Acta Physiol Scand 1979 106: 319-325
Muscle strength and muscle characteristics in monozygous and dizygous twins J. KARLSSON, P. V. KOMI and J. H. T. VIITASALO Laboratory for Human Performance (FOA 57), Karolinska Hospital, Stockholm, Sweden, and Kinesiology Laboratory, University of Jyvaskyla, Finland
KARLSSON, J., KOMI, P. V. & VIITASALO. J. H. T.: Muscle strength and muscle characteristics in monozygous and dizygous twins. Acta Physiol Scand 1979, 106: 319-325. Received 17 Jan. 1978. ISSN 0001-6772. Laboratory for Human Performance (FOA 57). Karolinska Hospital, Stockholm, Sweden and Kinesiology Laboratory, University of Jyvaskyla, Finland. Muscle strength and electrical activity were investigated on 31 pairs of young male and female monozygous (MZ) and dizygous (DZ) twins. The measurements included leg forces, force-time, running velocity, muscular power, maximal integrated electromyographic activity (IEMG) and chronaximetry of the quadriceps muscle group. In each parameter the intrapair variance was computed and the differences were tested between the MZ and DZ twins. The variance ratio (MZ vs. DZ) was statistically significant only for muscular power confirming an earlier finding which has demonstrated a genetic component for the variable. In addition to the various performance variables several key enzymes involved in ATP turnover during muscle contraction and in glucose residue metabolism were analyzed from the muscle biopsy samples (m. vastus lateralis). A genetic component could not be observed in any of their activities or their relationships to performance variables. Key words: Strength, anaerobic performance, muscle enzymes, electromyography, heredity
Man’s physical performance capacity is determined primarily by genetic and environmental factors. A major question arises to what extent these two components have an effect upon a particular trait of the physical performance measurements. Studies conducted on monozygous (MZ) and dizygous (DZ) twins have suggested the impact of the genetic component on the interindividual variation observed in maximal oxygen uptake and muscular power (Klissouras 1971, Komi et al. 1973). “The twin study concept” as developed by Holzinger (1929) has also been applied to human skeletal muscle and its histochemical and biochemical properties (Komi et al. 1977) and it was shown that the hereditary factors determined almost solely the variance in skeletal muscle fibre composition. The present study was undertaken to investigate on these same MZ and DZ pairs of both sexes the genetic factors involved in muscle strength performance and related muscle characteristics.
METHODOLOGY Twins Subjects for the study were obtained through the Population Register of Finland. The final sample was composed of 20 male (9 MZ and 11 DZ) and 11 female (6 MZ and 5 DZ) twin pairs in the age group 11-24 years. Determination of zygosity was performed by subjective observation of physical appearance and serological analyses. In the 16 cases of dizygosity, discordance was observed in more than five antigens or serum proteins (Komi et al. 1977). Maximal muscular power was determined using the method of Margaria et al. (1966). In this measurement the subject ran maximally for a few seconds in a staircase. The running velocity (V) was measured electronically and converted to the vertical component ( Vu).Mechanical power kgmls was computed on the basis of the subject’s V,, and his body weight. This test is referred to as an “anaerobic test” (Margaria et al. 1966). Muscular forces were measured as maximal voluntary isometric extensions of the right leg (quadriceps force) and of both legs (total leg forces), respectively. The type of dynamometers and testing specifications employed have been reported elsewhere (Komi 1973, Komi & Viitasalo 19751. Aclu Pk.vsiol S c u d I06
320
J . Korlsson et (11. MEAN
PERCENT
INTRAPAIR
DIFFERENCE
Fig. 1 . Mean percent intrapair difference in MZ and DZ twins for various performance data.
Force-rime curve was registered during the total leg force measurements and the time to reach 70% of the maximum force was taken as a force-time value. Integrated electromyogruphic activity (IEMG) was obtained from both m. rectus femoris and m. vastus
120-
lateralis during maximum right knee extension with Beckman miniature sized surface electrodes and amplified with Brookdeal 9432 preamplifiers. After storage of the data on magnetic tape (Philips Analog 7 tape recorder) the processing of the EMG signals was performed with
TWIN A
80 -
120-TWlN A
100
M A X I M A L MUSCULAR POWER (
kpm/ sec 1
0
Mi! pair
o Di! pair
Fig. 2 . Intrapair comparison of maximal muscular power for MZ and DZ twins. Acrir Physiol Scirittl 106
Muscle strength in twins
. TWIN 320 A13
*
1613
.
A
,
320
'
?LO
'
321
TWIN A
TWIN B
TWIN B 0
80
160
2LO
320
.TWIN A
240
-
TOTAL LEG FORCE (kg) 0 0
Mi! pair DZ pair
TWIN B 0
811
160
?LO
320
Fig. 3 . Intrapair comparison 0. .otal leg force for MZ and DZ twins.
Hewlett Packard 21 16C computer system (Viitasalo & Komi 1975). Chronaximetric measurements. Neuroton Model 626 stimulator was used to investigate the vastus lateralis muscle with constant current method using the following stimulus durations: 0.1 ms, 1 ms and 30 ms. The 30 rns duration was considered as a rheobase stimulus and was therefore used as a basis for calculation of the chronaxia value. Skeletal muscle fibre composition and muscle enzyme activities were determined in muscle biopsy samples taken
from the vastus lateralis muscle and analyzed according to Gollnick et al. (1972) and Lowry & Passoneau (1972). Fibre types were identified as slow twitch (ST) and fast twitch (FT) fibres. For details see Komi et al. (1977). The following enzyme activities were analyzed: creatine kinase (CPK),myokinase (MK), Caz+and Mg2+stimulated ATPases (Ca2+ ATPase, Mgz+ ATPase), hexokinase (HK), phosphorylase (Plase) and lactate dehydrogenase (LDH). Statistical analysis. Ordinary statistical procedures were employed to calculate means, standard deviation (S.D.) and linear correlation coefficients ( r ) , and to test the significance of differences in intrapair variances between the two twin types. If the variance ratio (F) was significant at 5 % level of probability a heritability estimate (Hest) was computed as originally described by Holzinger (1929): 21-795877
where S2MZ and SzDZ denote intrapair variability of an attribute in MZ and DZ twins, respectively, and S2e signifies the variance due to experimental error. For details see Komi et al. (1977).
RESULTS
Hereditary factors Functional tests. The intrapair variance for muscular power was significantly smaller in MZ pairs as compared to DZ pairs. The calculated He,, was 97.8. For the remaining functional tests no statistically significant difference within DZ pairs as compared to MZ could be demonstrated, but, with the exception of quadriceps force, the variation was slightly greater within DZ pairs (Figs. 1-4). IEMG and chronaximetric tests. The general finding from the different IEMG and chronaximetric data was that the variation between MZ twins was slightly, but not significantly, smaller than the variation between DZ twins (Fig. 5 ) . Acrtr Physiol Srrmd I06
322
J . Kcrrlsson et N I . 80
’
60
.
.,,ITWIN
TWIN A
A
/
/
TWIN B
TWIN B 20
60
LO
60
80
20
LO
eLl
OUADRICEPS FORCE ( k g )
.
0
o
20
60
LO
MZ pair DZ pair
60
F i g . 4. Intrapair comparison of quadriceps force for MZ and DZ twins.
MEAN
PERCENT
INTRAPAIR
DIFFERENCE
Fig. 5. Mean percent intrapair differences in MZ and DZ twins for integrated electromyographic (IEMG) data from rectus femoris (RF)and vastus lateralis (VL) muscles; and chronaximetric data of different stimulus durations (30 ms, 1 ms, 0.1 ms) from the vastus lateralis muscle.
Muscle strength in twins
323
FORCE TIME 70%
A 02 d n.50 0 MZ 9 r=-.67 ( p r 001) ' 6 w ~ ~ m s ~ c ~
A
n
A 0
40
€4
-i
A
-A
A M MUSCULAR POWER(kpm/wc)
A
100
80
120
Fig. 6. Relationship between value of force-time at 70% of maximum isometric contraction (total leg force measurement) and maximal muscular power for all twin groups.
Correlation analysis
Linear correlations between all the variables studied have been published elsewhere (Komi & Karlsson 1978). In the following only the most relevant information is given. Muscular power was in the whole material, as expected, best related to other strength variables as quadriceps force (r=0.82), total leg force (r=0.71) and running velocity (r=0.67) as well as force-time ( r = -0.67, Fig. 6). No significant relationship was, however, present between muscular power and per cent of ST fibres. Considering zygosity similar relationships were obtained for muscular power versus other strength performance variables as in the whole material. The DZ twins were the only group, however, to demonstrate correlations to total leg force (r=0.76) and IEMG of the vastus lateralis muscle (r=0.46). Quadriceps force was in the whole population best related to muscular power (r=0.82), total leg force (r=O.80) and force-time (r=-0.46). No differences in terms of correlations seemed to be present for quadriceps force when comparing monozygous (mean value 7 9 f 4 kp) and dizygous twins (mean value 5 2 f 1 6 kp). In the males quadriceps force was also related to per cent ST fibres (r=-0.55). Activities of enzymes involved in ATP turnover during muscle contraction. Among the enzymes studied only CPK demonstrated in the whole population correlations to strength performance var-
iables: running velocity (r=0.41) (Fig. 7 ) and forcetime (r=-0.40), respectively. The enzyme activities and their relations to performance variables were not influenced by zygosity. Activities of enzymes involved in glucose residue metabolism. Hexokinase (HK), phosphorylase
(Plase), and lactate dehydrogenase (LDH) were the enzymes selected to represent the glucose residue metabolism. They demonstrated no significant or vague relationships to performance variables. Per cent distribution of LDH-1 isozyme showed a correlation only in the MZ group versus a strength variable (muscular power, r= -0.62) in contrast to the DZ group as well as the whole material.
A
ONt
A
Oat
RUNNING,MLOCITV
om 09
10
I1
12
13
I4
IS
Fig. 7. Relationship between the activity of creatine phosphokinase (CPK)enzyme (moles * g-l. min-' lo-*)
and running velocity (mls) on staircase. Arrrr Physiol Srtrnd 106
324
J. Karlsson et al.
DISCUSSION training is affecting muscle fibre composition and An earlier report (Komi et al. 1977) with the same muscle enzyme activities and found no or small twin material had observed the strong heritability changes in spite of marked increases in perestimate for the ST fibre distribution. The results of formance (for reference and summary see Thorthe present report support earlier findings (Komi et stensson 1976). It was suggested that the neuroal. 1973) that also variance in muscular power is motoric control was relatively more affected algenetically determined and that muscular strength though no experimental evidence could be found and IEMG did not show heritability in the same for this hypothesis. statistically significant way as muscle fibre comWhen employing the isokinetic technique for position or muscular power. In these parameters a measurements of dynamic muscle strength, it could statistical comparison showed that the intrapair var- be demonstrated that the potential of a muscle to iance was not statistically significant at the required produce force during movements of high velocity confidence level of pCO.05. For this reason the was positively correlated to its proportion of FT computation of He&was abandoned. muscle fibres (Thorstensson 1976). Similarly, it has No special control of the socio-economic, health been shown that the time needed for production of or physical activity status was made in the present submaximal force levels is related to the muscle study. Although these environmental influences fibre composition (Viitasalo & Komi 1978). It seems probably have been only minor, they still might be then reasonable to suggest that even muscular the cause of individual adaptation to some other strength performance might be partly under the instrength performance thus masking a genetic com- fluence of genetic factors. This suggestion is basiponent present. Adaptation of similar kinds might cally motivated by the strong heritability estimate affect both neuromuscular functions and muscle found for the muscle fibre composition. Additional enzyme activities. support for not rejecting this hypothesis comes In the present study muscle performance has from a comparatively strong correlation between been interpreted from measurements of muscular muscular power and other strength variables. power, muscular forces, force-time and IEMG dur- Therefore it is encouraged that the problem will be ing maximal isometric knee extension. In addition, re-investigated with a larger twin material than what chronaximetric data were obtained from the vastus was possible in the present study. lateralis muscle. Muscular power as measured in the present study is sometimes referred to as an The study was supported by grants from the Swedish “anaerobic power” test (Margaria et al. 1966), Medical Research Council (project No. 4251). which would imply that it would indicate the magnitude of the available ATP and CP stores as well as the potential of the anaerobic glycolytic REFERENCES pathway (Karlsson 1971). Although this test easily GOLLNICK, P. D . , ARMSTRONG, B.. SAUBERT, G. differentiates power athletes from endurance athW., PIEHL, K. & SALTIN, B. 1972. Enzyme activity letes (Komi et al. 1977)it is the authors’ opinion that and fibre composition in skeletal muscle of untrained and trained man. J Appl Physiol33: 312-319. this hypothesis has not been satisfactorily tested. In both twin materials positive correlation coefi- Holzinger, K. J . 1929. The relative effect of nature and nurture influences on twin differences. J Educ Psycho1 cients were obtained, however, for muscular power 54: 231-237. versus lean body mass corresponding to 0.91 (MZ KARLSSON, J. 1971. Lactate and phosphagen concentrations in working muscle of man. Acta Physiol twins) and 0.97 (DZ twins). This is indicative that Scand, Suppl. 358. muscular power is directly related to the muscle volume. The individual anthropometric variables KLISSOURAS, V . 1971. Heritability of adaptive variation. J Appl Physiol31:338-341. have earlier been demonstrated to be dependent on KOMI, P. V . 1973. A new electromechanical ergometer. genetic factors (Komi et al. 1973). These consideraIn: Proc. 3. Internationales Seminar fur Ergometrie (ed. G . Hansen and H. Mellerowiz), pp. 173-176. tions would then support the original suggestion by Ergon-Verlag, Berlin. Markaria et al. (1%) that muscular power reflects KOMI, P. V. & KARLSSON, J . 1979. Physical quantitatively the anaerobic energy output. performance, skeletal muscle enzyme activities, and Thorstensson and co-workers have in a number fibre types in monozygous and dizygous twins of both sexes. Acta Physiol Scand Suppl. 462. of studies tried to evaluate how muscle strength A r m Phvsiol Scund 106
Muscle strength in twins KOMI, P. V. & VIITASALO, J. H. T. 1975. Signal characteristics of EMG at different levels of muscle tension. Acta Physiol Scand %: 267-276. KOMI, P. V., KLISSOURAS, V. & KARVINEN, E. 1973. Genetic variation in neuromuscular performance. Int Z angew Physiol31: 289-304. KOMI, P. V., RUSKO, H., VOS, J. & VIHKO, V. 1977. Anaerobic performance capacity in athletes. Acta Physiol Scand 100: 107-1 14. KOMI, P. V., VIITASALO, J. H. T., HAVU, M., THORSTENSSON, A., SJODIN, B. & KARLSSON, J. 1977. Skeletal muscle fibres and muscle enzyme activities in monozygous and dizygous twins of both sexes. Acta Physiol Scand 100: 385-392. LOWRY, 0. H. & PASSONEAU, J. V. 1972. A flexible
325
system of enzymatic analysis. Academic Press, New York. MARGARIA, R., AGHEMO. P. & ROVELLI, E. 1966. Measurement of muscular power (anaerobic) in man. J appl Physiol21: 1661-1669. THORSTENSSON, A. 1976. Muscle strength, fibre types and enzyme activities in man. Acta Physiol Scand, Suppl .443. VIITASALO, J. H. T. & KOMI, P. V. 1975. Signal characteristics of EMG with special reference to reproducibility of measurements. Acta Physiol Scand 93:531-536. VIITASALO, J. H. T. & KOMI, P. V. 1978. Force-time characteristics and fiber composition in human leg extensor muscles. Eur J appl Physiol40: 7-15.
Actti
Physiol Scand 106
Muscle strength and muscle characteristics in monozygous and dizygous twins J. KARLSSON, P. V. KOMI and J. H. T. VIITASALO Laboratory for Human Performance (FOA 57), Karolinska Hospital, Stockholm, Sweden, and Kinesiology Laboratory, University of Jyvaskyla, Finland
KARLSSON, J., KOMI, P. V. & VIITASALO. J. H. T.: Muscle strength and muscle characteristics in monozygous and dizygous twins. Acta Physiol Scand 1979, 106: 319-325. Received 17 Jan. 1978. ISSN 0001-6772. Laboratory for Human Performance (FOA 57). Karolinska Hospital, Stockholm, Sweden and Kinesiology Laboratory, University of Jyvaskyla, Finland. Muscle strength and electrical activity were investigated on 31 pairs of young male and female monozygous (MZ) and dizygous (DZ) twins. The measurements included leg forces, force-time, running velocity, muscular power, maximal integrated electromyographic activity (IEMG) and chronaximetry of the quadriceps muscle group. In each parameter the intrapair variance was computed and the differences were tested between the MZ and DZ twins. The variance ratio (MZ vs. DZ) was statistically significant only for muscular power confirming an earlier finding which has demonstrated a genetic component for the variable. In addition to the various performance variables several key enzymes involved in ATP turnover during muscle contraction and in glucose residue metabolism were analyzed from the muscle biopsy samples (m. vastus lateralis). A genetic component could not be observed in any of their activities or their relationships to performance variables. Key words: Strength, anaerobic performance, muscle enzymes, electromyography, heredity
Man’s physical performance capacity is determined primarily by genetic and environmental factors. A major question arises to what extent these two components have an effect upon a particular trait of the physical performance measurements. Studies conducted on monozygous (MZ) and dizygous (DZ) twins have suggested the impact of the genetic component on the interindividual variation observed in maximal oxygen uptake and muscular power (Klissouras 1971, Komi et al. 1973). “The twin study concept” as developed by Holzinger (1929) has also been applied to human skeletal muscle and its histochemical and biochemical properties (Komi et al. 1977) and it was shown that the hereditary factors determined almost solely the variance in skeletal muscle fibre composition. The present study was undertaken to investigate on these same MZ and DZ pairs of both sexes the genetic factors involved in muscle strength performance and related muscle characteristics.
METHODOLOGY Twins Subjects for the study were obtained through the Population Register of Finland. The final sample was composed of 20 male (9 MZ and 11 DZ) and 11 female (6 MZ and 5 DZ) twin pairs in the age group 11-24 years. Determination of zygosity was performed by subjective observation of physical appearance and serological analyses. In the 16 cases of dizygosity, discordance was observed in more than five antigens or serum proteins (Komi et al. 1977). Maximal muscular power was determined using the method of Margaria et al. (1966). In this measurement the subject ran maximally for a few seconds in a staircase. The running velocity (V) was measured electronically and converted to the vertical component ( Vu).Mechanical power kgmls was computed on the basis of the subject’s V,, and his body weight. This test is referred to as an “anaerobic test” (Margaria et al. 1966). Muscular forces were measured as maximal voluntary isometric extensions of the right leg (quadriceps force) and of both legs (total leg forces), respectively. The type of dynamometers and testing specifications employed have been reported elsewhere (Komi 1973, Komi & Viitasalo 19751. Aclu Pk.vsiol S c u d I06
320
J . Korlsson et (11. MEAN
PERCENT
INTRAPAIR
DIFFERENCE
Fig. 1 . Mean percent intrapair difference in MZ and DZ twins for various performance data.
Force-rime curve was registered during the total leg force measurements and the time to reach 70% of the maximum force was taken as a force-time value. Integrated electromyogruphic activity (IEMG) was obtained from both m. rectus femoris and m. vastus
120-
lateralis during maximum right knee extension with Beckman miniature sized surface electrodes and amplified with Brookdeal 9432 preamplifiers. After storage of the data on magnetic tape (Philips Analog 7 tape recorder) the processing of the EMG signals was performed with
TWIN A
80 -
120-TWlN A
100
M A X I M A L MUSCULAR POWER (
kpm/ sec 1
0
Mi! pair
o Di! pair
Fig. 2 . Intrapair comparison of maximal muscular power for MZ and DZ twins. Acrir Physiol Scirittl 106
Muscle strength in twins
. TWIN 320 A13
*
1613
.
A
,
320
'
?LO
'
321
TWIN A
TWIN B
TWIN B 0
80
160
2LO
320
.TWIN A
240
-
TOTAL LEG FORCE (kg) 0 0
Mi! pair DZ pair
TWIN B 0
811
160
?LO
320
Fig. 3 . Intrapair comparison 0. .otal leg force for MZ and DZ twins.
Hewlett Packard 21 16C computer system (Viitasalo & Komi 1975). Chronaximetric measurements. Neuroton Model 626 stimulator was used to investigate the vastus lateralis muscle with constant current method using the following stimulus durations: 0.1 ms, 1 ms and 30 ms. The 30 rns duration was considered as a rheobase stimulus and was therefore used as a basis for calculation of the chronaxia value. Skeletal muscle fibre composition and muscle enzyme activities were determined in muscle biopsy samples taken
from the vastus lateralis muscle and analyzed according to Gollnick et al. (1972) and Lowry & Passoneau (1972). Fibre types were identified as slow twitch (ST) and fast twitch (FT) fibres. For details see Komi et al. (1977). The following enzyme activities were analyzed: creatine kinase (CPK),myokinase (MK), Caz+and Mg2+stimulated ATPases (Ca2+ ATPase, Mgz+ ATPase), hexokinase (HK), phosphorylase (Plase) and lactate dehydrogenase (LDH). Statistical analysis. Ordinary statistical procedures were employed to calculate means, standard deviation (S.D.) and linear correlation coefficients ( r ) , and to test the significance of differences in intrapair variances between the two twin types. If the variance ratio (F) was significant at 5 % level of probability a heritability estimate (Hest) was computed as originally described by Holzinger (1929): 21-795877
where S2MZ and SzDZ denote intrapair variability of an attribute in MZ and DZ twins, respectively, and S2e signifies the variance due to experimental error. For details see Komi et al. (1977).
RESULTS
Hereditary factors Functional tests. The intrapair variance for muscular power was significantly smaller in MZ pairs as compared to DZ pairs. The calculated He,, was 97.8. For the remaining functional tests no statistically significant difference within DZ pairs as compared to MZ could be demonstrated, but, with the exception of quadriceps force, the variation was slightly greater within DZ pairs (Figs. 1-4). IEMG and chronaximetric tests. The general finding from the different IEMG and chronaximetric data was that the variation between MZ twins was slightly, but not significantly, smaller than the variation between DZ twins (Fig. 5 ) . Acrtr Physiol Srrmd I06
322
J . Kcrrlsson et N I . 80
’
60
.
.,,ITWIN
TWIN A
A
/
/
TWIN B
TWIN B 20
60
LO
60
80
20
LO
eLl
OUADRICEPS FORCE ( k g )
.
0
o
20
60
LO
MZ pair DZ pair
60
F i g . 4. Intrapair comparison of quadriceps force for MZ and DZ twins.
MEAN
PERCENT
INTRAPAIR
DIFFERENCE
Fig. 5. Mean percent intrapair differences in MZ and DZ twins for integrated electromyographic (IEMG) data from rectus femoris (RF)and vastus lateralis (VL) muscles; and chronaximetric data of different stimulus durations (30 ms, 1 ms, 0.1 ms) from the vastus lateralis muscle.
Muscle strength in twins
323
FORCE TIME 70%
A 02 d n.50 0 MZ 9 r=-.67 ( p r 001) ' 6 w ~ ~ m s ~ c ~
A
n
A 0
40
€4
-i
A
-A
A M MUSCULAR POWER(kpm/wc)
A
100
80
120
Fig. 6. Relationship between value of force-time at 70% of maximum isometric contraction (total leg force measurement) and maximal muscular power for all twin groups.
Correlation analysis
Linear correlations between all the variables studied have been published elsewhere (Komi & Karlsson 1978). In the following only the most relevant information is given. Muscular power was in the whole material, as expected, best related to other strength variables as quadriceps force (r=0.82), total leg force (r=0.71) and running velocity (r=0.67) as well as force-time ( r = -0.67, Fig. 6). No significant relationship was, however, present between muscular power and per cent of ST fibres. Considering zygosity similar relationships were obtained for muscular power versus other strength performance variables as in the whole material. The DZ twins were the only group, however, to demonstrate correlations to total leg force (r=0.76) and IEMG of the vastus lateralis muscle (r=0.46). Quadriceps force was in the whole population best related to muscular power (r=0.82), total leg force (r=O.80) and force-time (r=-0.46). No differences in terms of correlations seemed to be present for quadriceps force when comparing monozygous (mean value 7 9 f 4 kp) and dizygous twins (mean value 5 2 f 1 6 kp). In the males quadriceps force was also related to per cent ST fibres (r=-0.55). Activities of enzymes involved in ATP turnover during muscle contraction. Among the enzymes studied only CPK demonstrated in the whole population correlations to strength performance var-
iables: running velocity (r=0.41) (Fig. 7 ) and forcetime (r=-0.40), respectively. The enzyme activities and their relations to performance variables were not influenced by zygosity. Activities of enzymes involved in glucose residue metabolism. Hexokinase (HK), phosphorylase
(Plase), and lactate dehydrogenase (LDH) were the enzymes selected to represent the glucose residue metabolism. They demonstrated no significant or vague relationships to performance variables. Per cent distribution of LDH-1 isozyme showed a correlation only in the MZ group versus a strength variable (muscular power, r= -0.62) in contrast to the DZ group as well as the whole material.
A
ONt
A
Oat
RUNNING,MLOCITV
om 09
10
I1
12
13
I4
IS
Fig. 7. Relationship between the activity of creatine phosphokinase (CPK)enzyme (moles * g-l. min-' lo-*)
and running velocity (mls) on staircase. Arrrr Physiol Srtrnd 106
324
J. Karlsson et al.
DISCUSSION training is affecting muscle fibre composition and An earlier report (Komi et al. 1977) with the same muscle enzyme activities and found no or small twin material had observed the strong heritability changes in spite of marked increases in perestimate for the ST fibre distribution. The results of formance (for reference and summary see Thorthe present report support earlier findings (Komi et stensson 1976). It was suggested that the neuroal. 1973) that also variance in muscular power is motoric control was relatively more affected algenetically determined and that muscular strength though no experimental evidence could be found and IEMG did not show heritability in the same for this hypothesis. statistically significant way as muscle fibre comWhen employing the isokinetic technique for position or muscular power. In these parameters a measurements of dynamic muscle strength, it could statistical comparison showed that the intrapair var- be demonstrated that the potential of a muscle to iance was not statistically significant at the required produce force during movements of high velocity confidence level of pCO.05. For this reason the was positively correlated to its proportion of FT computation of He&was abandoned. muscle fibres (Thorstensson 1976). Similarly, it has No special control of the socio-economic, health been shown that the time needed for production of or physical activity status was made in the present submaximal force levels is related to the muscle study. Although these environmental influences fibre composition (Viitasalo & Komi 1978). It seems probably have been only minor, they still might be then reasonable to suggest that even muscular the cause of individual adaptation to some other strength performance might be partly under the instrength performance thus masking a genetic com- fluence of genetic factors. This suggestion is basiponent present. Adaptation of similar kinds might cally motivated by the strong heritability estimate affect both neuromuscular functions and muscle found for the muscle fibre composition. Additional enzyme activities. support for not rejecting this hypothesis comes In the present study muscle performance has from a comparatively strong correlation between been interpreted from measurements of muscular muscular power and other strength variables. power, muscular forces, force-time and IEMG dur- Therefore it is encouraged that the problem will be ing maximal isometric knee extension. In addition, re-investigated with a larger twin material than what chronaximetric data were obtained from the vastus was possible in the present study. lateralis muscle. Muscular power as measured in the present study is sometimes referred to as an The study was supported by grants from the Swedish “anaerobic power” test (Margaria et al. 1966), Medical Research Council (project No. 4251). which would imply that it would indicate the magnitude of the available ATP and CP stores as well as the potential of the anaerobic glycolytic REFERENCES pathway (Karlsson 1971). Although this test easily GOLLNICK, P. D . , ARMSTRONG, B.. SAUBERT, G. differentiates power athletes from endurance athW., PIEHL, K. & SALTIN, B. 1972. Enzyme activity letes (Komi et al. 1977)it is the authors’ opinion that and fibre composition in skeletal muscle of untrained and trained man. J Appl Physiol33: 312-319. this hypothesis has not been satisfactorily tested. In both twin materials positive correlation coefi- Holzinger, K. J . 1929. The relative effect of nature and nurture influences on twin differences. J Educ Psycho1 cients were obtained, however, for muscular power 54: 231-237. versus lean body mass corresponding to 0.91 (MZ KARLSSON, J. 1971. Lactate and phosphagen concentrations in working muscle of man. Acta Physiol twins) and 0.97 (DZ twins). This is indicative that Scand, Suppl. 358. muscular power is directly related to the muscle volume. The individual anthropometric variables KLISSOURAS, V . 1971. Heritability of adaptive variation. J Appl Physiol31:338-341. have earlier been demonstrated to be dependent on KOMI, P. V . 1973. A new electromechanical ergometer. genetic factors (Komi et al. 1973). These consideraIn: Proc. 3. Internationales Seminar fur Ergometrie (ed. G . Hansen and H. Mellerowiz), pp. 173-176. tions would then support the original suggestion by Ergon-Verlag, Berlin. Markaria et al. (1%) that muscular power reflects KOMI, P. V. & KARLSSON, J . 1979. Physical quantitatively the anaerobic energy output. performance, skeletal muscle enzyme activities, and Thorstensson and co-workers have in a number fibre types in monozygous and dizygous twins of both sexes. Acta Physiol Scand Suppl. 462. of studies tried to evaluate how muscle strength A r m Phvsiol Scund 106
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