This is the first study to comprehensively examine the relationship between gait speed and lower limb function (muscle strength and angular velocity of the knee and ankle) in older adults. The results of the study indicated two points: (1) the ankle function had a greater influence on gait speed than knee function, and (2) plantarflexion velocity was the most significant factor in determining gait speed.
The importance of ankle function can be explained by differences in the roles of each joint during gait. Knee extension mainly provides shock absorption during the loading response and controls body stability during mid-stance[22–26]. However, ankle plantarflexion provides forward propulsion during push-off in the late stance[22, 23, 27, 28], accounting for 67% of the total propulsion power during the gait. The forward propulsion is a major factor in determining gait speed. The ankle function, which provides most of the propulsion, has a greater impact on gait speed compared to knee function.
This study showed that the plantarflexion velocity is particularly crucial to gait speed. Angular velocities in various regions have a significant impact on mobility in older adults as suggested by previous studies[16–18,30−32]. Sayers et al. demonstrated that movement velocity in the leg press exercise is a stronger predictor of performance than muscle strength in lower-intensity tasks such as gait. Furthermore, Yamamoto et al. reported that knee extension velocity is more influential than knee extension strength as a determinant of gait speed in older adults. Our results that angular velocity is essential for gait speed are in line with the results of these studies.
The plantarflexion angular velocity during heel-raising was the only method that has been shown to be related to gait speed. Heel-raising is a high-load task for older adults. Therefore, the angular velocity at a high-load task is greatly affected by the force component, based on the force-velocity relationship. Several studies[16, 31] have suggested that lower limb angular velocity under low-load conditions is more strongly related to gait speed than that under high-load conditions. We hypothesized that the plantarflexion velocity under low-load conditions is more important for gait speed, and this study measured angular velocity under no-load conditions. As a result, the no-load plantarflexion velocity had a particularly strong relationship with gait speed in older adults.
This study has several limitations. First, the method of measuring muscle strength differed between knee extension and ankle plantarflexion. In the measurement of knee extension strength, the measured value was calculated as torque (Nm/kg) using an isokinetic dynamometer. On the other hand, in the measurement of ankle plantarflexion strength, the measured value was calculated as force (kgf/kg) instead of torque, using a handheld dynamometer. Differences in measurement methods may have affected the results of this study. Second, this examination included knee and ankle joints but did not evaluate the hip joint. The hip joint function might be related to gait speed and further studies are warranted to clarify this relationship.
In conclusion, the gait speed of community-dwelling older adults is significantly affected by the ankle plantarflexion function of the lower limb. Plantarflexion velocity is a critical determinant of gait speed. The results of this study suggest that it may be possible to increase gait speed by improving the ankle plantarflexion angular velocity.