Abstract
Isometric muscle force attained during isometric contractions decreases after active shortening compared to that attained during purely isometric contractions. This phenomenon is called residual force depression. The aim of this study was to examine whether residual force depression occurs in human plantar flexors in both plantar flexed and dorsiflexed region. In addition, the magnitude of fascicle shortening was evaluated because not only muscle force but also fascicle shortening during active shortening are considered to affect force depression. Eleven male subjects were recruited. All muscle contractions were evoked by muscle belly-electrical stimulation. In the reference trials, isometric plantar flexion (PF) was performed at 0° and 15° of PF. In the residual force depression trials, the following two contractions were conducted: (1) muscles were activated isometrically at 15° of dorsiflexion, then actively shortened to 0° of PF (long condition) and (2) muscles were activated isometrically at 0° of PF, then actively shortened to 15° of PF (short condition). Isometric joint torque obtained 4.9 s after the onset of contraction was compared between the reference and residual force depression trials at the same joint angle to calculate the magnitude of residual force depression. At the same time point, fascicle length and pennation angle were obtained from ultrasonographic images to examine whether the muscle architecture affected residual force depression. As a result, residual force depression was confirmed in both the long and short length conditions (long: 87.1 ± 9.1%, short: 92.1 ± 7.8%) while the magnitude was not different (p = 0.182). The fascicle length and pennation angle were not different between the reference and residual force depression trials (p = 0.291–0.906). These results indicate that residual force depression occurs in the physiological range of motion in the human plantar flexors, and this phenomenon is not related to muscle architecture. In addition, joint angle dependence of the residual force depression was not observed between long and short muscle length conditions.
Highlights
The isometric force attained at a given muscle length and activation level is smaller after active shortening compared to the isometric force attained at a pure isometric contraction (Abbott and Aubert, 1952)
The paired t-test revealed no significant difference between reference and residual force depression trials in both the short and long conditions (p = 0.782 for the short condition and p = 0.906 for the long condition) (Figure 4, upper panel)
We found a substantial decrease in joint torque in human plantar flexors in both conditions, but no significant differences in the magnitude of residual force depression were found between the two conditions, which seems to contradict our working hypothesis
Summary
The isometric force attained at a given muscle length and activation level is smaller after active shortening compared to the isometric force attained at a pure isometric contraction (Abbott and Aubert, 1952). Because actin filaments have small but some compliance (Huxley et al, 1994; Higuchi et al, 1995), and the actin filaments are pulled during active shortening due to muscle force, the shape of actin filaments is deformed This leads to dislocation of the binding site of the myosin head, which causes inhibition of crossbridge formation. This concept supported by previous studies that showed that the magnitude of residual force depression was strongly related to the magnitude of force produced during active shortening (Herzog and Leonard, 1997; De Ruiter et al, 1998) This concept is reasonable because larger force produces a larger deformation of the actin filament, which should cause a stronger inhibition. A larger magnitude of residual force depression is observed
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