313
Effect of Training on the Calf Muscle Energy Metabolism A 31P-NMR Study on Four Elite Downhill Skiers Challenged with a Standardized Exercise Protocol
j
, Alonso2 , J. F Lebas1, C'. Ards, J. M Gonzalez de Suso3 ,A. Rossi1 Institut de Resonance Magnétique en Biologic et Médecine (IRMBM), Université Joseph Fourier, 38041 Grenoble Cedex, France 2Departament de Bioquimica i Biologia Molecular, Facultat de Ciències, Universitat Autônoma de Barcelona, 08193 Bellaterra, Barcelona, Spain 3Unitat de Recerca, Centre d'Alt Rendiment (CAR), 08190 Sant Cugat del Vallés, Barcelona, Spain
D. Laurent1, G. Bern
Introduction D. Laurent, G. Bernüs, J. Alonso, .1. F. Lebas,
C. Arrs, J. M. Gonzalez de Suso and A. Rossi, Effect of Training on the Calf Muscle Energy Metabolism. A 31p. NMR Study on Four Elite Downhill Skiers Challenged with a Standardized Exercise Protocol. mt J Sports Med, Vo113,No4,pp313—318, 1992. Accepted after revision: December 3, 1991 This study evaluates the effects of a 6 months'
training period on the bioenergetics of the calf muscle of elite athletes by means of phosphorus-3 1 Nuclear Magnetic
Resonance Spectroscopy ( P-NMR). Four downhill skiers, belonging to the Spanish National Team, performed a standardized exercise protocol using their right leg inside a 2.35 Tesla magnet. The inorganic wide-bore
phosphate/phosphocreatine (Pi/PC) ratio and intracellular pH (pHi) were measured at steady-state during an exercise protocol composed of 5 work levels between 20% and 80% of the maximum voluntary contraction (MYC), before and after the training period. The measured values, which were markedly scattered at the beginning, regrouped after training. This was caused by a shift towards lower Pi/PC ratios and by a lower pHi acidification in three of the four subjects. This result suggests that 31P-NMR is a good tool to evaluate changes in the muscle aerobic capacity of athletes induced by training. Key words
Energy metabolism, localized exercise, 31PNMR spectroscopy, human skeletal muscle, training, oxidative capacity
Competitive downhill skiing requires good technical and physical qualities associating apparently incom-
patible apitudes for endurance and explosive power.
Downhill skiing events last between I and 3 minutes and consequently require a high endurance capacity during the exertion. This is why the training program for these athletes focuses on simultaneously improving their ability to sustain a high tension in their leg muscles during periods as long as 3 mm whilst delaying the aerobic exhaustion threshold. This is confirmed by their particularly high mean values for maximal oxygen up-
take (VO2max), about 70 ml x minx kg'(l), and maximal anaerobic power, 23.7 watt x kg 1(6).
Training-induced changes in the oxidative capacity of human skeletal muscle have already been described in the literature. Namely, an increase in the activity of mitochondrial enzymes upon training has been reported (20, 13).
Furthermore, an increase in free fatty acid use as a major energy source during long duration exercise has been demon-
strated upon training (12, 7). Invasive microbiopsy studies have shown that muscle energetic metabolism is also affected (16, 18). The main drawback of this technique being the impossibility of continuously monitoring the metabolite concentrations during an effort protocol. Phosphorus-3 1 Nuclear Magnetic Resonance (31 P-NMR) Spectroscopy is a well accepted methodology to investigate, in vivo, the bioenergetics of human skeletal muscle (4). This technique provides direct information of the response of skeletal muscle to exercise by continuously monitoring the concentration of inorganic phosphate (Pi), phosphocreatine (PC), and adenosine triphosphate (ATP), while following the
evolution of intracellular pH (pHi) (5). Studies carried out with exercising elite athletes have shown that the Pi/PC ratio for a given workoad, at metabolic steady-state, was lower than for untrained subjects. These results have been interpreted as suggesting an improvement of muscle oxidative capacity due to endurance training (8, 11). We wanted to individually evaluate by 31PNMR the metabolic effect of a 6 months' training period on the oxidative capacity of calf muscle of elite downhill (alpine)
lnt.J.SportsMed. 13(1992)313—318 GeorgThieme Verlag StuttgartNew York
skiers. Four skiers, members of the Spanish National Ski Team
and preselected for the 1992 Olympics, performed a stand-
Downloaded by: NYU. Copyrighted material.
Abstract
314 mt. J. Sports Med. 13(1992)
D. Laurent, G. Bernüs, J. Alonso, J. F. Lebas, C. Arts, J. M. Gonzalez de Suso, A. Rossi
ardized exercise protocol that consisted of a gradual increase of load in discrete steps leading to successive metabolic steadystates. The exercise-induced changes in muscle bioenergetics were measured by 31P-NMR before and after the training pe-
A 4
2
Place Alpes Alt. 3000m
Andes
2 wks
4 wks
Alpes
3000rn
ISOOm
July
Subjects Four elite downhill skiers from the National Spanish Team were selected to participate in this study. Individual values of age, morphological and metabolic characteristics are summarized in Table 1. Each subject gave his informed consent. Experimental procedures were in accordance with ethical standards of the Regional Committee on Human Experimentation. Table 1 Morphological and metabolic characteristics of the volunteers.
Age
Weight
Height
years
kg
m
A B
18
71 61
C D
18
1.74 1.68 1.72 1.76
16
17
62 72
CSA cm2
mlminkg
94.7
56.0
72.1
59.5 65.0 57.0
89.3 87.2
Aipes
Aipes 3000m
6 Aipes
3000m 2000m
September
October November Dcember
B
!OPW
AW May
August
I
October
SW
I December
Fig. 1 A: Calendar of training programs in altitude. BT: Basic technic; GS: Giant Slalom; SL: Special Slalom; PCI: Pre-Competition Training. During Stays 2 and 3, average recovery times between each race were about 20—25 mm. During both 4 and 5, intermediate recovery periods lasted about 10—15 mm. B: Training program followed during intermediate periods at CAR. AW: Aerobic work realized at the level of the 70—80% VO2max (intensive practice of bicycling); MAP: maximal aerobic power at 90—100% VO2max and SW: Speed Work.
(fO2max
CSA: Muscle Cross-Sectional Area calculated from leg muscles circumference and subcutaneous fat thickness measurements; VO2max: maximal 02 uptake.
Training Characteristics
Selected skiers intended to participate in the Olympic Games at Albertville scheduled for 1992. Their training out of the season of competitions in 1990—1991 consisted of 6 medium duration altitude stays especially devoted to ski practice and also, during the intermediate periods, in a whole
body physical preparation at the Centre d'Alt Rendiment (CAR) of Barcelona. Fig. 1 summarizes the training schedule followed by these athletes during the 1990 second semester (see legend for complete description). In summary, the first 3 altitude stays were devoted to proprioceptive work, and the remaining 3 to the development of the athlete's physical qualities. During the intermediate periods, from May to November, the skiers bicycled 2000—2200km. From May to June, the average sporting practice time was about 2—3 hours per day, with
70% of the time devoted to aerobic work, and the rest to anaerobic work. In July, sporting activities were extended to 4 hours per day. This training protocol is considered as being suitable for improving the athlete's aerobic capacity.
NMR Equipment, Propterties and Ergometer Experiments were carried out in a 2.35 Tesla, 35 cm effective bore diameter superconducting magnet operating at 40.6 MHz for31 P, linked to a BRUKER MSL- 100 con-
sole. Spectra were acquired with a transmitter/receiver 7 cm diameter circular surface coil double tuned to P and 1H. The surface coil was positioned against muscles of the posterior compartment of the right leg in the largest cross-sectional area at about 8 cm distance from the poplitealfossa for a subject of 1.75 m body height. The coil diameter (7 cm) allows examina-
tion of about 80 cm3 tissue volume comprising mainly medial and lateral heads of Gastrocnemius and Soleus muscles, all taking part in plantar flexion. A complete description of the
ergometer has been given in a recent paper by the present authors (17). This mechanical system allowed us to quantify the effort carried out by the subject by measuring the mean out-
put power (MOP) which was developed during plantar flex-
ion. In short, force, displacement and pedal movement frequency were transmitted to a graphic recorder and were sampled with an analogic to digital converter to be stored in a computer, allowing, for each load, the calculation of instan-
taneous output power (J/sec) and effective mean output power (J/min/cm2 of muscle). Cross-sectional muscular area of the calf muscle was obtained by a morphometric method (15). Functional data for four successive flexions were collected every 16 movements. Ergometer data acquisition and
associated calculations were performed by using a homemade program running in a PC computer.
Protocol The subjects, in a half-seated position, performed active plantar flexion and passive dorsiflexion of the right foot with the two legs inside the magnet. The maximum isometric voluntary contraction (MVC) was measured with the subject sitting without back support and pushing with maximal force for 3 s on the crank-gear system clamped in a 100 plantar flexion position. Such a position was chosen in order to avoid contribution of the back muscles to the movement. The subject was also verbally encouraged to maximize his effort. The MVC value used for each subject was the calculated mean of three MYC measurements. Four minutes were allowed for recovery between each successive maximal effort measurement. Then, the subjects were submitted to a standardized exercise protocol (17). Exercise was carried out in two stages separated by a 10 minutes' resting period. The first stage of exercise consisted of 3 consecutive periods of work, each being 120 plantar flexions, fixed at 20%, 35% and 50% respectively of the MYC value. The second part of the protocol consisted of working at 65% and 80% of the MVC. The timing for the plantar flexions was imposed by an acoustic system at a frequency of one flexion every 2 seconds. As stated above, the
Downloaded by: NYU. Copyrighted material.
Athletes
August
5
2 wko
nod. Material and Methods
3
Int.J.SportsMed. 13 (1992) 315
Effect of Training on the Calf Muscle Energy Metabolism
z
0 C
80% MVC
0,0
0
40
20
60
80
100
%MVC Fig. 2 Relationship between the imposed load-level (% MVC) and the corresponding mean output power (MOP) measured during the plantar flexion before ( July) and after training ( December) Values are mean SEM from the four skiers for whom a mean of 20 MOP measurements at each load-level were measured. There is no significative difference.
50% MVC PC
ously recorded and used to calculate the mean repetition time for each work load.
35% MVC
NMR acquisition After measuring the MVC, the magnetic field homogeneit1y was optimized by shimming on the proton water spectrum. P NMR spectra were acquired at rest, during exer-
20% MVC
Pi1L
ATP
cise and recovery using the 90° pulse at the center of the coil (40
ps pulse length). The data size was 4K and the sweep width used 6024 Hz. A total of 20 transients were accumulated for each spectrum. Before starting the exercise protocol, a fully relaxed spectrum was recorded at rest with a repetition time (TR) of 20 s to calculate the Pi/PC ratio under non saturating conditions. A second spectrum was recorded at rest with a TR of 2 s taken as a reference for spectra recorded during the exercise. During the protocol of exercise, pulses were synchronized with the movement so as to acquire data only during the resting time between flexions. Bach exercise period at a defined MVC level is made up of 6 NMR spectra of 20 transients each. The
acquisition of mechanical data was synchronized with the NMR recording (17). A 150 Hz exponential deconvolution function and a 15 Hz line broadening were applied prior to Fourier transformation. Spectra were manually phased and baseline corrected.
Data Analysis Individual resonance areas for Pi, PC and ATP were measured by integration from spectra processed in the
absolute intensity mode. The values obtained for each phosphorylated compound were multiplied by a specific coefficient to correct for partial saturation effects. These saturation coefficients were calculated from a preliminary experiment conducted on a resting subject (Laurent D., unpublished results). The experiment consisted in recording 20 pulses spectra from the calf muscle of the subject with varying TR: 1, 2, 5, 10 and 20s and adjusting the apparent resonance intensity to a monoexponential curve from which any required saturating coefficient for TR different than the ones used may be calculated. Accordingly, the equation used both for Pi and PC was
the same: saturation coefficient (x) 1/0956(13_TR/1.725);
Fig. 3 Changes in the 31 P-NMR spectra from the calf muscle of a downhill skier performing a gradual plantar flexion exercise. Spectra are recorded at rest and at different metabolic steady-states induced by successive regimes of work at 20%, 35%, 50%, 65% and 80% of the MVC. Spectra are scaled at constant height for the tallest peak (which is PC at rest, 20 and 35% of MVC and Pi at 65 and 80% of MVC).
while for b-ATP it was: saturation coefficient (y) 1/0.9 50 (1 -e TR/0.99) The TR value used was the mean calculated re-
petition time for each work load. Intracellular pHi was calcu-
lated by using the formula: pHi = 6.75 +log ((— 3.27)/ (5.69 —d)), where d is the Pi chemical shift relative to PC at 0 ppm (19). For each subject, steady-state Pi/ PC ratios and pHi values were taken from the last 4 spectra obtained at each work load in order to calculate average values. The variability of the measurements was less than 10%. Values are given as
mean SEM. Statistical analysis was made using a paired t-test for comparison of MVC values measured before and after the training period (paired observations) and an unpaired t-test for comparisons of MYC values obtained on athletes and sedentary subjects. A p-value of 0.05 was accepted as the level of statistical significance. Results
Biomechanical Results
The MVC value for calf muscles did not change significantly after the 6 months' training period
(5.42±0.63 N-cm2 in July vs 5.74±0.70 Ncm2 in
December). Nevertheless, both values of MVC are signifi-
Downloaded by: NYU. Copyrighted material.
real frequency, which ranged between 1.8 s—2.2 s, was continu-
316 mt. J. Sports Med. 13(1992)
D. Laurent, G. Bernis, J. Alonso, J. F. Lebas, C. Arüs, J. M. Gonzalez de Suso, A. Rossi
6
6
5
S
Fig. 4 Relationship between the Pi/PC ratio and the intensity of exercise before
C
4
U
(D July) and after training ( December).
4
U
3
Individual measurements for four subjects are shown. The PiIPC ratios were determined at the metabolic steady-state for each stage of maximum voluntary contraction j% MVC).
A
3
2
2
—I W
0
0
20
U
40 %
60
80
100
0
20
40
60
MVC
%
B
D
80
100
80
100
MVC
6 5
4
U
U
3
.;a
0 0
20
40
60
80
-,100
0
20
% MVC
40 %
60
Downloaded by: NYU. Copyrighted material.
2
MVC
can tly higher (p
Effect of Training on the Calf Muscle Energy Metabolism A 31P-NMR Study on Four Elite Downhill Skiers Challenged with a Standardized Exercise Protocol
j
, Alonso2 , J. F Lebas1, C'. Ards, J. M Gonzalez de Suso3 ,A. Rossi1 Institut de Resonance Magnétique en Biologic et Médecine (IRMBM), Université Joseph Fourier, 38041 Grenoble Cedex, France 2Departament de Bioquimica i Biologia Molecular, Facultat de Ciències, Universitat Autônoma de Barcelona, 08193 Bellaterra, Barcelona, Spain 3Unitat de Recerca, Centre d'Alt Rendiment (CAR), 08190 Sant Cugat del Vallés, Barcelona, Spain
D. Laurent1, G. Bern
Introduction D. Laurent, G. Bernüs, J. Alonso, .1. F. Lebas,
C. Arrs, J. M. Gonzalez de Suso and A. Rossi, Effect of Training on the Calf Muscle Energy Metabolism. A 31p. NMR Study on Four Elite Downhill Skiers Challenged with a Standardized Exercise Protocol. mt J Sports Med, Vo113,No4,pp313—318, 1992. Accepted after revision: December 3, 1991 This study evaluates the effects of a 6 months'
training period on the bioenergetics of the calf muscle of elite athletes by means of phosphorus-3 1 Nuclear Magnetic
Resonance Spectroscopy ( P-NMR). Four downhill skiers, belonging to the Spanish National Team, performed a standardized exercise protocol using their right leg inside a 2.35 Tesla magnet. The inorganic wide-bore
phosphate/phosphocreatine (Pi/PC) ratio and intracellular pH (pHi) were measured at steady-state during an exercise protocol composed of 5 work levels between 20% and 80% of the maximum voluntary contraction (MYC), before and after the training period. The measured values, which were markedly scattered at the beginning, regrouped after training. This was caused by a shift towards lower Pi/PC ratios and by a lower pHi acidification in three of the four subjects. This result suggests that 31P-NMR is a good tool to evaluate changes in the muscle aerobic capacity of athletes induced by training. Key words
Energy metabolism, localized exercise, 31PNMR spectroscopy, human skeletal muscle, training, oxidative capacity
Competitive downhill skiing requires good technical and physical qualities associating apparently incom-
patible apitudes for endurance and explosive power.
Downhill skiing events last between I and 3 minutes and consequently require a high endurance capacity during the exertion. This is why the training program for these athletes focuses on simultaneously improving their ability to sustain a high tension in their leg muscles during periods as long as 3 mm whilst delaying the aerobic exhaustion threshold. This is confirmed by their particularly high mean values for maximal oxygen up-
take (VO2max), about 70 ml x minx kg'(l), and maximal anaerobic power, 23.7 watt x kg 1(6).
Training-induced changes in the oxidative capacity of human skeletal muscle have already been described in the literature. Namely, an increase in the activity of mitochondrial enzymes upon training has been reported (20, 13).
Furthermore, an increase in free fatty acid use as a major energy source during long duration exercise has been demon-
strated upon training (12, 7). Invasive microbiopsy studies have shown that muscle energetic metabolism is also affected (16, 18). The main drawback of this technique being the impossibility of continuously monitoring the metabolite concentrations during an effort protocol. Phosphorus-3 1 Nuclear Magnetic Resonance (31 P-NMR) Spectroscopy is a well accepted methodology to investigate, in vivo, the bioenergetics of human skeletal muscle (4). This technique provides direct information of the response of skeletal muscle to exercise by continuously monitoring the concentration of inorganic phosphate (Pi), phosphocreatine (PC), and adenosine triphosphate (ATP), while following the
evolution of intracellular pH (pHi) (5). Studies carried out with exercising elite athletes have shown that the Pi/PC ratio for a given workoad, at metabolic steady-state, was lower than for untrained subjects. These results have been interpreted as suggesting an improvement of muscle oxidative capacity due to endurance training (8, 11). We wanted to individually evaluate by 31PNMR the metabolic effect of a 6 months' training period on the oxidative capacity of calf muscle of elite downhill (alpine)
lnt.J.SportsMed. 13(1992)313—318 GeorgThieme Verlag StuttgartNew York
skiers. Four skiers, members of the Spanish National Ski Team
and preselected for the 1992 Olympics, performed a stand-
Downloaded by: NYU. Copyrighted material.
Abstract
314 mt. J. Sports Med. 13(1992)
D. Laurent, G. Bernüs, J. Alonso, J. F. Lebas, C. Arts, J. M. Gonzalez de Suso, A. Rossi
ardized exercise protocol that consisted of a gradual increase of load in discrete steps leading to successive metabolic steadystates. The exercise-induced changes in muscle bioenergetics were measured by 31P-NMR before and after the training pe-
A 4
2
Place Alpes Alt. 3000m
Andes
2 wks
4 wks
Alpes
3000rn
ISOOm
July
Subjects Four elite downhill skiers from the National Spanish Team were selected to participate in this study. Individual values of age, morphological and metabolic characteristics are summarized in Table 1. Each subject gave his informed consent. Experimental procedures were in accordance with ethical standards of the Regional Committee on Human Experimentation. Table 1 Morphological and metabolic characteristics of the volunteers.
Age
Weight
Height
years
kg
m
A B
18
71 61
C D
18
1.74 1.68 1.72 1.76
16
17
62 72
CSA cm2
mlminkg
94.7
56.0
72.1
59.5 65.0 57.0
89.3 87.2
Aipes
Aipes 3000m
6 Aipes
3000m 2000m
September
October November Dcember
B
!OPW
AW May
August
I
October
SW
I December
Fig. 1 A: Calendar of training programs in altitude. BT: Basic technic; GS: Giant Slalom; SL: Special Slalom; PCI: Pre-Competition Training. During Stays 2 and 3, average recovery times between each race were about 20—25 mm. During both 4 and 5, intermediate recovery periods lasted about 10—15 mm. B: Training program followed during intermediate periods at CAR. AW: Aerobic work realized at the level of the 70—80% VO2max (intensive practice of bicycling); MAP: maximal aerobic power at 90—100% VO2max and SW: Speed Work.
(fO2max
CSA: Muscle Cross-Sectional Area calculated from leg muscles circumference and subcutaneous fat thickness measurements; VO2max: maximal 02 uptake.
Training Characteristics
Selected skiers intended to participate in the Olympic Games at Albertville scheduled for 1992. Their training out of the season of competitions in 1990—1991 consisted of 6 medium duration altitude stays especially devoted to ski practice and also, during the intermediate periods, in a whole
body physical preparation at the Centre d'Alt Rendiment (CAR) of Barcelona. Fig. 1 summarizes the training schedule followed by these athletes during the 1990 second semester (see legend for complete description). In summary, the first 3 altitude stays were devoted to proprioceptive work, and the remaining 3 to the development of the athlete's physical qualities. During the intermediate periods, from May to November, the skiers bicycled 2000—2200km. From May to June, the average sporting practice time was about 2—3 hours per day, with
70% of the time devoted to aerobic work, and the rest to anaerobic work. In July, sporting activities were extended to 4 hours per day. This training protocol is considered as being suitable for improving the athlete's aerobic capacity.
NMR Equipment, Propterties and Ergometer Experiments were carried out in a 2.35 Tesla, 35 cm effective bore diameter superconducting magnet operating at 40.6 MHz for31 P, linked to a BRUKER MSL- 100 con-
sole. Spectra were acquired with a transmitter/receiver 7 cm diameter circular surface coil double tuned to P and 1H. The surface coil was positioned against muscles of the posterior compartment of the right leg in the largest cross-sectional area at about 8 cm distance from the poplitealfossa for a subject of 1.75 m body height. The coil diameter (7 cm) allows examina-
tion of about 80 cm3 tissue volume comprising mainly medial and lateral heads of Gastrocnemius and Soleus muscles, all taking part in plantar flexion. A complete description of the
ergometer has been given in a recent paper by the present authors (17). This mechanical system allowed us to quantify the effort carried out by the subject by measuring the mean out-
put power (MOP) which was developed during plantar flex-
ion. In short, force, displacement and pedal movement frequency were transmitted to a graphic recorder and were sampled with an analogic to digital converter to be stored in a computer, allowing, for each load, the calculation of instan-
taneous output power (J/sec) and effective mean output power (J/min/cm2 of muscle). Cross-sectional muscular area of the calf muscle was obtained by a morphometric method (15). Functional data for four successive flexions were collected every 16 movements. Ergometer data acquisition and
associated calculations were performed by using a homemade program running in a PC computer.
Protocol The subjects, in a half-seated position, performed active plantar flexion and passive dorsiflexion of the right foot with the two legs inside the magnet. The maximum isometric voluntary contraction (MVC) was measured with the subject sitting without back support and pushing with maximal force for 3 s on the crank-gear system clamped in a 100 plantar flexion position. Such a position was chosen in order to avoid contribution of the back muscles to the movement. The subject was also verbally encouraged to maximize his effort. The MVC value used for each subject was the calculated mean of three MYC measurements. Four minutes were allowed for recovery between each successive maximal effort measurement. Then, the subjects were submitted to a standardized exercise protocol (17). Exercise was carried out in two stages separated by a 10 minutes' resting period. The first stage of exercise consisted of 3 consecutive periods of work, each being 120 plantar flexions, fixed at 20%, 35% and 50% respectively of the MYC value. The second part of the protocol consisted of working at 65% and 80% of the MVC. The timing for the plantar flexions was imposed by an acoustic system at a frequency of one flexion every 2 seconds. As stated above, the
Downloaded by: NYU. Copyrighted material.
Athletes
August
5
2 wko
nod. Material and Methods
3
Int.J.SportsMed. 13 (1992) 315
Effect of Training on the Calf Muscle Energy Metabolism
z
0 C
80% MVC
0,0
0
40
20
60
80
100
%MVC Fig. 2 Relationship between the imposed load-level (% MVC) and the corresponding mean output power (MOP) measured during the plantar flexion before ( July) and after training ( December) Values are mean SEM from the four skiers for whom a mean of 20 MOP measurements at each load-level were measured. There is no significative difference.
50% MVC PC
ously recorded and used to calculate the mean repetition time for each work load.
35% MVC
NMR acquisition After measuring the MVC, the magnetic field homogeneit1y was optimized by shimming on the proton water spectrum. P NMR spectra were acquired at rest, during exer-
20% MVC
Pi1L
ATP
cise and recovery using the 90° pulse at the center of the coil (40
ps pulse length). The data size was 4K and the sweep width used 6024 Hz. A total of 20 transients were accumulated for each spectrum. Before starting the exercise protocol, a fully relaxed spectrum was recorded at rest with a repetition time (TR) of 20 s to calculate the Pi/PC ratio under non saturating conditions. A second spectrum was recorded at rest with a TR of 2 s taken as a reference for spectra recorded during the exercise. During the protocol of exercise, pulses were synchronized with the movement so as to acquire data only during the resting time between flexions. Bach exercise period at a defined MVC level is made up of 6 NMR spectra of 20 transients each. The
acquisition of mechanical data was synchronized with the NMR recording (17). A 150 Hz exponential deconvolution function and a 15 Hz line broadening were applied prior to Fourier transformation. Spectra were manually phased and baseline corrected.
Data Analysis Individual resonance areas for Pi, PC and ATP were measured by integration from spectra processed in the
absolute intensity mode. The values obtained for each phosphorylated compound were multiplied by a specific coefficient to correct for partial saturation effects. These saturation coefficients were calculated from a preliminary experiment conducted on a resting subject (Laurent D., unpublished results). The experiment consisted in recording 20 pulses spectra from the calf muscle of the subject with varying TR: 1, 2, 5, 10 and 20s and adjusting the apparent resonance intensity to a monoexponential curve from which any required saturating coefficient for TR different than the ones used may be calculated. Accordingly, the equation used both for Pi and PC was
the same: saturation coefficient (x) 1/0956(13_TR/1.725);
Fig. 3 Changes in the 31 P-NMR spectra from the calf muscle of a downhill skier performing a gradual plantar flexion exercise. Spectra are recorded at rest and at different metabolic steady-states induced by successive regimes of work at 20%, 35%, 50%, 65% and 80% of the MVC. Spectra are scaled at constant height for the tallest peak (which is PC at rest, 20 and 35% of MVC and Pi at 65 and 80% of MVC).
while for b-ATP it was: saturation coefficient (y) 1/0.9 50 (1 -e TR/0.99) The TR value used was the mean calculated re-
petition time for each work load. Intracellular pHi was calcu-
lated by using the formula: pHi = 6.75 +log ((— 3.27)/ (5.69 —d)), where d is the Pi chemical shift relative to PC at 0 ppm (19). For each subject, steady-state Pi/ PC ratios and pHi values were taken from the last 4 spectra obtained at each work load in order to calculate average values. The variability of the measurements was less than 10%. Values are given as
mean SEM. Statistical analysis was made using a paired t-test for comparison of MVC values measured before and after the training period (paired observations) and an unpaired t-test for comparisons of MYC values obtained on athletes and sedentary subjects. A p-value of 0.05 was accepted as the level of statistical significance. Results
Biomechanical Results
The MVC value for calf muscles did not change significantly after the 6 months' training period
(5.42±0.63 N-cm2 in July vs 5.74±0.70 Ncm2 in
December). Nevertheless, both values of MVC are signifi-
Downloaded by: NYU. Copyrighted material.
real frequency, which ranged between 1.8 s—2.2 s, was continu-
316 mt. J. Sports Med. 13(1992)
D. Laurent, G. Bernis, J. Alonso, J. F. Lebas, C. Arüs, J. M. Gonzalez de Suso, A. Rossi
6
6
5
S
Fig. 4 Relationship between the Pi/PC ratio and the intensity of exercise before
C
4
U
(D July) and after training ( December).
4
U
3
Individual measurements for four subjects are shown. The PiIPC ratios were determined at the metabolic steady-state for each stage of maximum voluntary contraction j% MVC).
A
3
2
2
—I W
0
0
20
U
40 %
60
80
100
0
20
40
60
MVC
%
B
D
80
100
80
100
MVC
6 5
4
U
U
3
.;a
0 0
20
40
60
80
-,100
0
20
% MVC
40 %
60
Downloaded by: NYU. Copyrighted material.
2
MVC
can tly higher (p
