Passive elongation of muscle fascicles in human muscles with short and long tendons

Abstract

This study tested the hypothesis that the ratio of changes in muscle fascicle and tendon length that occurs with joint movement scales linearly with the ratio of the slack lengths of the muscle fascicles and tendons. We compared the contribution of muscle fascicles to passive muscle-tendon lengthening in muscles with relatively short and long fascicles. Fifteen healthy adults participated in the study. The medial gastrocnemius, tibialis anterior, and brachialis muscle-tendon units were passively lengthened by slowly rotating the ankle or elbow. Change in muscle fascicle length was measured with ultrasonography. Change in muscle-tendon length was calculated from estimated muscle moment arms. Change in tendon length was calculated by subtracting change in fascicle length from change in muscle-tendon length. The median (IQR) contribution of muscle fascicles to passive lengthening of the muscle-tendon unit, measured as the ratio of the change in fascicle length to the change in muscle-tendon unit length, was 0.39 (0.26-0.48) for the medial gastrocnemius, 0.51 (0.29-0.60) for tibialis anterior, and 0.65 (0.49-0.90) for brachialis. Brachialis muscle fascicles contributed to muscle-tendon unit lengthening significantly more than medial gastrocnemius muscle fascicles, but less than would be expected if the fascicle contribution scaled linearly with the ratio of muscle fascicle and tendon slack lengths.

Keywords: Brachialis; fascicle length; gastrocnemius; tendon; tibialis anterior; ultrasonography.

Figure 1

Ultrasound images of the medial gastrocnemius, tibialis anterior, and brachialis muscles (left) and schematic diagrams (right) showing how tendon lengths (L _t), fascicle lengths (L _f), and pennation angles (θ) were measured. The estimated ratio of fascicle slack length and muscle‐tendon slack length (L _fs:L _mtus) is given.

Figure 2

Individual participant data (two fascicles per participant) of change in fascicle length with change in muscle‐tendon unit length during passive lengthening. Data are shown for medial gastrocnemius (G: dashed lines), tibialis anterior (TA: solid lines), and brachialis (B: dotted lines). The main panel shows data for all three muscles and the smaller panels at right show the same data for each muscle. Muscle‐tendon length of 0 is nominally used to indicate ankle plantargrade and full elbow flexion.

Figure 3

The ratios of change in fascicle length with change in muscle‐tendon unit length for the three muscles. Data are (left) individual raw data and (right) medians with interquartile ranges. G, medial gastrocnemius, TA, tibialis anterior, B, brachialis.

Figure 4

A plot of the log of the ratio of changes in fascicle and tendon length against the log of the ratio of muscle fascicle and tendon slack lengths. G, medial gastrocnemius, TA, tibialis anterior, B, brachialis.