MULTIVARIATE GENETIC-ANALYSIS OF MAXIMAL ISOMETRIC MUSCLE FORCE AT DIFFERENT ELBOW ANGLES

The maximal isometric moment at five different elbow joint angles was measured in 25 monozygotic and 16 dizygotic male adult twin pairs (22. 4 +/- 3.7 yr). Genetic model fitting was used to quantify the genetic and environmental contributions to individual differences in isometric strength. Additive genetic factors explained 66-78% of the variance i n maximal torque at 170- 140- 110 and 80 degrees flexion (extension = 180 degrees). At 50 degrees flexion, common and subject-specific envir onmental factors contributed equally to the variation. The contributio n of unique environmental factors concurs with the level of variabilit y in muscle activation and (dis)-comfort of torque production in the s pecific angle. The relative contribution of lever arm and force-length relationship in torque varies according to the angle. Because these f actors might be genetic, this variability is reflected in the genetic contribution at the extreme angles of 170 and 50 degrees. Multivariate analyses suggested a general set of genes that control muscle area an d isometric strength, together with a more specific strength factor. G enetic correlations were high (0.82-0.99). Genes responsible for arm-s egment lengths did not contribute to muscle area nor to isometric stre ngth.