Abstract
Because it is difficult to measure tendon length changes directly in humans, tendon length changes during dynamic movement have been evaluated indirectly from changes in muscle fascicle length and joint angle. The purpose of this study was to examine the validity of the indirect method. Twitch contractions of the ankle plantar flexors were evoked isometrically in eight subjects. Twitch contractions evoked by singlet, doublet, and triplet stimulations were conducted at dorsiflexion 20° (DF20), plantar flexion 0° (PF0), and plantar flexion 20° (PF20). Muscle fascicle length and pennation angle were recorded by ultrasonography. The magnitude of muscle fascicle shortening was significantly smaller in DF20 than in PF0 and PF20, although the magnitude of joint torque was significantly larger in DF20 than in PF0 and PF20. Theoretically, the magnitude of tendon elongation is expected to be larger in larger joint torque conditions. However, we found that the magnitude of tendon elongation evaluated from muscle fascicle shortening was larger in a lower joint torque condition (PF20). These results suggest that the magnitude of muscle fascicle shortening does not necessarily represent tendon elongation. The larger muscle fascicle shortening in PF20 may be partly caused by eliminating slack of the muscle-tendon complex.
Highlights
During movements, a muscle generates force, which is transmitted to a bone via a tendon
The Achilles tendon length changes were estimated from changes in muscle-tendon complex length estimated from changes in knee and ankle joint angles[9, 10], and measured muscle fascicle length changes by ultrasonography, because measuring Achilles tendon length changes directly during dynamic movements is difficult in humans
No significant interaction was found in the muscle fascicle shortening, which is an index of tendon elongation (F value = 0.786, P = 0.50, η2 = 0.101), but a significant main effect was found in the joint angle (DF20, plantar flexion 0° (PF0), plantar flexion 20° (PF20)) and condition
Summary
A muscle generates force, which is transmitted to a bone via a tendon. Fukunaga et al.[5] reported that during walking, the Achilles tendon was elongated and shortened instead of the muscle, which would be beneficial from the point of view of using elastic energy and optimizing muscle shortening velocity[5,6,7,8] In these studies, the Achilles tendon length changes were estimated from changes in muscle-tendon complex length estimated from changes in knee and ankle joint angles[9, 10], and measured muscle fascicle length changes by ultrasonography, because measuring Achilles tendon length changes directly during dynamic movements is difficult in humans. If the influence of slack is substantial, the magnitude of tendon elongation evaluated through muscle fascicle shortening should be large, i.e., muscle fascicle shortening is larger when the ankle joint angle is flexed (slack is prominent) even though the joint torque is smaller
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