E., R. W. MOREADITH, AND K. M. VANDEGAER. Mitochondrial respiratory control. J. BioZ. Chem. 257: 2397-2402, 1982. 19. MACKIE, 8. G., AND R. L. TERJUNG, Influence of training on blood flow to different skeletal muscle fiber types. J. AppZ. Physiol. 55: 1072-1078,1983. 20. MINOTTI, J. R., E. C. JOHNSON, T. L. HUDSON, R. R. SIBBITT, L. E. WISE, E. FUKUSHIMA, AND M. V. ICENOGLE. Forearm metabolic asymmetry detected by 31P NMR during submaximal exercise. J. AppZ. Physiol. 67: 324-329, 1989. 21. MORITANI, T., AND H. A. DEVRIES. Neural factors versus hypertrophy in the time course of muscle strength gain. Am. J. Phys. Med.58:115-130,1979. 22. SALE, D. Influence of exercise and training on motor unit activation. Exercise Sport Sci. Rev. 15: 95-151, 1987. 23. SALTIN, B., K. NAZAR, D. L. COSTILL, E. STEIN, E. JANSSON, B. ES&N, AND P. D. GOLLNICK. The nature of the training response; peripheral and central adaptations to one-legged exercise. Actu Physiol. Stand. 96: 289-305,1976. 24. SINOWAY, L., J. MINOTTI, D. DAVIS, J. PENNOCK, J. BURG, T. MUSCH, AND R. ZELIS, Delayed reversal of impaired vasodilation in congestive heart failure after heart transplantation. Am. J. Cardiol. 61: 1076-1079, 1988. 25. SINOWAY, L., J. MINOTTI, T. MUS~H, D. GOLDNER, D. DAVIS, AND R. ZELIS. Enhanced metabolic vasodilation secondary to diuretic therapy in decompensated congestive heart failure. An. J. Curdiol. 60: 107-111, 1987. 26. SINOWAY, L,, T, MUSCH, J. MINOTTI, AND R. ZELIS, Enhanced maximal metabolic vasodilation in the dominant arm of tennis players. J. AppZ. Physiol. 61: 673-678, 1986. 27. VARNAUSKAS, E., P. BJ~RNTORP, M. FAHL$N, I. PREROVSK~, AND J. STENBERG. Effects of physical training on exercise blood flow and enzymatic activity in skeletal muscle. Curdiovusc. Res. 4: 418422,197O. 28. WHITNEY, R, J. Measurement of volume changes in human limbs. J. Physiol. Land. 121: l-27, 1953. 29. WILLIAMS, C. A., AND A. R. LIND. Measurement of forearm blood flow by venous occlusion plethysmography: influence of hand blood flow during sustained and intermittent isometric exercise. Eur. J. AppZ. Physiol. Occup. Physiol. 42: 141-149, 1979. l
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skeletal muscle adaptations of systemic adaptations
JOHN R. MINOTTI, EMILY C. JOHNSON, TRACEE L. HUDSON, EIICHI FUKUSHIMA, GLEN MURATA, LOIS E. WISE, THOMAS W. CHICK, AND MILTON V. ICENOGLE
GLEN
ZUROSKE,
Department of Internal Medicine, Veterans Administration Medical Center, and Research Division, Lovelace Medical Foundation, Albuquerque, New Mexico 87108
MINOTTI, JOHN R., EMILY C. JOHNSON, TRACEE L. HUDSON, GLEN ZUROSKE, EIICHI FUKUSHIMA, GLEN MURATA, LOIS E. WISE, THOMAS W. CHICK, AND MILTON V. ICENOGLE. Training-induced skeletal muscle adaptations are independent of systemic adaptations. J. Appl. Physiol. 68(I): 289-294,
troscopy (31P-MRS) was used to determine muscle pH and the inorganic phosphate-to-phosphocreatine ratio (Pi/P(%) as a function of submaximal work load, in the dominant (D) and nondominant (ND) forearms of normal subjects (20). The D forearm demonstrated lower PJPCr and higher pH at the same submaximal work load, implying an improved metabolic response to exercise. Because these subjects were not involved in unilateral arm training, the asymmetry in Pi/PCr and pH were attributed to adaptations induced by differences in daily use of the forearms. This observation suggests that training adaptations of forearm skeletal muscle are sensitive to repetitive activity and could possibly be trained independently of systemic training. To isolate the peripheral components of the training response, a small muscle group was trained at an intensity below the threshold necessary for systemic circulatory adaptations. This study determined the effects of localized forearm training on forearm PJPCr, pH, blood flow, muscle mass, and exercise endurance independent of cardiovascular adaptations. Such training may provide a means of conditioning patients who are too debilitated to undergo systemic training.
1990.-To isolate the peripheral adaptations to training, five normal subjectsexercisedthe nondominant (ND) wrist flexors for 41 & 11 days, maintaining an exerciseintensity below the threshold required for cardiovascular adaptations. Before and after training, intracellular pH and the ratio of inorganic phosphate to phosphocreatine(PiJPCr) were measuredby 31Pmagnetic resonancespectroscopy.Also maximal O2 consumption (VO, max),musclemass,and forearm blood flow were determined by gradedsystemic exercise,magnetic resonanceimaging, and venous occlusion plethysmography, respectively. Blood flow, PJPCr, and pH were measuredin both forearms at rest and during submaximalwrist flexion at 5,23, and 46 J/min. Training did not affect voz max,exerciseblood flow, or musclemass. RestingpH, PJPCr, and blood flow were alsounchanged.After training, the ND forearm demonstratedsignificantly lower Pi/ PCr at 23 and 46 J/min. Endurance, measuredas the number of contractions to exhaustion, also was increasedsignificantly (63%) after training in the ND forearm. We conclude that I) forearm training resultsin a lower PJPCr at identical submaximal work loads;2) this improvement is independentof changes in iTop maxy muscle mass, or limb blood flow; and 3) these differences are associatedwith improved endurance and may reflect improved oxidative capacity of skeletal muscle. MATERIALS AND METHODS
Study design. The same studies were performed on the nuclear magnetic resonancespectroscopy;musclemetabolism; subjects before and after 40 days of forearm training. endurancetraining; forearm blood flow ADAPTATIONS to endurance training of large muscle groups have been well characterized. These adaptations include greater maximal exercise cardiac output, maximal oxygen consumption (VOW&, and peak exercise muscle blood flow (4). In conjunction with these central effects, peripheral skeletal muscle demonstrates increased mitochondrial density and oxidative phosphorylation enzyme activity (11, 16). Muscles with high mitochondrial density exhibit increased oxidative capacity and augmented capacity for ATP production via oxidative phosphorylation (9, 16). These peripheral adaptations have been suggested as the primary determinant of endurance during submaximal exercise (23). Little is known about the response of human skeletal muscle to training independent of systemic cardiovascular adaptations. Recently 31P magnetic resonance specTHE CARDIOVASCULAR
Measured variables were obtained over 3 days in the following order: day I, resting forearm blood flow followed by systemic exercise testing; day 2, forearm muscle cross-sectional area and maximal voluntary contraction; day 3, forearm Pi/PCr, pH, and exercise blood flow during submaximal exercise and assessment of maximal forearm endurance. Subjects. Six normal, right-handed, nonsmoking males free of cardiovascular, pulmonary, and musculoskeletal disease were recruited from the house staff, students, and faculty of the University of New Mexico School of Medicine. Subjects were 24-40 yr old, had no physical limitations to exercise, and were not involved in any unilateral arm training (Table 1). The protocol was approved by the Human Research Review Committee of the University of New Mexico School of Medicine and the Institutional Review Board of the Lovelace Medical Foundation. The risks and benefits of the study were 289
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TRAINING-INDUCED
290 TABLE
1. Physical characteristics Subject
PP BW MG LG DC Mean * There
difference
Age, Yr
Height, cm
Weight, kg
39 40 33 38 24
166.6 178.8 169.0 154.3 166.6
71.3 73.1 88.6 54.0 68.1
167.Ok8.7
between
the nondominant
Superconducting
ADAPTATIONS
Nondominant Flexor Muscle Area, * cm2
71.0zk12.3 forearm
explained to each participant, and informed written consent was obtained. IYxercise protocol. Subjects arrived at the laboratory postprandially. Before exercise, each subject rested with his forearm in the magnet for -15 min to allow field shimming. Exercise consisted of wrist flexion over an arc (linear distance 96 mm) against an exercise handle connected to a customized computer-controlled dynamometer (Kin-Corn, Chattecx). Flexion repetition was cued every 5 s with a flashing light synchronized to 31P-MRS data collection, and exercise spectrum was obtained after each wrist flexion. Metabolic changes during exercise were obtained from the forearm flexors positioned over the spectroscopy coil (Fig. 1). After an 8-min resting spectra and warm-up, the ND forearm was exercised at power levels of 5, 23, and 46 J/min. Each work load, including warm-up, lasted 8 min. The ND forearm endurance test (see Enduranceperformance) was completed before progressing to the same warm-up and submaximal exercise in the D forearm. One subject complained of mild ND forearm fatigue at 24 J/min. To facilitate completion of the protocol, his highest work load was adjusted to 34 J/min. The ND 31P-MRS spectra at this work load demonstrated PCr depletion, so these data were grouped with the 46-J/min data from the other subjects. After training, subjects were tested using the same work loads as their pretest. Endurance performance. To test ND endurance, subjects performed wrist flexion at 69 J/min in the magnet on the same apparatus used for submaximal exercise. Each subject performed wrist flexion at his own pace until unable to complete a subsequent flexion within 5 s of the previous contraction. The number of contractions performed was defined as exercise endurance. 31P-MRS. A magnetic resonance spectrometer/imager (Nalorac) and a 1.9-T superconducting magnet (Oxford) with a 3O-cm-diam, horizontal, room temperature bore 1.97
MUSCLE
of subjects
34.8k6.6
,t SD
was no significant
SKELETAL
Magnet
am
flexor
muscle
Before
After
29.8 25.5 24.3 17.7 19.2
27.2 26.5 25.8 18.8 20.3
23.3t4.9 area before
and after
23.723.8 training.
were used to collect spectra. The subject’s grounded forearm rested over a 4-cm, two-turn surface coil made of 14-gauge copper wire under a 3-mm acrylic sheet. Guides were attached to the probe on either side of the forearm to facilitate reproducible positioning. Magnetic field homogeneity was optimized by shimming with the phosphorus coil tuned to the proton frequency, initially using a phantom and then the subject’s arm, The optimum forearm proton line width for the magnitude spectrum was 60 Hz. Free induction decays (FIDs) were collected after 45-ps pulses, which were 180" pulses at the sample surface and 90” pulses 12 mm into the sample. Sweep width and filter settings were 3,000 Hz. The last 60 FIDs (5 min) at rest and each exercise load were used for analysis. FIDs were multiplied by a decaying exponential to give 7 Hz line broadening. Relative Pi, PCr, and ,&ATP spectral peak heights were measured from the base line. Normalized ATP concentration was calculated as ATP = Muscle intracellular cal shift difference scribed (2).
P-ATP Pi + PCr + P-ATP
(0
pH was calculated from the chemibetween Pi/PCr, as previously de-
Resting skeletal muscle blood flow. Venous occlusion plethysmography was used to measure skeletal muscle blood flow (5, 20, 28). This technique utilizes two inflation cuffs with the upper cuff secured as far from the
olecranon process as possible and the lower cuff secured around the wrist. When the upper cuff is briefly inflated to 40 mmHg (venous occlusive pressure), arterial blood enters the forearm but venous outflow is obstructed, resulting in an increase in the volume of the forearm. If it is assumed that the shape of the forearm approximates a cylinder, changes in circumference are directly proportional to changes in volume and, therefore, blood flow. A single-strand mercury-in-Silastic strain gauge positioned 10 cm distal to the olecranon process measured changes in forearm circumference. To ensure adequate venous drainage, the arm rested on a board and was raised 10 cm above the level of the right atrium.
FIG. 1. Magnetic resonance nous occlusion plethysmography
spectroscopy magnet, flow apparatus.
exercise,
and ve-
Resting blood flows were obtained as follows. The wrist cuff was inflated to 240 mmHg for 1 min before flow measurements to eliminate venous return from the hand. Arterial inflow tracings were then obtained by inflating the upper cuff to 40 mmHg to occlude the venous system. Three resting blood flow determinations were averaged.
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TRAINING-INDUCED
SKELETAL
Resting blood flow was calculated as milliliters per 100 ml of volume per minute. Exercise skeletal muscle blood flow. Forearm blood flow during submaximal exercise was also measured with venous occlusion plethysmography. For exercise measurements, the strain gauge was secured on the forearm 1 cm proximal to the region where the 31P-MRS signal was obtained. A rapid-inflation cuff was then secured around the upper arm, and the arm was positioned in the bore of the magnet at approximately the same height as the shoulder to ensure no obstruction to venous return. The cuff was inflated to 40 mmHg, and arterial inflow tracings were obtained within 2 s after completion of the final exercise contraction at each work load. Exercise blood flow was measured at all work loads in both forearms and was calculated and expressed similarly to resting flows. Forearm mu&e mass. Relative muscle mass in the ND forearm, before and after training, was determined by magnetic resonance imaging (20) with a 1.5 T. imaging system (Signa, General Electric). Two-dimensional transverse images were taken using a Helmholtz extremity coil. The images were obtained from 3-mm-thick slices in the same location on the forearm, approximately one-third the ulnar length distal to the olecranon process. A spin echo sequence with a pulse repetition time (TR) of 300 ms, an echo time (TE) of 20 ms, acquisition matrix of 256 x 256, and a lo-cm field of view was used without signal averaging. Images were analyzed with the GE areatracing software. Cross-sectional area of the flexor muscle group was measured before and after training. Bone and subcutaneous fat were excluded from area measurements; vascular and nerve areas were small and not separated from surrounding tissue (see Fig. 4). It was assumed, before and after training, that within the same forearm, the cross-sectional area was proportional to muscle mass. Maximal voluntary contraction (MVC). MVC strength of the wrist flexors was tested on an arm exercise apparatus identical with the one used in the magnet. With the arm positioned as it was for the magnet studies, subjects performed a maximal isometric wrist flexion contraction with the handle at the midpoint of the range of motion. Three maximal efforts were performed, separated by at least 15 s of rest. The highest force of these three contractions was defined as MVC strength. Systemic exercise testing. Upright exercise testing was performed using an electronically braked cycle ergometer (Erich Jaeger, Rockford, IL) which maintains constant work load at pedal frequencies of 40-100 rpm. Testing was performed in an air-conditioned laboratory with an ambient temperature of 22-24°C and humidity of 3040%. Before exercise subjects rested upright on the cycle ergometer for 3-5 min. Exercise was then performed at 40 W for 3 min, succeeded by increments of 40 W every 3 min. Exercise was continued until exhaustion, which was defined as the inability to maintain critical pedal frequency (>40 rpm). Standard verbal encouragement was used for all subjects. Respiratory gas exchange was measured continuously (Ergupneumotest, Erich Jaeger). VoZmax was defined as a plateau of Vop with increasing work load in association with a respiratory exchange
MUSCLE
291
ADAPTATIONS
ratio X.15. These criteria were met in all tests. Training program. The training sessions consisted of wrist flexion exercise using a hand-held weight performed with the ND forearm. The sessions were performed an average of six times per week, for a mean total of 41 -i- 11 training sessions. The weights were plastic containers measuring 5.5 x 11 cm filled with lead shot to attain the desired weight and padding to stabilize the shot. The initial work loads were the same (1.9 kg) for each subject. The work load was increased once during training (3.3 t 1.5 kg). All work loads were selected to maintain a heart rate within 5% of base line. Each exercise session was performed in an interval fashion with three sets of 8 min each separated by a 5min rest period. Exercise was performed with the dorsal aspect of the forearm stabilized on a flat surface below the level of the shoulder so that the wrist could hyperextend. The weight was lifted through the entire range of motion once every 5 s and then returned to the original position. Subjects were instructed to place the nonexercising arm in a comfortable position and to avoid any muscular contraction in that arm. Each subject completed an exercise log, recording the frequency of training and the heart rate response during training. They were also encouraged to continue their normal activities and not to initiate any additional exercise program during their training period. Statistical analysis. Data from subjects at rest and during submaximal exercise for each forearm before and after training were compared by using the Wilcoxon signed ranks test. P < 0.05 was considered indicative of a significant difference. All data are reported as means t SD. RESULTS
All subjects completed the training protocol without complications. PiJPCr and pH were measured at rest and at work loads of 5, 23, and 46 J/min. One subject was excluded from the study because of an increase in heart rate during training that was ~5% over his base line. Table 2 and Fig. 2 show the effect of forearm training on P;/PCr, blood flow, and pH in the ND forearm of our TABLE 2. Effects of training
PH Rest 5 J/min 23 J/min 46 J/min Forearm blood flow, ml 100 ml-l min-’ Rest 5 J/min 23 J/min 46 J/min Maximal endurance,* contractions MVC strength, N l
Values * Statistical 0.05).
are means difference
on nondominant
forearm
Before Training
After Training
7.06zkO.08 6.73zk0.29 6.701kO.21 6.53kO.27
6.96&O. 14 6.9OkO. 11 7.02zko.09 6.87kO.10
3.1kO.7 5.8t1.8 7.3t2.9 12.5t5.1 51.6k23.4
3.4tl.2 5.524.1 7.7tl.4 10.9t4.0 83.2k52.0
l
284.8MO
265.8&64
t SD. MVC, maximal voluntary between before- and after-training
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contraction. values (P C
292
TRAINING-INDUCED 2.5
SKELETAL
MUSCLE 2.5
epIB 3EFORE IIAINING m AFfER TRAJNING
ADAPTATIONS m -
BEFORE TRAINING AFTER TRAINING
2.0 1 $
1.5
\ cl-
1.0 0.5 0.0 RfSt
23
3
40
WORKLOAD (JMN’I)
WORKLOAD (JMN-‘) FIG, 2. Inorganic phosphate to phosphocreatine ratio (Pi/PCr) before and after training at rest and during submaximal exercise in nondominant forearm. * Statistically significant difference between before- and after-training Pi/PCr.
FIG. 3. Inorganic phosphate to phosphocreatine ratio {Pi/PCr) before and after training at rest and during submaximal exercise in dominant forearm. * Statistically significant difference between beforeand after-training Pi/PCr.
3. Effects of nondominunt training on dominant untrained forearm
TABLE
4. Relative ATP concentrations before and after training in nondominant forearm
TABLE
Before Training
PII 7.02kO.07 Rest 6.86kO.07 5 J/min 23 Jjmin 6.91kO.09 46 J/min 6.95kO.09 Forearm blood flow, ml. 100 ml-’ min-l Rest 3.1k1.2 5 J/min 3.521.0 9.2k3.3 23 J/min 12.7k3.1 46 J/min Values are means + SD. No statistical difference and after-training values.
Before Training
After Training
7.05t0.05 6.99kO.07 7.01~0.05 7.Olt,O.O5
Rest 5 J/min 23 J/min 46 J/min Values are means k SD. and after-training values.
After Training
0.13t_O.O6 0.13t,o.o3 0.15zk0.01 0.16kO.03 0.16zk0.03 0.15kO.03 0.14t0.02 0.16t0.01 No statistical difference between before-
l
3.0tO.9 3.6t2.5 7.5k1.8 8.6kl.l between before-
subjects. After training, Pi/PCr was decreased in every subject at each work load compared with before-training values. These decreases were significant at work loads of 23 and 46 J/min. The greatest changes were seen in subjects with the highest pretraining exercise values for Pi/PCr. Endurance was also increased in the ND forearm after training in our subjects (Table 2). Training had no significant effect on blood flow or relative ATP concentration at rest or at any level of submaximal exercise in the ND forearm (Tables 2 and 4). Forearm muscle cross-sectional area and MVC were unaffected by the training protocol (Tables 1 and 2). VO 2 max also remained unchanged by forearm training (before 42.9 t 4.3 vs. after 43.0 k 3.7 ml O2 gkg-’ gmin-l). Table 3 and Fig. 3 show the effect of ND forearm training on the nontrained D forearm in our subjects. A significant decrease in Pi/PCr was seen at 23 J/min. l
DISCUSSION
This study demonstrated lower Pi/PCr during submaximal wrist flexion in the ND forearm of normal subjects after 41 t 11 days of exercise training. This program, designed to train the wrist flexor muscles, did not cause changes in 602 max)limb blood flow, or forearm flexor muscle mass. Previous studies of peripheral skeletal muscle adapta-
tions to endurance exercise typically have investigated responses to training with large muscle groups (10, 11). The effects of such endurance training include systemic (cardiac output and VO 2max) and peripheral (circulatory and muscle metabolic) adaptations which enhance exercise performance. Peripheral skeletal muscle adaptations to endurance training of this type include increases in mitochondrial density and oxidative phosphorylation enzyme activity (11, 16). Because most of these training studies have been conducted at a level above 60% of VOzrnax or 70% of peak heart rate to maximize improvements in cardiovascular fitness (1, 12), it is difficult to separate peripheral and central changes. In the present study, systemic cardiovascular training effects were avoided by training a small muscle group and by maintaining the training heart rate at a level -(5% above rest values (1). The unchanged VOWmaxafter training in these subjects supports the assumption that systemic adaptations did not contribute to the changes seen in forearm metabolism and endurance. This strategy allows the peripheral component of the training response to be evaluated independent of systemic changes. The energy required for skeletal muscle contraction is supplied by ATP hydrolysis to ADP via myofibrillar adenosinetriphosphatase (ATPase). During repetitive contractions, ADP and Pi, the breakdown products of ATP and PCr (16), are elevated in proportion to work intensity. The relationship of cytosolic ADP concentration to a given absolute submaximal work load reflects the oxidative capacity of the muscle. Unfortunately, ADP concentrations cannot be measured noninvasively. However, Pi/PCr is proportional to ADP at submaximal work loads (6.8), thereby providing an index of oxidative
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TRAINING-INDUCED
SKELETAL
capacity that can be measured by 31P-MRS. During steady-state submaximal exercise the 60, required for a given work load is constant. Therefore, the available mitochondrial respiratory chains m.ust be stimulated to the extent necessary to achieve the VoZ required to produce ATP aerobically. Endurance training increases the muscle mitochondrial content and oxidative capacity without changing the O2 requirements at a given steady-state submaximal exercise. Because the number of respiratory chains increases with training, each individual chain is required to work at a lower intensity after training to produce the ATP required for work performance (22). This lower intensity results in less ADP at a given submaximal work load and would be reflected by a lower Pi/PCr. We suspect that the lower Pi/PCr demonstrated after training in this study during submaximal exercise most likely reflects an increase in the oxidative capacity of the exercising muscle. However, several other mechanisms could also explain the reduced Pi/PCr and these were examined. Increases in muscle mass or strength presumably would decrease the Pi/PCr at a given submaximal work load because the sampled tissue would be working at a lower relative intensity. In the present study, neither muscle mass, estimated by magnetic resonance imaging, nor maximal strength, measured as MVC strength, was affected by training. Therefore, neither of these mechanisms is likely to account for the observed decrease in Pi/PCr. Another mechanism that might explain the lower Pi/ PCr after training would be differences in neural recruitment patterns before and after training. Although some types of exercise training, such as weight lifting, produce alterations in motor unit recruitment patterns (14, 21), ND Forearm post-training
ND Forearm pre-training
FIG. 4. Representative of nondominant forearm
magnetic resonance cross-sectional before and after training.
images
MUSCLE
ADAPTATIONS
293
the effect of endurance training on this parameter is not well defined (22). To minimize neural contributions to changes in Pi/PCr with training, each forearm was exercised in an identical manner at the same absolute work loads before and after training. Because motor unit recruitment proceeds in an orderly fashion depending on the force and orientation of the movement (22), we believe that any possible neural contribution to the observed decrease in Pi/PCr was minimal. The MRS measurement of muscle Pi/PCr during exercise is restricted to the sample volume directly over the surface coil. Inconsistent placement of the forearm over the coil could lead to evaluation of slightly different regions of muscle before and after training, which could influence Pi/PCr. To facilitate collecting data from the same muscle sample volume, great care was taken to position the forearm identically with each test. Furthermore, the 31P-MRS probe was shaped to comfortably support, stabilize, and minimize movement of the forearm during exercise. Although the investigated sample volume probably includes both working and nonworking muscle, a comparable fraction of working muscle was evaluated in the forearms before and after training because there were no significant differences between forearms in cross-sectional area (Fig. 4). Thus metabolic comparisons of forearms should be valid. Another variable that might cause a reduction in Pi/ PCr would be an increase in blood flow. The effect of training on submaximal exercise blood flow in normal subjects is not clearly defined. Researchers have reported that submaximal limb blood flow is unchanged (23) or reduced after training (3, 27). Our data are consistent with the finding that training does not alter total limb blood flow during an absolute submaximal exercise. Nevertheless, blood flow distribution may be altered by training (19), which could result in more effective matching of perfusion to tissue requirements, thereby reducing Pi/PCr. Although venous occlusion plethysmography provides an approximate index of flow to exercising skeletal muscles (24-26), changes in inter- and intramuscular blood flow distribution would not be detected by this technique. A clearer understanding of the training response obviously requires the determination of how training affects blood flow distribution within active muscle. However, total limb perfusion in itself is an important concept when dealing with a compromised circulatory system. If submaximal muscle metabolism and endurance can be achieved without altering overall limb perfusion, and therefore without requiring an elevated cardiac output, such training may offer a clinical benefit with minimal risk. Importantly, exercise performance could be improved in patients with debilitating cardiovascular disease without the hazards associated with systemic training. An unexpected finding was the trend toward a lower Pi/PCr after training in the untrained D forearm, which reached significance only at 23 J/min. Although the physiological relevance of these small changes in the D forearm is unclear, it warrants discussion. Training of one limb may elicit improvements in performance in the untrained, as well as the trained, limb. This phenome-
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294
TRAINING-INDUCED
SKELETAL
non, termed “cross-transfer,” is well documented in strength training (14) and has been attributed to neural adaptations (22), with little or no contribution by muscle hypertrophy (21) or metabolic adaptations (17). Although less well characterized, endurance training may also produce a cross-transfer effect on the untrained limb. Hardman et al. (13) found that one-leg training resulted in improved endurance in the untrained leg although, in contrast to the trained limb, parallel improvements in skeletal muscle oxidative capacity were not found. Another possible explanation for the lower P;/PCr after training is that contractions of the nonexercising forearm occurred during training in an attempt to stabilize body position. This could have resulted in a small training effect, which would be reflected by a reduction in PJPCr in the D forearm. Because this protocol was not designed to address training effects in the untrained limb, conclusions cannot be drawn. A protocol to specifically examine this observation is ongoing. We conclude that I) forearm training results in lower PJPCr at identical submaximal work loads; 2) this improvement is independent of changes in ~~~~~~~muscle mass, or limb blood flow; and 3) these differences are associated with improved endurance and may reflect improved oxidative capacity of skeletal muscle. The authors thank Deanna Adams and Julia Martin for excellent secretarial support. This work was supported in part by the New Mexico Affiliate of the American Heart Association and General Clinical Research Program Division of Research Resources Grant 5-MOl-RR-00997-14 from the National Institutes of Health. Address for reprint requests: J. R. Minotti, Dept. of Internal Medicine, Section of Cardiology, 1 llC, Veterans Administration Medical Center, University of California, 4150 Clement, San Francisco, CA 94121. Received 28 March 1989; accepted in final form 28 August 1989. REFERENCES 1. AMERICAN COLLEGE OF SPORTS MEIXINE. Position paper on the recommended quantity and quality of exercise for developing and maintaining fitness in healthy adults. 1Med. Sci. Sports 10: VII-X, 1978. 2. ARNOLD, D. L., P. M. MATTHEWS, AND G. K. RADDA. Metabolic recovery after exercise and assessment of mitochondrial function in vivo in human skeletal muscle by means of 31P NMR. Magn. Reson. Med. 1: 307-315,1984. 3. BERGMAN, H., P. BJ~RNTORP, T.-B. CONRADSON, M. FAHLEN, J. STENBERG, AND E, VARNAUSKAS. Enzymatic and circulatory adjustments to physical training in middle-aged men. Eur. J. CLin. hvest. 3: 414-418, 1973. 4. BLOMQVIST, C. G., AND B. SALTIN. Cardiovascular adaptations to physical training. Annu. Reu. Physiol. 45: 169-189, 1983. 5. BURGER, H. C., H. W. HOREMAN, AND A. J. M. BRAKKEE. Comparison of some methods for measuring peripheral blood flow. Phys. Med. BioZ. 4: 168-175, 1959. 6. CHANCE, B., J. S. LEIGH, JR., B. J. CLARK, J. MARIS, J. KENT, S. NIOKA, AND D. SMITH. Control of oxidative metabolism and oxygen delivery in human skeletal muscle: a steady-state analysis of the work/energy cost transfer function. Proc. NC&L.Acad. Sci. USA 82: $384~8388,1985. 7. CHANCE, B., J. S. LEIGH, JR., J. KENT, AND K. MCCULLY. Metabolic control principles and 31P NMR. Federation Proc. 45: 29152920,1986. 8. CHANCE. B.. AND G. R. WILLIAMS. Respiratorv enzvmes in oxida-
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