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

Abstract

The purpose of this study was to assess the effect of low-frequency force steadiness practice in the plantar flexor muscles on postural sway during quiet standing. Healthy young 21 men (21Ϯ1 yrs) were randomly assigned to a practice group (nϭ14) and a nonexercising control group (nϭ7). Practice groups were divided by frequency of practice: 7 participants practiced once a week, and the other 7 twice a week, for 4 weeks. Steadiness practice required practice group to 5 sets of 60-s contraction at levels corresponding to 10% and 20% maximal voluntary contraction (MVC) in the plantar flexor muscles. The 4-week-long practice period reduced the force fluctuations (assessed as the standard deviation (SD) of the outputted force during steady isometric plantar flexion) and postural sway (assessed as SD of the center of mass velocity during quiet standing). However, these practice effects were not significantly affected by the practice frequencies (1 vs. 2 sessions per week) examined in this study. Further, a linear regression analysis revealed the association between prepractice postural sway and the relative change in postural sway by the practice (rϭϪ0.904) in the practice group. These results suggest that the steadiness practice in plantar flexor muscles improves postural stability during quiet standing, even though the practice is low-frequency (once a week) and low-intensity (within 20% MVC). These practice effects are dependent on prepractice postural stability. Further, the present results have provided the functional significance of force fluctuation in lower limb muscles.

Functional Significance of Force Fluctuation During Voluntary Muscle Contraction

“…Table 1. Changes in muscle strength and force fluctuation due to steadiness practice in the practice group (Oshita and Yano, 2011c) These previous findings corroborate those of the present study. Both the existing literature and present results indicate that strength and steadiness training interventions consistently reduce fluctuations during the low-to-moderate-intensity contractions of the distal muscles of the upper and lower limbs.…”

“…This finding is also consistent with the findings of other reports demonstrating that improvements in steadiness are more frequent in subjects with low initial steadiness levels (Manini et al, 2005;Tracy et al, 2004); this strengthens the notion that the effectiveness of training is dependent on the initial level of steadiness. (Oshita and Yano, 2011c) As indicated in section 4, multiple factors influence the relationship between force steadiness and postural stability. Although we focused on whether force steadiness practice in the plantar flexor muscles improves postural stability during quiet standing, we demonstrated the functional significance of force fluctuations during voluntary contraction.…”

“…This is because the results presented in section 4 suggest that the neural strategies employed by the plantar flexor muscles during quiet standing are similar to those controlling plantar flexion forces in young adults. Thus, in this section, we discuss the effects of steadiness practice in the plantar flexor muscles on force fluctuation during isometric contraction and postural sway during quiet standing (Oshita and Yano, 2011c).…”

Postural Steadiness and Ankle Force Variability in Peripheral Neuropathy

“…Four weeks of steadiness training consisting of 60-s constant-force tasks at 10% and 20% MVC, was focused on just the plantarflexors (Oshita & Yano, 2011). The standard deviation of the center of mass velocity was reduced by ~15% during quiet standing trials as a result of the training.…”

Later stages of diabetic neuropathy affect the complexity of the neuromuscular system at the knee during low‐level isometric contractions

“…The amount of torque variability during isometric contractions has been shown to be correlated with motor performance, with larger plantar flexor torque variability being associated with, for example, a less stable balance . Additionally, the practice of isometric plantar flexion contractions within 20% of MVC has been shown to diminish the amount of plantar flexion torque variability and improve the postural stability …”