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Although the phenomenon of residual force depression has been known for decades, the mechanisms remain elusive. In the present study, we investigated mechanisms of residual force depression by measuring the stiffness to force ratio during force redevelopment after shortening at different velocities. The results showed that the slope of the relationship between muscle stiffness and force decreased with decreasing shortening velocity, and the y-intercept increased with decreasing shortening velocity. The differing slopes and y-intercepts indicate that the stiffness to force ratio during isometric force redevelopment depends on the active shortening velocity at a given muscle length and activation level. The greater stiffness to force ratio after active shortening can potentially be explained by weakly-bound cross bridges in the new overlap zone. However, weakly-bound cross bridges are insufficient to explain the reduced slope at the slowest shortening velocity because the reduced velocity should increase the proportion of weakly- to strongly-bound cross bridges, thereby increasing the slope. In addition, if actin distortion caused by active shortening recovers during the force redevelopment period, then the resulting slope should be similar to the non-linear slope of force redevelopment over time. Alternatively, we suggest that a tunable elastic element, such as titin, could potentially explain the results.

Asymmetry in involuntary trunk motion may be related to core stability. If core stability properly controls postural stability during distal segment movements, trunk motion should be symmetric. Based on the assumption that head motion partially represents trunk motion, we developed a system that measures head angle and provides feedback music on head motion. If the head angle exceeds a predefined threshold, the music is muted in the earbud on the side of the head tilt. To investigate the relationship between core stability and asymmetric trunk motion, we measured mediolateral head angles during cycling using a self-developed system and compared them between individuals with good and poor core stability, and we investigated the effects of feedback music on mediolateral head motion during cycling. The poor-core-stability group showed significantly greater amplitude of, and less symmetry in, mediolateral head motion than the good-core-stability group. Furthermore, feedback music significantly reduced the amplitude of mediolateral head motion in the good- and poor-core-stability groups and improved symmetry in mediolateral head motion in the poor-core-stability group. Our findings imply that core stability is important for maintaining symmetry in head motion during cycling. Feedback music could be useful for establishing symmetry in head motion during cycling.