Relationship between Force Fluctuation in the Plantar Flexor and Sustainable Time for Single-leg Standing

Abstract

The purpose of this study was to investigate the relationships between force fluctuation during isometric plantar flexion and the sustainable time for single-leg standing. Fourteen healthy males (21Ϯ1 years) performed unilateral (preferred leg) force matching tasks and single-leg quiet standing. Force matching tasks were performed to maintain isometric plantar flexion for 15 s at levels corresponding to 10% and 20% maximal voluntary contraction (MVC) with the visual feedback of force. Force fluctuation during force matching tasks was quantified as the standard deviation of force. Sustainable time for single-leg quiet standing was performed to maintain a single-leg quiet standing barefoot on a platform using the preferred leg with their eyes closed. Force fluctuation was significantly greater in 20% MVC task compared to 10% MVC task. The sustainable time for singleleg quiet standing was strongly correlated with force fluctuation in 20% MVC task (rϭϪ0.56, pϭ0.04). However, it was not related to force fluctuation in 10% MVC task (rϭ0.19, pϭ0.52) or MVC value (rϭ0.13, pϭ0.65). These results suggest that a specificity of contraction intensity is observed between force steadiness and the posture stability during single-leg quiet standing; force steadiness during 20% MVC plantar flexion is one of the important components for posture stability during single-leg quiet standing.

Functional Significance of Force Fluctuation During Voluntary Muscle Contraction

“…Certainly, strength of the main working muscles to support self body weight is thought to be the most important factor for postural stability. However, MVC in the plantar flexor did not relate with posture sway in our report (Oshita and Yano, 2010c). Further, Kouzaki et al (2007) reported that postural sway during bipedal quiet standing increases following bed rest despite maintenance of the muscle volume of the main working muscle for human postural standing by strength training.…”

“…These results indicate that the force steadiness in plantar flexor muscles is important for postural stability during quiet standing; this is somewhat consistent with the results of previous studies. Oshita and Yano (2010c) report that force fluctuation during plantar flexion at 20% MVC is associated with postural stability during single-leg quiet standing. Although they report that postural stability is associated with force fluctuation at 20% MVC, the present results show a significant positive correlation between postural sway and force fluctuation at 10% MVC.…”

“…The discrepancy between these results is thought to be due to the difference in the state of quiet standing. Oshita and Yano (2010c) report that force fluctuation is associated with postural stability during single-leg quiet standing. The activity level of the plantar flexor muscles is approximately 15-20% of MVC during single-leg standing (Sawai et al, 2004); however, it is approximately 10% of MVC during bipedal standing (Ohnishi et al, 2005;Sawai, et al, 2004).…”

Low-frequency Force Steadiness Practice in Plantar Flexor Muscle Reduces Postural Sway during Quiet Standing

“…The normalized force fluctuation (measured by the standard deviation (SD) of force) during isometric contractions is referred to as "steadiness," which influences functional human movement (Carville et al, 2007;Kornatz et al, 2005;Kouzaki and Shinohara, 2010;Marmon et al, in press;Oshita and Yano, 2010a;Salonikidis et al, 2009). Recently, force fluctuation during isometric plantar flexion was reported to be associated with postural stability during quiet standing (Kouzaki and Shinohara, 2010;Oshita and Yano, 2010a).…”

“…Recently, force fluctuation during isometric plantar flexion was reported to be associated with postural stability during quiet standing (Kouzaki and Shinohara, 2010;Oshita and Yano, 2010a). For example, Oshita and Yano (2010a) reported that the force fluctuation of the plantar flexion muscles was associated with postural stability during single-leg quiet standing. Further, Kouzaki and Shinohara (2010) reported that the force fluctuation of the plantar flexion muscles was associated with postural sway during bipedal quiet standing in young and older adults.…”