INHERITANCE OF STATIC AND DYNAMIC ARM STRENGTH AND SOME OF ITS DETERMINANTS

Maximal static, eccentric and concentric torques and arm components es timated by anthropometry and measured by computed tomography were eval uated in 25 male monozygotic twins and 16 dizygotic twins (22.4 +/- 3. 7 years). The importance of genetic and environmental factors in the o bserved variation in these measurements was estimated by genetic model -fitting techniques. In this sample of young adult male twins, genetic factors were significant in most of the strength measurements, arm mu scle components and muscle activation variables. The contribution of g enetic factors in strength measures depended on the angle, contraction type and to some extent on contraction velocity. For isometric streng th, angle-specificity in genetic and environmental variation could be attributed to the degree of variability in muscle activation and perfo rmance discomfort at each specific angle, with the highest unique envi ronmental impact at extreme angles. The high genetic contribution at 1 70 degrees, but not at 50 degrees, possibly expressed different contri butions of genetic factors in the muscle-length factor and moment arm in torques at both angles. The importance of genetic factors in eccent ric arm flexor strength (62-82%) was larger than for concentric flexio n (29-65%), as the pattern of genetic determination followed the torqu e-velocity curve. Genetic variations in contractile and elastic compon ents, contributing differently to eccentric and concentric torques, to gether with velocity-dependent actin-myosin binding factors, could acc ount for the observed differences. The broad heritability was very hig h for all anthropometric and arm cross-sectional area measurements (>8 5%) and common environmental factors were only significant for anthrop ometrically estimated mid-arm muscle tissue area (48%). Heritability e stimates of different arm muscularity measurements were comparable.