Muscle membrane excitability is thought to decline with aging; the extent of this decline may be noninvasively assessed by measurement of the electrically evoked compound muscle action potential (M-wave). The intent of this study was two-fold: (1) to compare the M-wave in the brachioradialis (BR), tibialis anterior (TA), and thenar (TH) muscles of elderly (mean age = 66.3 r 3.7 years) and young (mean age = 31.2 5 4.9 years) adults, and (2) to determine the effects of 12 weeks of resistance training on M-wave characteristics in elderly adults. Prior to training, the elderly subjects had significantly smaller (P < 0.05) resting M-waves than the young aduits in the BR (4.8 mV vs. 8.7 mV), TA (8.8 mV vs. 11.0 mV), and TH (5.2 mV vs. 10.2 mV) muscles. During a 2-minute voluntary fatigue paradigm (3 seconds MVC per 2 seconds rest for 2 minutes), there was no evidence of excitability failure in either group. Following training, there was a significant increase (P < 0.05) in the size of the M-wave of the TH (pretraining: 5.2 mV; posttraining: 8.96 mV) and BR (pretraining: 4.8 mV; posttraining: 6.1 mV), and a nonsignificant increase in the M-wave of the TA, but there was no change in the relative behavior of the M-wave during the 2-minute voluntary fatigue paradigm. It is suggested that the decline in muscle membrane excitation with aging may be due, at least in part, to the effects of a decreased membrane potential on the muscle fiber action potential. Furthermore, our results suggest that exercise training may improve muscle excitability in elderly adults, possibly reflecting a training-induced increase in the resting membrane potential due to an increased activity and/or concentration of the skeletal muscle Na+-K+-ATPase. Key words: skeletal muscle M-wave aging training MUSCLE & NERVE 15:87-93 1992
MUSCLE EXClTATlON IN ELDERLY ADULTS: THE EFFECTS OF TRAINING AUDREY L. HICKS, PhD, CYNTHIA M. CUPIDO, BPE, JOAN MARTIN, MSc, and JENNIFER DENT, MSc
It is well established that muscular performance decreases with aging. Morphological changes, such as a decrease in muscle fiber number and muscle fiber size7714922 may account for a significant part of this decline in performance, however, changes in muscle membrane electrical properties with aging may also play a role. Animal studies have suggested that there is a progressive loss of muscle membrane excitability during aging which will limit the functional activity of the muscle. Periph~~
~
From the Department of Physical Education, McMaster University, Hamilton, Ontario, Canada. Acknowledgments: The authors gratefully acknowledge the technical assistance of John Moroz. This work was supported by the Science and Engineering Research Board at McMaster University. Address reprint requests to Audrey L. Hicks, Department of Physical Education, McMaster University, 1280 Main St. West, Hamilton, Ontario, L8s 4K1, Canada. Accepted for publication February 13, 1991. CCC 0148-639W92/01087-07 $04.00 0 1992 John Wiley & Sons, Inc.
M-Wave Characteristics in the Elderly
era1 factors which contribute to this loss of excitability include a decrease in chloride conductance, which prolongs the duration of the action potential, a decreased resting membrane potential, an increase in the critical polarization threshold, and a decreased activity andlor efficiency of the Na+-K+ T h e peak-to-peak amplitude of the compound muscle action potential (M-wave) is representative of the amount of muscle membrane excitation, as it is dependent on both the resting membrane potential and the extent of the overshoot of the action potential of the single muscle fibers. l 8 Significant decrements in the size of the M-waves have been reported in the ankle dorsi- and plantar-flexon2' and the extensor digitorum brevis4 after the age of 60 years, but there are no other reports of changes in M-wave size with aging in other muscle groups. In addition to offering information on muscle excitability at rest, the M-wave is also commonly
MUSCLE & NERVE
January 1992
87
used during fatigue experiments to assess excitability; specifically, it provides information on the effectiveness of electrical propagation across the neuromuscular junction and along the surface membrane of the muscle fibers. During the course of maximal voluntary contractions in young adults, most studies have shown that the M-wave undergoes little or no decrement, suggesting that muscle activation is maintained and neuromuscular transmission is More recently, Hicks et a1.8 noted that the M-wave actually increased in both amplitude and area during the course of intermittent maximal voluntary contractions of the thenar and extensor digitorum brevis muscles. It was suggested that the potentiation of the M-wave was likely due to an increase in the peak-to-peak amplitude of single fiber action potentials resulting from a hyperpolarization of the muscle fiber membranes. This has recently been shown to occur in rat soleus muscles coincidentally with M-wave potentiation, following tetanic stimul a t i ~ nIn . ~the latter study, increased activity of the Na+-K+ pump, which increases the resting membrane potential of the muscle fibers, was found to be the cause of the enlargement of the action potentials and, hence, the M-wave. Thus, under certain conditions, the M-wave may be used as a noninvasive index of Na+- K+-ATPase activity in skeletal muscle. With this background, the purpose of this study was to compare the M-waves at rest and during a voluntary fatigue paradigm in elderly and young adults. In addition, the effects of 12 weeks of resistance training on M-wave characteristics in elderly adults was examined.
posable Ag/AgCl discs (3M, No. 2248) were taped approximately 4 cm apart over the radial nerve, just proximal to the radial-humeral joint; for T H studies, 2 silver discs (9 mm diameter) mounted in a Plexiglas holder were taped over the median nerve at the wrist; and, for T A studies, a lead plate cathode was secured over the common peroneal nerve just proximal to the head of the fibula, and the anode on the superior border of the tibia. The stimuli were rectangular 5 0 - ~ spulses delivered from a high-voltage stimulator (Grass Instruments, Model S11B); the latter being triggered by a computerized timing device. For BR and TA studies, the recording electrodes were disposable Ag/AgCl discs (as above); the stigmatic electrode being placed on the muscle belly (at the point where the largest M-wave was recorded) and the reference on the musculo-tendonous junction. In T H experiments, the recording electrodes were silver strips (5 cm x 6 mm); the stigmatic electrode was placed perpendicular to the first metacarpal bone at the junction of the proximal and middle thirds of TH, and the reference was attached to the end of the little finger. A similar silver strip served as the ground electrode for all muscle groups. The signals from the recording electrodes were fed into amplifiers with bandwidths of 20 Hz to 1.5 kHz, and were displayed on a storage oscilloscope (Hewlett-Packard, Model 1201B). At the same time, data were streamed continuously to disc by means of a Dataq waveform scrolling board (WFS-2OOPC; Dataq Instruments) in an IBM-compatible computer. Analysis of peak-to-peak amplitude, duration, and area of the M-wave was done by CODAS analysis software with custom calibrations.
METHODS
T h e elderly group consisted of 7 women and 4 men, with a mean age of 66.3 & 3.7 years, none having prior experience with weightlifting training. The young group consisted of 5 women and 6 men, with a mean age of 3 1.2 -+ 4.9 years. All subjects were volunteers, and the investigation carried the approval from the University Ethics Committee. Subjects.
Stimulating and Recording. Experiments were conducted on the brachioradialis (BR), the tibialis anterior (TA), and the thenar (TH) muscles, with the subjects sitting. In all studies, care was taken to ensure that the skin was properly prepared before electrode application and that the electrodes were coated with conducting gel. The stimulating electrodes were of several types; for BR studies, dis-
88
M-Wave Characteristics in the Elderly
For each muscle, baseline recordings of resting M-waves were made with the muscle relaxed; once the maximal M-wave was determined, the voltage of the stimulator was increased approximately 10% to ensure maximality of activation. For experiments on T H , subjects had their wrist and hand strapped into a moulded plastic cast, for BR and TA studies, custom stabilizing apparatuses were at joint angles of 100" of elbow flexion (semipronated) and 20" of plantar flexion. Subjects were instructed to perform maximal 3-second isometric contractions followed by a 2-second rest for a total time of 2 minutes. This protocol resulted in a 17% to 23% reduction in MVC torque over the course of the 2 minutes. M-waves were evoked every 5 seconds in the rest interval between contractions; care was Testing Procedure.
MUSCLE & NERVE
January 1992
taken to ensure that the positions of the particular muscle groups remained constant whenever the testing stimuli were delivered (see ref. 8). T h e voluntary contractions were timed by a light which was controlled by a computer timing device. Recovery measurements of the M-wave were made 1, 3, 5, and 10 minutes following the 2-minute fatigue paradigm. The elderly subjects participated in a 12-week strength and endurance training program which involved dynamic strength training of several muscle groups and submaximal cycle ergometry (15 to 25 minutes at 60% to 70% maximum HR), however, only the training of the T H , BR, and T A will be described. Custom-designed apparatuses were utilized for the training of the T H and TA; free-weight dumbbells were utilized for the training of the BR. For training of T H , subjects sat in a chair with their right forearm resting on a table, palmar surface down. The fingers and wrist were secured to the table with velcro strapping. Subjects inserted their right thumb into a soft cast-mould, which was attached to a weight pan by means of‘ an adjustable cable and pulley. Subjects were instructed to abduct their thumb in a downward motion against the resistance provided by the placement of weights in the pan. The TA training device required that the subject perform a complete dorsiflexion movement, while sitting in an adjustable chair with the knee at a 90” angle, and their foot strapped to a rotatable steel plate. A post at the front of the plate could be stacked with circular weights to provide resistance. For training of the BR, subjects sat at a “preacher’s” bench, and were instructed to perform full-range elbow flexions while holding dumbbells in a semipronated position. Subjects trained the BR of both arms, but for the T H and TA only the right limb was trained to provide a within-subject control. Training was done on 2 alternate days each week for 12 weeks. Exercises were done in a circuit set system, with 2-minute pauses between sets. Each set comprised 10 to 15 repetitions; training progressed from 2 sets of each exercise at 50% of the initial 1 RM to 4 sets at 70% to 80% of 1 RM over the course of the 12-week training period. Weights were adjusted every other week to accommodate for improvement in strength and to restrict each set to the 10 to 15 RM limit. Training Regimen.
All of the data were computer analyzed using analysis of variance (ANOVA).
Statistical Treatment.
M-Wave Characteristics in the Elderly
Significant differences between means were deiermined using a Tukey A post hoc test. A level of P < 0.05 was considered to be statistically significant. RESULTS Comparison of M-waves Between Elderly and Young Subjects. Baseline M-waves in all 3 muscle
groups were significantly smaller in amplitude and in area in the elderly versus the young adults, whereas M-wave duration was similar between groups. These values are summarized in Table 1. During the 2 minutes of intermittent MVCs, there was no evidence of excitation failure in either group (i.e., no decline in M-wave size), in fact, there was significant enlargement of the M-wave during the fatigue paradigm in T A and T H in the young adults. These results are illustrated in Figure 1. M-wave duration showed no consistent changes during the 2-minute Fatigue paradigm in either group, however, after the first minute o f MVCs in BK and TA, the M-wave duration was significantly greater in the elderly versus the young adults. Training. The changes in baseline M-wave size following the 12 weeks of training in the elderly subjects are also sunimarized in Table 1. There was a significant increase in M-wave size (both amplitude and area) in the T H and BK and a nonsignificant increase in the TA M-wave. Apart from the increase in baseline amplitude, the relative behavior of the M-wave during the voluntary fatigue paradigm did not change after training, as seen in Figure 1. In Figure 2, the M-waves in the trained T H are compared with those in the untrained T H within the elderly group; this figure clearly illustrates the specificity of the training effect in both M-wave amplitude and area. T h e training regimen was of sufficient intensity to induce significant increases in muscle strength. In the T H , BR, and T A muscles, there were increases in the 1 RM of 182%, 52%, and 48%, respectively. Isometric MVC torque also increased significantly after training. In the BR, MVC torque increased from 35 (k4.6) Nm pretraining to 41.7 (26.1) Nm posttraining, and in the T A , MVC torque increased from 30.5 (23.1) Nm to 35.0 (*:3.2) Nm. I t should be clarified here that both of‘ these MVC torques represent the summed action of all the elbow flexors and all of the ankle dorsiflexors; it would be impossible to voluntarily activate only the BK or the TA. Isometric torques of the T H were not measured due Effects of
MUSCLE & NERVE
January 1992
89
10.0
-
8.0
-
LD-
4.0-
120
-
Ion
-
w -
FIGURE 1. Changes in the mean M-wave amplitude ( S E ) during and following a 2-minute voluntary fatigue paradigm in (a) brachioradialis, (b) thenar, and (c) tibialis anterior muscles. Symbols represent pretrained elderly (o),posttrained elderly (a), and young adults (0).
w U -
?a-
0 -
,
1
"11 0
I
5
I6
. .. . . . .
. .
n I U W W l I O IW ma0
.
m
6-ol&
5.0
0
->
40.0
9
3
4
I
I
I
1
5
15
1' - i
30.0
-
20.0
-
I
I
I
I
I
I
I
I
1
1
25 35456080110 180 300420 720
Voluntary Contractlons
da
< w
>
2 I
I
O J I 1
I
I
5
1s
I I I I I I I I 1 I 25 35456080110 180 300420 720
TIME
(8)
FIGURE 2. Changes in the mean thenar M-wave amplitude (a) and area (b) during a 2-minute voluntary fatigue paradigm in elderly adults. The trained muscles are illustrated with squares; (Wm) preposttraining, and the untrained muscles are illustrated with circles; ( o h )pre-posttraining.
to the lack of a suitable stabilizing device equipped with a force transducer. DISCUSSION
The results from this study have drawn attention to the fact that the loss of muscle membrane excit-
M-Wave Characteristics in the Elderly
ability associated with aging may be noninvasively assessed by measurement of the M-wave. It has been demonstrated that the M-waves in 3 different muscle groups, the brachioradialis (BR), the thenar (TH), and the tibialis anterior (TA), are significantly smaller in elderly versus young
MUSCLE & NERVE
January 1992
91
~
~~
Table 1. Summary of the M-wave characterlstlcs in elderly adults before (pre) and after (post) 12 weeks of exercise traintng Values obtained in young adults included for comparison. M-wave amplitude (mv) Pre
BR Elderly Young
TH Elderly
4.8 t 0.5 8.3 t 1.3
M-wave duration (ms)
Post 6.1
?
M-wave area (CLV s )
Pre
Post
Pre
Post
0.9*
27.2 ? 1.0 28.2 2 0.8
28.0 -+ 1.3
31.9 2 3.9 57.3 ? 7.8
42.0
5.2 ? 1.2 5.8 t 1.0 10.2 t 1.o
9.0 2 1.4* 5.3 2 1.1
16.5 t 0.8 16.0 1.0 18.6 2 1 2
16.4 ? 0.9 17.6 ? 0.7
19.9 f 5.4 19.5 ? 2.7 40.8 ? 6.0
29.7 2 4.0' 18.1 2 3.1
Tr. con:
8.8 ? 0.8 8.4 t 0.5 11.0 t 0.6
9.5 ? 0.6 9.2 2 0.7
24.8 22.7 23.9
23.9 ? 1.4 23.0 2 1.5
55.3 ? 5.4 45.5 -+ 3,9 64.0 3.7
56.6 51.3
*
TA
Young
5.6*
Tr: Con:
Young
Elderly
2
? ? ?
1.4 1.2 12
*
* 4.5 * 4.6
Values are expressed as means ? SE of the mean. 'Significant change from pretraining value (P < 0.05)
adults. It is possible that the smaller M-waves may be due to factors related to tissue ultrastructure and conductivity, however, we are proposing that an age-associated change in the size of the muscle action potential is contributing to the decreased M-wave. The present observations of smaller M-waves in the older subjects are in agreement with previous s t ~ d i e s . ~ In . ~ their , ~ ' investigation of the changes in the human ankle musculature with aging, Vandervoort and McComas reported an 18% decrease in the TA M-wave between the third decade and seventh decade," which is comparable with the 22% decrease observed in the present study. As in nerve fibers, excitability of a muscle fiber is determined by the resting membrane potential. The Na+- K f pump, through its electrogenic nature, contributes approximately 10 mV to the resting membrane potential under normal steady state conditions.2 ) Elderly individuals, who have been sedentary for a number of years, may experience a down-regulation in their Na+-K+ transport system through various hormonal factors,'""2 which may partly explain the ageassociated decline in resting membrane potential and concomitant decrease in action potential amplitude. Evidence suggests, however, that under conditions of heightened pump activity, i.e., during acute exercise or after chronic exercise training, the electrogenic contribution of the pump to the resting membrane potential may increase up to three-fo1d."I3 Furthermore, in a recent crosssectional study, Klitgaard and Clausen" observed that there were more Nat-K+ pump sites in eld-
92
M-Wave Characteristics in the Elderly
erly individuals who were physically active than in those who were sedentary. 'Thus, it appears that at least one component of muscle excitability may be amenable to exercise training. During the course of maximal voluntary contractions in adults, most studies have shown that the M-wave undergoes little or no decrement, suggesting that muscle activation is maintained and neuromuscular transmission is i n t a ~ t . ' " " ~T h e results from the present investigation suggest that the same is true for elderly adults. In spite of a smaller excitable muscle mass, there is no evidence of activation failure during this type of maximal exercise. It is noteworthy that while there appeared to be some M-wave potentiation during the fatigue paradigm in the young adults, there was no such pattern in the elderly group (see Fig. 1). This may be reflective of an age-related change in the ability of the Na+-K+ pump to cope with the ionic perturbations associated with muscle activity. In the present study, the M-waves in the elderly subjects increased significantly in 2 muscle groups after training, a finding that has not been reported previously. We cannot exclude the possibility that changes in tissue ultrastructure (e.g., decreased intramuscular and subcutaneous fat) andlor skin impedence after training may have contributed to this increase. It is noteworthy, however, that the muscle group which showed the greatest change (TH) is probably the one with the least amount of subcutaneous or intramuscular fat. An intrasmuscular recording of the M-wave would have controlled for any skin or subcutane-
MUSCLE & NERVE
January 1992
ous impedence changes, and should, therefore, be considered for future studies. T h e fact that a significant increase was not found in the TA muscle could possibly be due to the fact that this muscle is used daily for ambulation, and thus may not experience the same decline in activity with aging as the other 2 muscle groups (BR and T H ) examined in this study. At this stage, the mechanism(s) responsible for this increased M-wave posttraining can only be speculated upon. In a previous study, Hicks and McComasg found muscle fiber hyperpolarization in the rat soleus to be associated with, and implicitly responsible for, the enlargement of the M-wave and of single fiber action potentials following repetitive stimulation. T h e involvement of the Na+-K+ pump in this response was supported by the observation that inhibition of Na+- K+ pump activity (by ouabain, cooling, or
K+-free solutions) abolished the hyperpolarization and the enlargement of the M-wave. The posttraining increase in M-wave size in the present study could be explained entirely by an increased resting membrane potential due to enhanced Na+- K+ pump activity. Moss and co-workers19 reported significantly higher resting membrane potentials in the tibialis anterior of trained versus sedentary adults, and Knochel et al.'?' reported a significant increase in the resting membrane potential of dog gracilis muscle after 6 weeks of training. Furthermore, there are numerous reports of increased Na+-K+ pump activity after exercise training. l ' , ' " Clearly, future studies should further examine the effects of training on muscle membrane excitation in older adults, specifically by investigating any changes in Na+-K+ pump activity and/or resting membrane potentials in muscle fibers.
REFERENCES 1. Bigland-Ritchie B, Jones DA, Woods JJ: Excitation fre-
quency and muscle fatigue: Electrical responses during human voluntary and stimulated contractions. Rxp Neurol 1979;64:414-427. 2. Bigland-Ritchie B, Kukulka CG, Lippold OCJ, Woods JJ: The absence of neuroniuscular transniission failure in sustained maximal voluntary contractions. ,I Physiol (Lond) 1982;330:265-278. 3. Brown WF: A method for estimating the number of motor units in thenar muscles and the changes in the motor unit count with aging. ,J Neurol Neurosurg Psychiatry 1972;35:845-852. 4. Campbell MJ, McComas AJ, Petito F: Physiological changes in ageing muscles. J Neurol Neurosurg P.\ychiatry 1973;36:174- 182. 5. DeLuca A, Mambrini M, Camerino DC: Changes in niembrane ionic conductances and excitability characteristics of rat skeletal muscle during aging. P j u g Arch 1990;415:642644. 6. Frolkis VV, Martynenko OA, Zamostyan VP: Aging of the neuromuscular apparatus. Gerontology 1976;22:244-279. 7. Grimby G, Saltin B: T h e ageing muscle. Clin Physiol (Oxford) 1983;3 :209 - 2 18. 8. Hicks A, Fenton J, Garner S, McComas AJ: M wave potentiation during and after muscle activity. J Appl Physiol I989;66:2606- 2610. 9. Hicks A, McComas AJ: Increased sodium pump activity following stimulation of rat soleus muscles. J Physiol (Lond) 1989;414:337-349. 10. Kjeldson K: Regulation of the concentration of 'H-ouabain binding sites in mammalian skeletal muscle-Effects of age, K-depletion, thyroid status and hypertension. Dan Med Bull 1987;34:15-46. 11. Kjeldsen K, Richter EA, Galbo H , Lortie G, Clausen T: Training increases the concentration of ['Hlouabain-bind-
M-Wave Characteristics in the Elderly
ing sites in rat skeletal muscle. Biochzm Bzophys Actu 1986;860:708- 7 12. 12. Klitgaard H, Clausen T : Increased total concentration of Na-K pumps in vastus lateralis muscle of old trained human subjects. J Appl Physiol 1%9;67:2491-2494. 13. Knochel JP, Rlachley JU, Johnson J H , Carter NW: Muscle cell electrical hyperpolarization and rcduc-ed exercise hyperkalemia in physically conditioned dogs. J Clin Inumt 1985;75:740-745. 14. 1.exell J , Henriksson-Larsen K, Winblad B, Sjostrom M: Distribution of different fibre types in human skeletal niuscles: Effects of aging studied in whole muscle cross sections. M w c l e Nerue 1983;6:588-595. 5. Marsh E, Sale D, McComas AJ, Quinlan J: Influence of joint position on ankle darsiflexion in humans. J Appl P h y 201 1981;51: 160- 167. 6. McCartney N , Moroz D, Garner SH, McComas A]: The effects of strength training in patients with selected neuromuscular disorders. Med Sci Sports Exer 1988;20:362-368. 7. Merton PA: Voluntary strength and fatigue. J Physiol (Lond) 1954; 128:553-563. 18. Milner-Brown HS, Miller RG: Muscle membrane excitation and impulse propagation velocity are reduced during muscle fatigue. Muscle Nerve 1986;9:367-374. 19. Moss RF, Raven PB, Knochel JP, Peckham JK, Blachley JD: The effect of training on resting muscle membrane potentials. Znt Ser Sports Scz 1983;13:806-811. 20. ?'tiomas RC: Electrogenic sodium pump in nerve and muscle cells. Physiol Rev 1972;52:563-594. 2 1. Vandervoort AA, McComas AJ: Contractile changes in opposing muscles of the human ankle joint with aging. J Appl Physiol 1986;61:361-367. 22. Wokke JHJ, Jennekens FGI, van den '3rd CJM, Veldman H, Smit LME, Leppink GJ: Morphological changes in the human end plate with age.J Neurol Sci 1990;95:291-310.
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MUSCLE EXClTATlON IN ELDERLY ADULTS: THE EFFECTS OF TRAINING AUDREY L. HICKS, PhD, CYNTHIA M. CUPIDO, BPE, JOAN MARTIN, MSc, and JENNIFER DENT, MSc
It is well established that muscular performance decreases with aging. Morphological changes, such as a decrease in muscle fiber number and muscle fiber size7714922 may account for a significant part of this decline in performance, however, changes in muscle membrane electrical properties with aging may also play a role. Animal studies have suggested that there is a progressive loss of muscle membrane excitability during aging which will limit the functional activity of the muscle. Periph~~
~
From the Department of Physical Education, McMaster University, Hamilton, Ontario, Canada. Acknowledgments: The authors gratefully acknowledge the technical assistance of John Moroz. This work was supported by the Science and Engineering Research Board at McMaster University. Address reprint requests to Audrey L. Hicks, Department of Physical Education, McMaster University, 1280 Main St. West, Hamilton, Ontario, L8s 4K1, Canada. Accepted for publication February 13, 1991. CCC 0148-639W92/01087-07 $04.00 0 1992 John Wiley & Sons, Inc.
M-Wave Characteristics in the Elderly
era1 factors which contribute to this loss of excitability include a decrease in chloride conductance, which prolongs the duration of the action potential, a decreased resting membrane potential, an increase in the critical polarization threshold, and a decreased activity andlor efficiency of the Na+-K+ T h e peak-to-peak amplitude of the compound muscle action potential (M-wave) is representative of the amount of muscle membrane excitation, as it is dependent on both the resting membrane potential and the extent of the overshoot of the action potential of the single muscle fibers. l 8 Significant decrements in the size of the M-waves have been reported in the ankle dorsi- and plantar-flexon2' and the extensor digitorum brevis4 after the age of 60 years, but there are no other reports of changes in M-wave size with aging in other muscle groups. In addition to offering information on muscle excitability at rest, the M-wave is also commonly
MUSCLE & NERVE
January 1992
87
used during fatigue experiments to assess excitability; specifically, it provides information on the effectiveness of electrical propagation across the neuromuscular junction and along the surface membrane of the muscle fibers. During the course of maximal voluntary contractions in young adults, most studies have shown that the M-wave undergoes little or no decrement, suggesting that muscle activation is maintained and neuromuscular transmission is More recently, Hicks et a1.8 noted that the M-wave actually increased in both amplitude and area during the course of intermittent maximal voluntary contractions of the thenar and extensor digitorum brevis muscles. It was suggested that the potentiation of the M-wave was likely due to an increase in the peak-to-peak amplitude of single fiber action potentials resulting from a hyperpolarization of the muscle fiber membranes. This has recently been shown to occur in rat soleus muscles coincidentally with M-wave potentiation, following tetanic stimul a t i ~ nIn . ~the latter study, increased activity of the Na+-K+ pump, which increases the resting membrane potential of the muscle fibers, was found to be the cause of the enlargement of the action potentials and, hence, the M-wave. Thus, under certain conditions, the M-wave may be used as a noninvasive index of Na+- K+-ATPase activity in skeletal muscle. With this background, the purpose of this study was to compare the M-waves at rest and during a voluntary fatigue paradigm in elderly and young adults. In addition, the effects of 12 weeks of resistance training on M-wave characteristics in elderly adults was examined.
posable Ag/AgCl discs (3M, No. 2248) were taped approximately 4 cm apart over the radial nerve, just proximal to the radial-humeral joint; for T H studies, 2 silver discs (9 mm diameter) mounted in a Plexiglas holder were taped over the median nerve at the wrist; and, for T A studies, a lead plate cathode was secured over the common peroneal nerve just proximal to the head of the fibula, and the anode on the superior border of the tibia. The stimuli were rectangular 5 0 - ~ spulses delivered from a high-voltage stimulator (Grass Instruments, Model S11B); the latter being triggered by a computerized timing device. For BR and TA studies, the recording electrodes were disposable Ag/AgCl discs (as above); the stigmatic electrode being placed on the muscle belly (at the point where the largest M-wave was recorded) and the reference on the musculo-tendonous junction. In T H experiments, the recording electrodes were silver strips (5 cm x 6 mm); the stigmatic electrode was placed perpendicular to the first metacarpal bone at the junction of the proximal and middle thirds of TH, and the reference was attached to the end of the little finger. A similar silver strip served as the ground electrode for all muscle groups. The signals from the recording electrodes were fed into amplifiers with bandwidths of 20 Hz to 1.5 kHz, and were displayed on a storage oscilloscope (Hewlett-Packard, Model 1201B). At the same time, data were streamed continuously to disc by means of a Dataq waveform scrolling board (WFS-2OOPC; Dataq Instruments) in an IBM-compatible computer. Analysis of peak-to-peak amplitude, duration, and area of the M-wave was done by CODAS analysis software with custom calibrations.
METHODS
T h e elderly group consisted of 7 women and 4 men, with a mean age of 66.3 & 3.7 years, none having prior experience with weightlifting training. The young group consisted of 5 women and 6 men, with a mean age of 3 1.2 -+ 4.9 years. All subjects were volunteers, and the investigation carried the approval from the University Ethics Committee. Subjects.
Stimulating and Recording. Experiments were conducted on the brachioradialis (BR), the tibialis anterior (TA), and the thenar (TH) muscles, with the subjects sitting. In all studies, care was taken to ensure that the skin was properly prepared before electrode application and that the electrodes were coated with conducting gel. The stimulating electrodes were of several types; for BR studies, dis-
88
M-Wave Characteristics in the Elderly
For each muscle, baseline recordings of resting M-waves were made with the muscle relaxed; once the maximal M-wave was determined, the voltage of the stimulator was increased approximately 10% to ensure maximality of activation. For experiments on T H , subjects had their wrist and hand strapped into a moulded plastic cast, for BR and TA studies, custom stabilizing apparatuses were at joint angles of 100" of elbow flexion (semipronated) and 20" of plantar flexion. Subjects were instructed to perform maximal 3-second isometric contractions followed by a 2-second rest for a total time of 2 minutes. This protocol resulted in a 17% to 23% reduction in MVC torque over the course of the 2 minutes. M-waves were evoked every 5 seconds in the rest interval between contractions; care was Testing Procedure.
MUSCLE & NERVE
January 1992
taken to ensure that the positions of the particular muscle groups remained constant whenever the testing stimuli were delivered (see ref. 8). T h e voluntary contractions were timed by a light which was controlled by a computer timing device. Recovery measurements of the M-wave were made 1, 3, 5, and 10 minutes following the 2-minute fatigue paradigm. The elderly subjects participated in a 12-week strength and endurance training program which involved dynamic strength training of several muscle groups and submaximal cycle ergometry (15 to 25 minutes at 60% to 70% maximum HR), however, only the training of the T H , BR, and T A will be described. Custom-designed apparatuses were utilized for the training of the T H and TA; free-weight dumbbells were utilized for the training of the BR. For training of T H , subjects sat in a chair with their right forearm resting on a table, palmar surface down. The fingers and wrist were secured to the table with velcro strapping. Subjects inserted their right thumb into a soft cast-mould, which was attached to a weight pan by means of‘ an adjustable cable and pulley. Subjects were instructed to abduct their thumb in a downward motion against the resistance provided by the placement of weights in the pan. The TA training device required that the subject perform a complete dorsiflexion movement, while sitting in an adjustable chair with the knee at a 90” angle, and their foot strapped to a rotatable steel plate. A post at the front of the plate could be stacked with circular weights to provide resistance. For training of the BR, subjects sat at a “preacher’s” bench, and were instructed to perform full-range elbow flexions while holding dumbbells in a semipronated position. Subjects trained the BR of both arms, but for the T H and TA only the right limb was trained to provide a within-subject control. Training was done on 2 alternate days each week for 12 weeks. Exercises were done in a circuit set system, with 2-minute pauses between sets. Each set comprised 10 to 15 repetitions; training progressed from 2 sets of each exercise at 50% of the initial 1 RM to 4 sets at 70% to 80% of 1 RM over the course of the 12-week training period. Weights were adjusted every other week to accommodate for improvement in strength and to restrict each set to the 10 to 15 RM limit. Training Regimen.
All of the data were computer analyzed using analysis of variance (ANOVA).
Statistical Treatment.
M-Wave Characteristics in the Elderly
Significant differences between means were deiermined using a Tukey A post hoc test. A level of P < 0.05 was considered to be statistically significant. RESULTS Comparison of M-waves Between Elderly and Young Subjects. Baseline M-waves in all 3 muscle
groups were significantly smaller in amplitude and in area in the elderly versus the young adults, whereas M-wave duration was similar between groups. These values are summarized in Table 1. During the 2 minutes of intermittent MVCs, there was no evidence of excitation failure in either group (i.e., no decline in M-wave size), in fact, there was significant enlargement of the M-wave during the fatigue paradigm in T A and T H in the young adults. These results are illustrated in Figure 1. M-wave duration showed no consistent changes during the 2-minute Fatigue paradigm in either group, however, after the first minute o f MVCs in BK and TA, the M-wave duration was significantly greater in the elderly versus the young adults. Training. The changes in baseline M-wave size following the 12 weeks of training in the elderly subjects are also sunimarized in Table 1. There was a significant increase in M-wave size (both amplitude and area) in the T H and BK and a nonsignificant increase in the TA M-wave. Apart from the increase in baseline amplitude, the relative behavior of the M-wave during the voluntary fatigue paradigm did not change after training, as seen in Figure 1. In Figure 2, the M-waves in the trained T H are compared with those in the untrained T H within the elderly group; this figure clearly illustrates the specificity of the training effect in both M-wave amplitude and area. T h e training regimen was of sufficient intensity to induce significant increases in muscle strength. In the T H , BR, and T A muscles, there were increases in the 1 RM of 182%, 52%, and 48%, respectively. Isometric MVC torque also increased significantly after training. In the BR, MVC torque increased from 35 (k4.6) Nm pretraining to 41.7 (26.1) Nm posttraining, and in the T A , MVC torque increased from 30.5 (23.1) Nm to 35.0 (*:3.2) Nm. I t should be clarified here that both of‘ these MVC torques represent the summed action of all the elbow flexors and all of the ankle dorsiflexors; it would be impossible to voluntarily activate only the BK or the TA. Isometric torques of the T H were not measured due Effects of
MUSCLE & NERVE
January 1992
89
10.0
-
8.0
-
LD-
4.0-
120
-
Ion
-
w -
FIGURE 1. Changes in the mean M-wave amplitude ( S E ) during and following a 2-minute voluntary fatigue paradigm in (a) brachioradialis, (b) thenar, and (c) tibialis anterior muscles. Symbols represent pretrained elderly (o),posttrained elderly (a), and young adults (0).
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40.0
9
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-
20.0
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25 35456080110 180 300420 720
Voluntary Contractlons
da
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TIME
(8)
FIGURE 2. Changes in the mean thenar M-wave amplitude (a) and area (b) during a 2-minute voluntary fatigue paradigm in elderly adults. The trained muscles are illustrated with squares; (Wm) preposttraining, and the untrained muscles are illustrated with circles; ( o h )pre-posttraining.
to the lack of a suitable stabilizing device equipped with a force transducer. DISCUSSION
The results from this study have drawn attention to the fact that the loss of muscle membrane excit-
M-Wave Characteristics in the Elderly
ability associated with aging may be noninvasively assessed by measurement of the M-wave. It has been demonstrated that the M-waves in 3 different muscle groups, the brachioradialis (BR), the thenar (TH), and the tibialis anterior (TA), are significantly smaller in elderly versus young
MUSCLE & NERVE
January 1992
91
~
~~
Table 1. Summary of the M-wave characterlstlcs in elderly adults before (pre) and after (post) 12 weeks of exercise traintng Values obtained in young adults included for comparison. M-wave amplitude (mv) Pre
BR Elderly Young
TH Elderly
4.8 t 0.5 8.3 t 1.3
M-wave duration (ms)
Post 6.1
?
M-wave area (CLV s )
Pre
Post
Pre
Post
0.9*
27.2 ? 1.0 28.2 2 0.8
28.0 -+ 1.3
31.9 2 3.9 57.3 ? 7.8
42.0
5.2 ? 1.2 5.8 t 1.0 10.2 t 1.o
9.0 2 1.4* 5.3 2 1.1
16.5 t 0.8 16.0 1.0 18.6 2 1 2
16.4 ? 0.9 17.6 ? 0.7
19.9 f 5.4 19.5 ? 2.7 40.8 ? 6.0
29.7 2 4.0' 18.1 2 3.1
Tr. con:
8.8 ? 0.8 8.4 t 0.5 11.0 t 0.6
9.5 ? 0.6 9.2 2 0.7
24.8 22.7 23.9
23.9 ? 1.4 23.0 2 1.5
55.3 ? 5.4 45.5 -+ 3,9 64.0 3.7
56.6 51.3
*
TA
Young
5.6*
Tr: Con:
Young
Elderly
2
? ? ?
1.4 1.2 12
*
* 4.5 * 4.6
Values are expressed as means ? SE of the mean. 'Significant change from pretraining value (P < 0.05)
adults. It is possible that the smaller M-waves may be due to factors related to tissue ultrastructure and conductivity, however, we are proposing that an age-associated change in the size of the muscle action potential is contributing to the decreased M-wave. The present observations of smaller M-waves in the older subjects are in agreement with previous s t ~ d i e s . ~ In . ~ their , ~ ' investigation of the changes in the human ankle musculature with aging, Vandervoort and McComas reported an 18% decrease in the TA M-wave between the third decade and seventh decade," which is comparable with the 22% decrease observed in the present study. As in nerve fibers, excitability of a muscle fiber is determined by the resting membrane potential. The Na+- K f pump, through its electrogenic nature, contributes approximately 10 mV to the resting membrane potential under normal steady state conditions.2 ) Elderly individuals, who have been sedentary for a number of years, may experience a down-regulation in their Na+-K+ transport system through various hormonal factors,'""2 which may partly explain the ageassociated decline in resting membrane potential and concomitant decrease in action potential amplitude. Evidence suggests, however, that under conditions of heightened pump activity, i.e., during acute exercise or after chronic exercise training, the electrogenic contribution of the pump to the resting membrane potential may increase up to three-fo1d."I3 Furthermore, in a recent crosssectional study, Klitgaard and Clausen" observed that there were more Nat-K+ pump sites in eld-
92
M-Wave Characteristics in the Elderly
erly individuals who were physically active than in those who were sedentary. 'Thus, it appears that at least one component of muscle excitability may be amenable to exercise training. During the course of maximal voluntary contractions in adults, most studies have shown that the M-wave undergoes little or no decrement, suggesting that muscle activation is maintained and neuromuscular transmission is i n t a ~ t . ' " " ~T h e results from the present investigation suggest that the same is true for elderly adults. In spite of a smaller excitable muscle mass, there is no evidence of activation failure during this type of maximal exercise. It is noteworthy that while there appeared to be some M-wave potentiation during the fatigue paradigm in the young adults, there was no such pattern in the elderly group (see Fig. 1). This may be reflective of an age-related change in the ability of the Na+-K+ pump to cope with the ionic perturbations associated with muscle activity. In the present study, the M-waves in the elderly subjects increased significantly in 2 muscle groups after training, a finding that has not been reported previously. We cannot exclude the possibility that changes in tissue ultrastructure (e.g., decreased intramuscular and subcutaneous fat) andlor skin impedence after training may have contributed to this increase. It is noteworthy, however, that the muscle group which showed the greatest change (TH) is probably the one with the least amount of subcutaneous or intramuscular fat. An intrasmuscular recording of the M-wave would have controlled for any skin or subcutane-
MUSCLE & NERVE
January 1992
ous impedence changes, and should, therefore, be considered for future studies. T h e fact that a significant increase was not found in the TA muscle could possibly be due to the fact that this muscle is used daily for ambulation, and thus may not experience the same decline in activity with aging as the other 2 muscle groups (BR and T H ) examined in this study. At this stage, the mechanism(s) responsible for this increased M-wave posttraining can only be speculated upon. In a previous study, Hicks and McComasg found muscle fiber hyperpolarization in the rat soleus to be associated with, and implicitly responsible for, the enlargement of the M-wave and of single fiber action potentials following repetitive stimulation. T h e involvement of the Na+-K+ pump in this response was supported by the observation that inhibition of Na+- K+ pump activity (by ouabain, cooling, or
K+-free solutions) abolished the hyperpolarization and the enlargement of the M-wave. The posttraining increase in M-wave size in the present study could be explained entirely by an increased resting membrane potential due to enhanced Na+- K+ pump activity. Moss and co-workers19 reported significantly higher resting membrane potentials in the tibialis anterior of trained versus sedentary adults, and Knochel et al.'?' reported a significant increase in the resting membrane potential of dog gracilis muscle after 6 weeks of training. Furthermore, there are numerous reports of increased Na+-K+ pump activity after exercise training. l ' , ' " Clearly, future studies should further examine the effects of training on muscle membrane excitation in older adults, specifically by investigating any changes in Na+-K+ pump activity and/or resting membrane potentials in muscle fibers.
REFERENCES 1. Bigland-Ritchie B, Jones DA, Woods JJ: Excitation fre-
quency and muscle fatigue: Electrical responses during human voluntary and stimulated contractions. Rxp Neurol 1979;64:414-427. 2. Bigland-Ritchie B, Kukulka CG, Lippold OCJ, Woods JJ: The absence of neuroniuscular transniission failure in sustained maximal voluntary contractions. ,I Physiol (Lond) 1982;330:265-278. 3. Brown WF: A method for estimating the number of motor units in thenar muscles and the changes in the motor unit count with aging. ,J Neurol Neurosurg Psychiatry 1972;35:845-852. 4. Campbell MJ, McComas AJ, Petito F: Physiological changes in ageing muscles. J Neurol Neurosurg P.\ychiatry 1973;36:174- 182. 5. DeLuca A, Mambrini M, Camerino DC: Changes in niembrane ionic conductances and excitability characteristics of rat skeletal muscle during aging. P j u g Arch 1990;415:642644. 6. Frolkis VV, Martynenko OA, Zamostyan VP: Aging of the neuromuscular apparatus. Gerontology 1976;22:244-279. 7. Grimby G, Saltin B: T h e ageing muscle. Clin Physiol (Oxford) 1983;3 :209 - 2 18. 8. Hicks A, Fenton J, Garner S, McComas AJ: M wave potentiation during and after muscle activity. J Appl Physiol I989;66:2606- 2610. 9. Hicks A, McComas AJ: Increased sodium pump activity following stimulation of rat soleus muscles. J Physiol (Lond) 1989;414:337-349. 10. Kjeldson K: Regulation of the concentration of 'H-ouabain binding sites in mammalian skeletal muscle-Effects of age, K-depletion, thyroid status and hypertension. Dan Med Bull 1987;34:15-46. 11. Kjeldsen K, Richter EA, Galbo H , Lortie G, Clausen T: Training increases the concentration of ['Hlouabain-bind-
M-Wave Characteristics in the Elderly
ing sites in rat skeletal muscle. Biochzm Bzophys Actu 1986;860:708- 7 12. 12. Klitgaard H, Clausen T : Increased total concentration of Na-K pumps in vastus lateralis muscle of old trained human subjects. J Appl Physiol 1%9;67:2491-2494. 13. Knochel JP, Rlachley JU, Johnson J H , Carter NW: Muscle cell electrical hyperpolarization and rcduc-ed exercise hyperkalemia in physically conditioned dogs. J Clin Inumt 1985;75:740-745. 14. 1.exell J , Henriksson-Larsen K, Winblad B, Sjostrom M: Distribution of different fibre types in human skeletal niuscles: Effects of aging studied in whole muscle cross sections. M w c l e Nerue 1983;6:588-595. 5. Marsh E, Sale D, McComas AJ, Quinlan J: Influence of joint position on ankle darsiflexion in humans. J Appl P h y 201 1981;51: 160- 167. 6. McCartney N , Moroz D, Garner SH, McComas A]: The effects of strength training in patients with selected neuromuscular disorders. Med Sci Sports Exer 1988;20:362-368. 7. Merton PA: Voluntary strength and fatigue. J Physiol (Lond) 1954; 128:553-563. 18. Milner-Brown HS, Miller RG: Muscle membrane excitation and impulse propagation velocity are reduced during muscle fatigue. Muscle Nerve 1986;9:367-374. 19. Moss RF, Raven PB, Knochel JP, Peckham JK, Blachley JD: The effect of training on resting muscle membrane potentials. Znt Ser Sports Scz 1983;13:806-811. 20. ?'tiomas RC: Electrogenic sodium pump in nerve and muscle cells. Physiol Rev 1972;52:563-594. 2 1. Vandervoort AA, McComas AJ: Contractile changes in opposing muscles of the human ankle joint with aging. J Appl Physiol 1986;61:361-367. 22. Wokke JHJ, Jennekens FGI, van den '3rd CJM, Veldman H, Smit LME, Leppink GJ: Morphological changes in the human end plate with age.J Neurol Sci 1990;95:291-310.
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