Masanori Sakamoto

Leg stiffness adjustment for a range of hopping frequencies in humans

The purpose of the present study was to determine how humans adjust leg stiffness over a range of hopping frequencies. Ten male subjects performed in place hopping on two legs, at three frequencies (1.5, 2.2, and 3.0Hz). Leg stiffness, joint stiffness and touchdown joint angles were calculated from kinetic and/or kinematics data. Electromyographic activity (EMG) was recorded from six leg muscles. Leg stiffness increased with an increase in hopping frequency. Hip and knee stiffnesses were significantly greater at 3.0Hz than at 1.5Hz. There was no significant difference in ankle stiffness among the three hopping frequencies. Although there were significant differences in EMG activity among the three hopping frequencies, the largest was the 1.5Hz, followed by the 2.2Hz and then 3.0Hz. The subjects landed with a straighter leg (both hip and knee were extended more) with increased hopping frequency. These results suggest that over the range of hopping frequencies we evaluated, humans adjust leg stiffness by altering hip and knee stiffness. This is accomplished by extending the touchdown joint angles rather than by altering neural activity.

Knee stiffness is a major determinant of leg stiffness during maximal hopping

Understanding stiffness of the lower extremities during human movement may provide important information for developing more effective training methods during sports activities. It has been reported that leg stiffness during submaximal hopping depends primarily on ankle stiffness, but the way stiffness is regulated in maximal hopping is unknown. The goal of this study was to examine the hypothesis that knee stiffness is a major determinant of leg stiffness during the maximal hopping. Ten well-trained male athletes performed two-legged hopping in place with a maximal effort. We determined leg and joint stiffness of the hip, knee, and ankle from kinetic and kinematic data. Knee stiffness was significantly higher than ankle and hip stiffness. Further, the regression model revealed that only knee stiffness was significantly correlated with leg stiffness. The results of the present study suggest that the knee stiffness, rather than those of the ankle or hip, is the major determinant of leg stiffness during maximal hopping.

Combining observation and imagery of an action enhances human corticospinal excitability.

The present study investigated whether combining observation and imagery of an action increased corticospinal excitability over the effects of either manipulation performed alone. Corticospinal excitability was assessed by motor-evoked potentials in the biceps brachii muscle following transcranial magnetic stimulation over the motor cortex during observation, imagery or both. The action utilized was repetitive elbow flexion/extension. Simultaneous observation and imagery of the elbow action facilitated corticospinal excitability as compared to that recorded during observation or imagery alone. However, facilitation due to the combination of observation and imagery was not obtained when the participants imagined the action pattern while they observed the same action presented out of phase. These findings suggest that a combination of observation and imagery can enhance corticospinal excitability. This enhancement depends on phase consistency between the observed and imagined actions.

Continuous change in spring-mass characteristics during a 400 m sprint

The purpose of the present study was to utilise a spring-mass model to (1) continuously measure vertical stiffness (K(vert)) and leg stiffness (K(leg)) over an entire 400 m sprint, and (2) investigate the relationship between leg spring stiffness (K(vert) and K(vert)) and the performance characteristics of mean forward running velocity (V(forwad)), mean stride frequency (f(stride)), and mean stride length (L(stride)). Eight well-trained male athletes performed a 400 m sprint with maximal effort on an outdoor field track. K(vert) was calculated from the subjects body mass, ground contact time and flight time at each step. V(forwad), f(stride) and L(stride) were determined from video images. K(vert) and V(forwad) peaked at the 50-100 m interval, and consistently decreased from the middle to the later part of the sprint. K(leg) peaked at first 50 m interval, and remained constant from next 50 m interval to finish. As compared with peak values, K(vert) and V(forward) in the last 50 m decreased by about 40% and 25%, respectively. A significant positive linear relationship existed between the K(vert) and V(forward). While K(vert) was significantly correlated with f(stride), it had no correlation with L(stride). Further, no significant positive linear relationship was found between K(leg) and V(forward), f(stride), or L(stride). This result indicates that in order to keep V(forward) at later stage of a 400 m sprint, maintaining the higher f(stride) through retaining a higher K(vert) would be necessary.

Differences in lower extremity stiffness between endurance-trained athletes and untrained subjects

An understanding of lower extremity stiffness is important for evaluation of sports performance and injury prevention. The aim of this study was to investigate whether stiffness regulation during hopping differed between endurance-trained athletes and untrained subjects. Eight endurance-trained athletes and eight untrained subjects performed two-legged hopping at 2.2 Hz. We determined leg and joint stiffness of hip, knee and ankle from kinetic and kinematics data. The endurance-trained athletes demonstrated significantly higher leg stiffness than untrained subjects. Further, the differences in leg stiffness were attributable to differences in ankle and knee joint stiffness. This study demonstrates a possibility that endurance training, like power training, increases leg and joint stiffness.

Brain activity during motor imagery of an action with an object: a functional magnetic resonance imaging study.

We utilized functional magnetic resonance imaging to investigate the brain regions activated during motor imagery of an action with an object both with and without passively holding the object. Participants performed the following tasks: (1) Imagery with Ball condition: subjects imagined squeezing a foam ball (7cm diameter) while holding the ball, (2) Imagery condition: subjects imagined squeezing a ball without holding the ball, and (3) Ball condition: subjects held the ball without motor imagery. Regions activated by the Imagery with Ball condition were located in the left dorsolateral prefrontal cortex (DLPFC), supplemental motor areas (SMA), inferior parietal lobule (IPL), superior parietal lobule (SPL), insula, cerebellum and basal ganglia. A direct comparison revealed that the right DLPFC and the right IPL showed a higher level of activation during the Imagery with Ball than during the Imagery+Ball conditions. Our studies suggested that the right front-parietal networks were involved in the motor imagery of an action with an object.

Influence of touching an object on corticospinal excitability during motor imagery

We investigated whether corticospinal excitability during the imagery of an action involving an external object was influenced by actually touching the object. Corticospinal excitability was assessed by monitoring motor evoked potentials (MEPs) in the first dorsal interosseous muscle following transcranial magnetic stimulation over the motor cortex during imagery of squeezing a ball—with or without passively holding the ball. The MEPs amplitude during imagery when the ball was held was larger than that when the ball was not held. The MEPs amplitude was not modulated just by holding the ball. In the same experimental condition, the somatosensory evoked potentials (SEPs) in response to the stimulation of median nerve were not modulated by motor imagery or by holding the ball. These results suggest that the corticospinal excitability during imagery of squeezing a ball is enhanced with the real touch of the ball, and the enhancement would be caused by some changes along the corticospinal pathway itself and not by the change in responsiveness along the afferent pathway to the primary somatosensory cortex.

The Modulation of Corticospinal Excitability during Motor Imagery of Actions with Objects

We investigated whether corticospinal excitability during motor imagery of actions (the power or the pincer grip) with objects was influenced by actually touching objects (tactile input) and by the congruency of posture with the imagined action (proprioceptive input). Corticospinal excitability was assessed by monitoring motor evoked potentials (MEPs) in the first dorsal interosseous following transcranial magnetic stimulation over the motor cortex. MEPs were recorded during imagery of the power grip of a larger-sized ball (7 cm) or the pincer grip of a smaller-sized ball (3 cm)—with or without passively holding the larger-sized ball with the holding posture or the smaller-sized ball with the pinching posture. During imagery of the power grip, MEPs amplitude was increased only while the actual posture was the same as the imagined action (the holding posture). On the other hand, during imagery of the pincer grip while touching the ball, MEPs amplitude was enhanced in both postures. To examine the pure effect of touching (tactile input), we recorded MEPs during imagery of the power and pincer grip while touching various areas of an open palm with a flat foam pad. The MEPs amplitude was not affected by the palmer touching. These findings suggest that corticospinal excitability during imagery with an object is modulated by actually touching an object through the combination of tactile and proprioceptive inputs.

Influence of somatosensory input on corticospinal excitability during motor imagery.

Our previous studies showed that corticospinal excitability during imagery of squeezing a foam ball was enhanced by somatosensory input generated by passively holding the ball. In the present study, using the same experimental model, we investigated whether corticospinal excitability was influenced by holding the object with the hand opposite to the imagined hand. Corticospinal excitability was assessed by monitoring motor evoked potentials (MEPs) in the first dorsal interosseous muscle following transcranial magnetic stimulation over the motor cortex during motor imagery. Subjects were asked to imagine squeezing a foam ball with the right hand (experiment 1) or the left hand (experiment 2), while either holding nothing (Null condition), a ball in the right hand (Right condition) or a ball in the left hand (Left condition). The MEPs amplitude during motor imagery was increased, only when the holding hand and the imagined hand were on the same side. These results suggest that performance improvement and rehabilitation exercises will be more effective when somatosensory stimulation and motor imagery are done on the same side.

Remote facilitation of supraspinal motor excitability depends on the level of effort.

Stretch reflexes and motor‐evoked potentials (MEPs) of a muscle are facilitated when performing intensive contraction of muscles located in a different segment (remote effect). We investigated to what extent the remote effect on MEPs in the flexor carpi radialis (FCR) in humans is modulated during sustained maximal and submaximal voluntary contractions of the ipsilateral quadriceps (remote muscle). We found that even when the force of maximal voluntary contraction (MVC) of the remote muscle declined during sustained MVC, the magnitude of the remote effect on MEPs remained constant. Maximal electrical stimulation of the remote muscle and transcranial magnetic stimulation of the corresponding motor cortex revealed that the level of voluntary activation gradually decreased during the sustained MVC. The motor response in the FCR following magnetic stimulation at the level of the foramen magnum, which preferentially elicits muscle response as a direct response of the corticospinal tract, was not modified by the remote effect during the sustained MVC. This finding suggested that the excitability of the spinal motoneuron pool remained constant. In contrast to the sustained MVC, during sustained submaximal contraction of the remote muscle, the magnitude of the remote effect on MEPs gradually increased as muscle fatigue developed. These findings suggest that the remote effect on MEPs was dependent on the level of effort driving the remote muscle, but not on the actual level of force output of the remote muscle, and that the origin of the remote effect was supraspinal, putatively upstream of the primary motor cortex.