Kohei Omuro

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.

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.

Effects of muscle cooling on the stiffness of the human gastrocnemius muscle in vivo

Background/Aims: The effects of muscle cooling on the stiffness of the human gastrocnemius muscle (GAS) were examined in vivo. Methods: The knee joint was passively extended from 90 to 0° (0° = full knee extended position) with a constant ankle angle of 10° dorsiflexed position (0° = the sole of the foot is approximately perpendicular to the anterior margin of the shaft of the tibia) in a control condition (room temperature of 18–23°C) and a cooling condition (muscle temperature decreased by 5.8 ± 1.7°C after cooling using a cold water bath at a temperature of 5–8°C for 60 min). The change in passive Achilles tendon force, muscle fascicle length of GAS and muscle temperature were measured (n = 6) during the motion. Results and Conclusion: GAS stiffness was significantly greater in the cooling condition (20 ± 8 N/mm) than the control condition (18 ± 8 N/mm). There was no cooling effect on the muscle slack length, beyond which passive muscle force arises. The maximum passive Achilles tendon force significantly increased by 19 ± 20% after cooling. These results suggested that cooling increased the passive muscle force due to the increase in the muscle stiffness rather than the shift of the muscle slack length.

Influence of muscle cooling on the passive mechanical properties of the human gastrocnemius muscle

The purpose of the present study was to examine the influence of muscle cooling on the passive mechanical properties of the human gastrocnemius muscle (GAS) in vivo. In a thermoneutral (a room temperature of 18-23degC) and a local cooling (placing the right lower leg into cold water with a temperature of 5-8degC for 60 min) conditions, the change in passive plantarflexion force (F), which is produced only by the GAS length change, was taken in five subjects during passive knee extension from 90deg to 0deg with a constant ankle joint angle of 10deg dorsiflexion. To evaluate an elastic component of the passive plantarflexion force of GAS, the subject held full knee extended position for 1 min (i.e. relaxation period). Skin and muscle temperature of GAS were also measured using a core temperature thermistor. The peak value of F (Fve) that was measured at the end of the knee extension phase, the decrease of F (DeltaF) during the relaxation period, and the F at the end of the relaxation period (Fe) were measured in the two conditions. Muscle cooling decreased the skin and muscle temperature by 6.7 plusmn 1.1degC and 8.1 plusmn 2.5degC, respectively. Fve increased by 24% plusmn 22% by muscle cooling. DeltaF in the thermoneutral and local cooling conditions were 11.5 plusmn 4.9 N and 12.5 plusmn 2.9 N, respectively. Fe increased by 28% plusmn 21% by muscle cooling. These results suggested that muscle cooling increased an elastic component of the passive force arisen from GAS and seemed not to affect its viscous component

HUMAN HOPPING WITHOUT VISION: COMPENSATORY STRATEGY BY STIFFNESS REGULATION

Bouncing gait (hopping, running, trotting) is the typical locomotion used by a number of species including humans. In such movements, the leg stiffness is adjusted to offset the changes in external environments [4]. Leg stiffness depends on a combination of individual joint stiffness [4], which is regulated by muscle activation [5]. Thus, we hypothesized that stiffness regulation could compensate for the lack of visual input during bouncing gait.