Force-length characteristics of the in vivo human gastrocnemius muscle.

In this study, the force-length characteristics of the in vivo medial (GM) and lateral (GL) heads of the human gastrocnemius muscle were estimated from measurements in eight healthy male subjects. This involved: 1) dynamometry-based measurements of the moment generated during maximal isometric plantar flexion; 2) ultrasound-based measurements of fascicular length and pennation angle; and 3) ultrasound-based calculations of moment arm lengths. All measurements were taken over the ankle angle range from 20 degrees of dorsiflexion to 30 degrees of plantar flexion. Tendon forces were calculated by dividing the moments recorded by the muscle moment arm lengths, and fascicular forces were calculated by dividing the tendon forces estimated by the cosine of pennation angle. In the transition from 30 degrees of plantar flexion to 20 degrees of dorsiflexion, the GM muscle fascicular length and force increased linearly from 24 to 39 mm and from 222 to 931 N, respectively. Over the same ankle angle range, the GL muscle fascicular length and force increased linearly from 30 to 47 mm and from 139 to 393 N, respectively. Estimates of the sarcomeric lengths corresponding to the fascicular lengths measured indicated that the two muscles operated in the range 1.4-2.2 microm, below the optimal length region for force generation according to the cross-bridge mechanism of contraction. These results indicate that the force-length relation of the in vivo human gastrocnemius muscle is limited to the ascending limb of the bell-shaped force-length curve obtained from experiments on isolated material.