Frequency features of mechanomyographic signals of human soleus muscle during quiet standing

Attenuated spontaneous postural sway enhances diastolic blood pressure during quiet standing

PURPOSE

Spontaneous postural sway during quiet standing has been considered a simple output error of postural control. However, as postural sway and inherent body orientation evoke compensatory activity of the plantar flexors, they might contribute to blood circulation under gravitational stress via the muscle pump. Hence, the present study employed an external support device to attenuate the plantar flexor activity in supported standing (SS), to experimentally identify its physiological impact on blood circulation.

METHODS

Eight healthy young subjects performed two 5-min quiet standing trials (i.e., normal standing (NS) and SS), and the beat-to-beat interval (RRI) and blood pressure (BP) were compared between trials. We confirmed that postural sway and corresponding plantar flexor activity, quantified by the anteroposterior displacement of the foot center of pressure and lower back position with respect to the wall, and by the amplitude of electromyography and mechanomyography, respectively, were attenuated in SS, while mean body orientation angle and relative position of the BP sensor were comparable to NS.

RESULTS

The 5-min averages of diastolic BP and mean arterial pressure during SS were significantly higher than during NS, while RRI and systolic BP did not change. These could be interpreted as an increase in peripheral vascular resistance; meanwhile, in NS, this effect was replaced by the muscle pump of the plantar flexors.

CONCLUSION

The muscle contractions related to spontaneous postural sway and body orientation produce substantial physiological impact on blood circulation during quiet standing.

Development of a novel 6-DOF parallel mechanism moving platform for investigating the effects of multimodal sensory feedback information regarding postural stability in humans

The purpose of this study was to develop a six-degree-of-freedom parallel mechanism moving platform to investigate the effects of multimodal sensory feedback information while standing upright. We constructed a custom-designed disturbance-applying instrument (DAI) consisting of a support surface suspended from eight pneumatic artificial muscles. The posture of the support surface was controlled with a proportional-integral-derivative (PID) feedback system using an infra-red camera-based real-time 3D motion capture system, and was estimated by step and frequency responses. Head trajectories of two healthy subjects were recorded to evaluate the effect of the differences in the impedance of the support surface during upright standing with their eyes opened/closed. The results demonstrated that the step and frequency responses of the DAI were good enough to enhance the bodily oscillation and head motion relative to the support surface. Indeed, the head-swaying space, using the DAI in a 0.2-Hz air-supplying condition during upright standing with their eyes closed, was larger than in the other standing condition. These results suggest that the specific medio-lateral head swaying was caused by the effects of the multimodal sensory feedback information using the DAI. In conclusion, we developed a novel moving platform to investigate the effects of multimodal sensory feedback information upon upright postural control. The DAI would be particularly valuable to enhance the head-swaying space during upright standing by changing the impedance of the support surface or stimulating the translational and/or rotational sensory feedback integration.

Medial gastrocnemius is a key muscle for involuntary alternate muscle activity of plantar flexor synergists

Redundant and/or complicated muscle activations between synergist muscles have been demonstrated during low-level sustained contractions. Identification of a key muscle for this phenomenon allows for the simplification of motor control during prolonged contraction. In this study, we have identified a key muscle for involuntary alternate muscle activity of plantar flexor muscles based on a physiological tremor sequence that was recorded over 120 min. Two epochs where the muscle activity of medial gastrocnemius abruptly increased with decrease in other synergists (case ON) and vise verse (case OFF) were analyzed. Our results indicated that involuntary alternate muscle activity was associated with changes in physiological tremor of ankle angular acceleration when the muscle activity of medial gastrocnemius decreased in case OFF. In particular, the activity of the medial gastrocnemius muscle, but not the activity of other synergists, was accompanied by physiological tremor, demonstrating that the medial gastrocnemius is a key muscle for involuntary alternate muscle activity in plantar flexor synergists. In addition, weaker correlations between muscle activities and physiological tremor were found in case ON than case OFF. We suggest that, if the central nervous system can employ this unique muscle strategy, redundant and/or complicated neuromuscular activities will be reduced because of the existence of the key muscle.

Index finger position fluctuations reflect multi-muscle coordination

We hypothesized that movement fluctuations in the index finger reflect the integrated result of the coordination of multiple muscles because index finger movements are determined by the cooperation of multiple muscles spanning the metacarpophalangeal (MCP) joint. To evaluate this hypothesis, the aim of the present study was to examine the fluctuations of the index finger in abduction-adduction and extension-flexion directions during a position-holding task using two laser displacement sensors. Eleven healthy men maintained their index finger position while supporting a load at 5% of the maximal voluntary contraction force. To maintain the position of the index finger, displacement of the index finger in the abduction-adduction and extension-flexion directions was measured from a distance with two laser displacement sensors that were positioned to the lateral side of and above the index finger. The index finger movements fluctuated around the target position in not only the abduction-adduction direction but also the extension-flexion direction. The path length of finger displacement and the standard deviation of finger acceleration were significantly greater in the extension-flexion direction than in the abduction-adduction direction. These results suggest that the index finger movements quantified by two laser displacement sensors reflect the coordination of multiple muscles spanning the MCP joint.

Postural sway during quiet standing is related to physiological tremor and muscle volume in young and elderly adults

To examine the age-related deterioration in postural control, we investigated the association between postural sway during quiet standing and either amplitude of physiological tremor or muscle volume of the plantar flexors in 20 young and 20 elderly adults. They maintained a quiet standing position on a force platform for 60s with their eyes open or closed. During quiet standing, physiological tremors detected using a piezoresistive accelerometer were recorded from the soleus muscle, and the center of pressure (COP) displacement and body acceleration in the antero-posterior direction were calculated using the ground reaction forces as an assessment of postural sway. Muscle volume was predicted from muscle thickness by an ultrasonographic image. The physiological tremor of the soleus muscle during quiet standing was significantly greater in elderly than in young adults, and a positive association between physiological tremor and the amplitude of postural sway was found for young and elderly adults combined. Furthermore, physiological tremor was positively correlated with the high-frequency component of COP sway during quiet standing. A significantly negative relation between the muscle volume of the plantar flexors and postural sway was found in both age groups. These results suggest that physiological tremor reflects high-frequency fluctuations in postural sway during quiet standing in young and elderly adults, and age-related increases in the postural sway amplitude in the antero-posterior direction may be related to a decrease in muscle volume of the plantar flexors for maintaining an upright posture.

Asymmetry of force fluctuation during low and moderate intensity isometric knee extensions

The purpose of this study was to investigate the asymmetry of force fluctuation during isometric knee extension at low and moderate intensities. 11 healthy men (M age = 21 yr., SD = 1) performed unilateral force matching tasks; sustained isometric knee extension at 20% and 30% maximal voluntary contraction (MVC). During the tasks, a mechanomyogram was measured by an accelerometer arrangement placed on the vastus lateralis. Although force fluctuation was not significantly different between the two legs at 20% MVC, it was higher in the left (weaker) leg than in the right (stronger) leg at 30% MVC. A significant difference in mean power frequency of the mechanomyographic signal between the two legs was also observed only at 30% MVC. These results suggest that the asymmetry of force fluctuation during isometric knee extension was not statistically significant at low intensity; however, it was significant at moderate intensity. These differences in force fluctuation between intensities might be influenced by different motor-unit firing rates in active muscle.

Steadiness in plantar flexor muscles and its relation to postural sway in young and elderly adults

To investigate the functional significance of force fluctuations during voluntary contraction with a select muscle group, we examined the association between force fluctuations during voluntary contraction with plantar flexor muscles and postural sway during quiet standing in 20 young and 20 elderly adults. Young and elderly subjects maintained a quiet standing position on a force platform. They also performed a force-matching task with unilateral isometric plantar flexion. A positive correlation was found in young and elderly adults between the coefficient of variation (CV) of center of pressure during quiet standing and the CV of force during plantar flexion only at contraction intensities of < or =5% maximum voluntary contraction that corresponded to muscle activity during quiet standing. The electromyogram power in the medial gastrocnemius was greater in the elderly than in young adults by approximately 10 Hz during quiet standing and at low contraction intensities during plantar flexion. Fluctuations in motor output during low-intensity plantar flexion were associated with postural sway during quiet standing in both young and elderly adults.