Change in length of relaxed muscle fascicles and tendons with knee and ankle movement in humans

Abstract

Ultrasonography was used to measure changes in length of muscle fascicles in relaxed human tibialis anterior and gastrocnemius during passively imposed changes in joint angle. Changes in the length of muscle fascicles were compared to changes in the length of the whole muscle-tendon units calculated from joint angles and anthropometric data. Relaxed muscle fascicles underwent much smaller changes in length than their muscle-tendon units. On average, muscle fascicles in tibialis anterior 'saw' 55 +/- 13 % (mean +/- S.D.) of the total change in muscle-tendon length. This indicates nearly half of the total change in muscle-tendon length was taken up by stretch of tendon. In gastrocnemius, which has relatively long tendons, only 27 +/- 9 % of the total change in muscle-tendon length was transmitted to muscle fascicles. Thus, the tendency for passive movement to be taken up by the tendon was greater for gastrocnemius than tibialis anterior (P = 0.002). For these muscles, the relatively large changes in tendon length across much of the physiological range of muscle-tendon lengths could not wholly be explained by tendon slackness, changes in fibre pennation, or stretch or contraction history of the muscle. Our data confirm that when joints are moved passively, length changes 'seen' by muscle fascicles can be much less than changes in the distance between muscle origin and insertion. This occurs because tendons undergo significant changes in length, even at very low forces.

Figure 1. Relationship between change in length of muscle-tendon units and joint angle

Data on changes in length of whole muscle-tendon units were obtained from measurements in cadavers. A, change in tibialis anterior length with ankle angle. B, change in gastrocnemius length with ankle angle. C, change in gastrocnemius length with knee angle. Continuous lines are polynomial regressions through the data from Spoor et al. (1990) (S in A and B); Spoor & van Leeuwen (1992) (S in C); Klein et al. (1996) (K); Grieve et al. (1978) (G); and Visser et al. (1990) (V). Dotted lines are best fit linear regressions to all data. The absolute values of the slopes of the three linear regressions are 0.53 mm deg⁻¹ (tibialis anterior), 0.83 mm deg⁻¹ (gastrocnemius at ankle) and 0.29 mm deg⁻¹ (gastrocnemius at knee). See text for details.

Figure 2. Relationship between muscle fascicle length and change in length of muscle-tendon units

Each panel shows data from one subject. A, medial gastrocnemius. B, tibialis anterior. Continuous lines are least-squares linear regressions and 95 % confidence intervals. The dashed lines indicate a slope of 1. (Scales differ in panels A and B, but the ratio of horizontal and vertical scales is the same.)

Figure 3. Linear regressions of muscle fascicle length on change in muscle-tendon length

Each line is the linear regression through all data for one subject (mean of 22 measurements per subject; see Fig. 2). The regression line for each subject has been extended from the smallest to the largest muscle-tendon length measured. A, medial gastrocnemius. B, tibialis anterior. (Note that scales and ratio of horizontal and vertical scales differ in A and B.)

Figure 4. Effect of muscle contraction on relationship between muscle fascicle length and change in muscle-tendon length for tibialis anterior

Data from one subject. A, subject relaxed. B, subject contracting the ankle dorsiflexor muscles to 5 % of the maximal isometric torque that could be produced with the ankle dorsiflexed to 90 deg. C, subject contracting the ankle dorsiflexor muscles to 100 % of the maximal isometric torque that could be produced with the ankle dorsiflexed to 90 deg. The slope of the regressions is greater during contraction than at rest, indicating that muscle fascicles undergo relatively greater changes in length during contraction.