Motoki Kouzaki

Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max.

This study consists of two training experiments using a mechanically braked cycle ergometer. First, the effect of 6 wk of moderate-intensity endurance training (intensity: 70% of maximal oxygen uptake (VO2max), 60 min.d-1, 5 d.wk-1) on the anaerobic capacity (the maximal accumulated oxygen deficit) and VO2max was evaluated. After the training, the anaerobic capacity did not increase significantly (P > 0.10), while VO2max increased from 53 +/- 5 ml.kg-1 min-1 to 58 +/- 3 ml.kg-1.min-1 (P < 0.01) (mean +/- SD). Second, to quantify the effect of high-intensity intermittent training on energy release, seven subjects performed an intermittent training exercise 5 d.wk-1 for 6 wk. The exhaustive intermittent training consisted of seven to eight sets of 20-s exercise at an intensity of about 170% of VO2max with a 10-s rest between each bout. After the training period, VO2max increased by 7 ml.kg-1.min-1, while the anaerobic capacity increased by 28%. In conclusion, this study showed that moderate-intensity aerobic training that improves the maximal aerobic power does not change anaerobic capacity and that adequate high-intensity intermittent training may improve both anaerobic and aerobic energy supplying systems significantly, probably through imposing intensive stimuli on both systems.

Reduced postural sway during quiet standing by light touch is due to finger tactile feedback but not mechanical support

It is well known that a light and voluntary touch with a fingertip on a fixed surface improves postural stability during quiet standing. To determine whether the effect of the light touch is due to the tactile sensory input, as opposed to mechanical support, we investigated the light touch effect on postural stability during quiet standing with and without somatosensory input from the fingertip. Seven young subjects maintained quiet standing on a force platform with (LT) and without (NT) lightly touching a fixed surface, and with (TIS) and without (CON) the application of tourniquet ischemia, which removed the tactile sensation from the fingertip. The mean velocity of centre of pressure (CoP) was calculated to assess the postural sway in each condition. The mean velocity of CoP was significantly smaller in the LT condition compared to the NT condition only under the CON condition, whereas the light touch effect was not significant under the TIS condition. We found that the reduction of the horizontal ground reaction force due to the light touch was about 20%, which was approximately equivalent to the reduction of mean velocity of CoP in the LT condition compared to the NT condition. Since the fingertip contact force was relatively large compared to the horizontal ground reaction force, one could say that the light touch effect might be due to the mechanical support provided by the contact itself. However, we demonstrated experimentally that light touch effects were diminished due to loss of finger tactile feedback induced by the tourniquet ischemia, but not due to the mechanical support provided by the light touch. One possible reason is the lack of feedback information in controlling posture, and the other is the altered control of the arm induced by the loss of tactile feedback.

Larger center of pressure minus center of gravity in the elderly induces larger body acceleration during quiet standing.

When an inverted pendulum approximates quiet standing, it is assumed that the distance between the center of pressure and the vertical projection of the center of mass on the ground (COP-COG) reflects the relationship between the controlling and controlled variables of the balance control mechanism, and that the center of mass acceleration (ACC) is proportional to COP-COG. As aging affects the control mechanism of balance during quiet standing, COP-COG must be influenced by aging and, as a result, ACC is influenced by aging as well. The purpose of this study was to test the hypotheses that aging results in an increased COP-COG amplitude and, as a consequence, that ACC becomes larger in the elderly than the young. Fifteen elderly and 11 young subjects stood quietly on a force platform with their eyes open or closed. We found that (1) the standard deviations of COP-COG and ACC were larger in the elderly than in the young, irrespective of the eye condition; (2) COP-COG is proportional to ACC in both age groups, i.e., the inverted pendulum assumption holds true for quiet standing. The results suggest that a change in the control strategy that is due to aging causes a larger COP-COG in the elderly and, as a consequence, that ACC becomes larger as well.

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 ≤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 ∽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. Muscle Nerve, 2010

Task-dependent spatial distribution of neural activation pattern in human rectus femoris muscle

Compartmentalization of skeletal muscle by multiple motor nerve branches, named as neuromuscular compartment (NMC), has been demonstrated in animals as well as humans. While different functional roles among individual NMCs were reported in the animal studies, no studies have clarified the region-specific functional role within a muscle related with NMCs arrangement in human skeletal muscle. It was reported that the rectus femoris (RF) muscle is innervated by two nerve branches attached at proximal and distal parts of the muscle. The purpose of the present study is to clarify the possible region-specific functional role in the human RF muscle. Multi-channel surface electromyography (SEMG) were recorded from the RF muscle by using 128 electrodes during two different submaximal isometric contractions that the muscle contributes, i.e. isometric knee extension and hip flexion, at 20%, 40%, 60% and 80% of maximal voluntary contraction (MVC). Results indicated that the central locus activation for the amplitude map of SEMG during hip flexion located at more proximal region compared with that during knee extension. Significant higher normalized root mean square (RMS) values were observed at the proximal region during the hip flexion in comparison to those at middle and distal regions at 60% and 80% of MVC (p<0.05). In while, significant higher normalized RMS values were demonstrated at the distal region comparing with that at the proximal region at 80% of MVC (p<0.05). The results of the present study suggest possible region-specific functional role in the human RF muscle.

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.

Effect of the hip motion on the body kinematics in the sagittal plane during human quiet standing.

Human quiet stance is often modeled as a single-link inverted pendulum pivoting only around the ankle joints in the sagittal plane. However, several recent studies have shown that movement around the hip joint cannot be negligible, and the body behaves like a double-link inverted pendulum. The purpose of this study was to examine how the hip motion affects the body kinematics in the sagittal plane during quiet standing. Ten healthy subjects were requested to keep a quiet stance for 30s on a force platform. The angular displacements of the ankle and hip joints were measured using two highly sensitive CCD laser sensors. By taking the second derivative of the angular displacements, the angular accelerations of both joints were obtained. As for the angular displacements, there was no clear correlation between the ankle and hip joints. On the other hand, the angular accelerations of both joints were found to be modulated in a consistent anti-phase pattern. Then we estimated the anterior-posterior (A-P) acceleration of the center of mass (CoM) as a linear summation of the angular acceleration data. Simultaneously, we derived the actual CoM acceleration by dividing A-P share force by body mass. When we estimated CoM acceleration using only the angular acceleration of the ankle joint under the assumption that movement of the CoM is merely a scaled reflection of the motion of the ankle, it was largely overestimated as compared to the actual CoM acceleration. Whereas, when we take the angular acceleration of the hip joint into the calculation, it showed good coincidence with the actual CoM acceleration. These results indicate that the movement around the hip joint has a substantial effect on the body kinematics in the sagittal plane even during quiet standing.

Spatial EMG potential distribution pattern of vastus lateralis muscle during isometric knee extension in young and elderly men.

The aim of the present study was to compare spatial electromyographic (EMG) potential distribution during force production between elderly and young individuals using multi-channel surface EMG (SEMG). Thirteen elderly (72-79years) and 13 young (21-27years) healthy male volunteers performed ramp submaximal contraction during isometric knee extension from 0% to 65% of maximal voluntary contraction. During contraction, multi-channel EMG was recorded from the vastus lateralis muscle. To evaluate alteration in heterogeneity and pattern in spatial EMG potential distribution, coefficient of variation (CoV), modified entropy and correlation coefficients with initial torque level were calculated from multi-channel SEMG at 5% force increment. Increase in CoV and decrease in modified entropy of RMS with increase of exerted torque were significantly smaller in elderly group (p<0.05) and correlation coefficients with initial torque level were significantly higher in elderly group than in young group at moderate torque levels (p<0.05). These data suggest that the increase of heterogeneity and the change in the activation pattern are smaller in elderly individuals than in young individuals. We speculated that multi-channel SEMG pattern in elderly individual reflects neuromuscular activation strategy regulated predominantly by clustering of similar type of muscle fibers in aged muscle.

Alterations in synergistic muscle activation impact fluctuations in net force.

Net muscle force recorded during voluntary contractions with multiple agonist muscles is the summated result of individual muscle forces. The purpose of this article is to synthesize recent findings from several studies on the modulations of muscle activity and force fluctuations during steady voluntary contractions with multiple agonist muscles in humans. During a sustained low-force contraction with the knee extensor muscles, fluctuations in knee extension force changed concurrently with the involuntary alternate muscle activity between the rectus femoris muscle and the vasti muscles. After prolonged bed rest, the amount of change in the fluctuations in net force was associated with the amount of change in the EMG distribution among individual muscles within the knee extensor group and the ankle extensor group. As an adaptation to bed rest, an increase in the fluctuations in plantarflexion force accompanied an increase in the relative contribution of the medial gastrocnemius muscle to net force. In contrast, an increased contribution of the medial gastrocnemius by an acute increase in muscle length reduced force fluctuations. The discrepancy in the changes in force fluctuations due to an increased contribution of the medial gastrocnemius between two interventions (bed rest and muscle length change) is associated with increased low-frequency power in the EMG of the medial gastrocnemius after bed rest. In summary, the fluctuations in net force were influenced by the alteration in muscle activity among agonist muscles in terms of relative contributions to net force and frequency characteristics.

Identification of muscle synergies associated with gait transition in humans.

There is no theoretical or empirical evidence to suggest how the central nervous system (CNS) controls a variety of muscles associated with gait transition between walking and running. Here, we examined the motor control during a gait transition based on muscle synergies, which modularly organize functionally similar muscles. To this end, the subjects walked or ran on a treadmill and performed a gait transition spontaneously as the treadmill speed increased or decreased (a changing speed condition) or voluntarily following an experimenter’s instruction at constant treadmill speed (a constant speed condition). Surface electromyograms (EMGs) were recorded from 11 lower limb muscles bilaterally. We then extracted the muscle weightings of synergies and their activation coefficients from the EMG data using non-negative matrix factorization. As a result, the gait transition was controlled by approximately 9 muscle synergies, which were common during a walking and running, and their activation profiles were changed before and after a gait transition. Near a gait transition, the peak activation phases of the synergies, which were composed of plantar flexor muscles, were shifted to an earlier phase at the walk-to-run transition, and vice versa. The shifts were gradual in the changing speed condition, but an abrupt change was observed in the constant speed condition. These results suggest that the CNS low-dimensionally regulate the activation profiles of the specific synergies based on afferent information (spontaneous gait transition) or by changing only the descending neural input to the muscle synergies (voluntary gait transition) to achieve a gait transition.