Is the descending limb of the force–length relation in the human gastrocnemius muscle ever used?

In a recent article in this journal, Maganaris (2003) presented data on the force–length relation of human gastrocnemius muscle, measured in isometric contractions in vivo. Fascicle lengths of the muscle were measured with ultrasound imaging. In combination with earlier work on other muscles in vivo (Kawakami et al., 1998; Maganaris, 2001), we consider these data highly relevant for assessment and understanding of anatomical and physiological muscle properties in human movement. In our opinion his experimental results are entirely correct, but it is against his conclusion: “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” that we want to raise objection. We found (Hof et al., 2002) that in functional activities, walking, running, and jumping, the descending limb of the force–length curve is used extensively. Whereas Maganaris used ultrasound imaging to measure muscle fascicle lengths, we used mechanical measurements. First, the force-stretch curve of the series elastic component (SEC, about equal to tendon plus aponeurosis) was determined experimentally using controlled-release experiments. The length of the contractile component (CC, i.e., fascicle length) was then calculated at each force level as the difference between origin-insertion length and SEC length. This method does not discriminate between fascicle lengths of gastrocnemius and soleus, but the advantage is that the method causes little impediment in movements. Figure 1 shows the force–length relation in some isometric contractions with electrical stimulation (circles, the same as in Figure 1 of Hof et al. (2002)). Our results are in agreement with Maganaris (squares and triangles), and with the force–length relation to be expected in muscle fibers of the relevant length (dashed line). The important point is that to reach the steady state isometric situation from a resting situation, the fascicles need to stretch the SEC and hence need to shorten themselves considerably. To reach the maximum isometric force of 4,000 N, for example, the fascicles shorten over 30 mm, where the force increased from the resting value of 200–500 N to a maximum of 4,000 N (solid lines). In the course of this shortening the greater part of the descending limb of the force–length curve has been traversed. This means that CC initially starts out at the descending limb of its force–length curve, but in building up isometric force, shortens to the ascending limb, thereby stretching the SEC. It was shown in our study (Hof et al., 2002) that in the non-isometric movements of real life a similar effect occurs. Figure 2 might clarify the issue further. Figure 2A shows the three force–length relations: active CC (dashed); passive parallel-elastic component (PEC, drawn line); and SEC (-.-.). The latter is drawn reversed, because SEC lengthening corresponds to CC shortening. The situation is presented when the ankle is at 10° of dorsiflexion. At rest, CC length is determined by the intersection of SEC and PEC curves (Fig. 2B) and settles around 60 mm. In an isometric contraction the SEC is stretched by 30 mm and the CC shortens eventually to 60 30 30 mm, the intersection of SEC and active CC force–length relation (Fig. 2A, Point 1). Similar fascicle lengths have been found with the ultrasound method (Kawakami et al., 1998): 40–59 mm passive and 26–38 mm active for medial gastrocnemius, depending on the ankle angle. Figure 2B shows the functional importance of the PEC. Although its force is low over most of the range of the CC force–length relation, it increases steeply at lengths corresponding to the end of the CC curve (end of overlap between actin and myosin filaments). In this way, independent of ankle or knee angle, muscle fibers will never be stretched much beyond this length (Herbert et al., 2002). Figure 2A shows that in human triceps surae the slope of the SEC curve is less than the (negative)