Effects of walking velocity on vertical head and body movements during locomotion

4Trunk and head movements were characterized over a wide range of walking s peeds to determine the relationship between stride length, stepping frequen cy, vertical head translation, pitch rotation of the head, and pitch trunk rotation as a function of gait velocity. Subjects (26-44 years old) walked on a linear treadmill at velocities of 0.6-2.2 m/s. The head and trunk were modeled as rigid bodies, and rotation and translation were determined usin g a video-based motion analysis system. At walking speeds up to 1.2 m/s the re was little head pitch movement in space, and the head pitch relative to the trunk was compensatory for trunk pitch. As walking velocity increased, trunk pitch remained approximately invariant, but a significant head transl ation developed. This head translation induced compensatory head pitch in s pace, which tended to point the head at a fixed point in front of the subje ct that remained approximately invariant with regard to walking speed. The predominant frequency of head translation and rotation was restricted to a narrow range from 1.4 Hz at 0.6 m/s to 2.5 Hz at 2.2 m/s. Within the range of 0.8-1.8 m/s, subjects tended to increase their stride length rather than step frequency to walk faster, maintaining the predominant frequency of he ad movement at close to 2.0 Hz. At walking speeds above 1.2 m/s, head pitch in space was highly coherent with, and compensatory for, vertical head tra nslation. In the range 1.2-1.8 m/s, the power spectrum of vertical head tra nslation was the most highly tuned, and the relationship between walking sp eed and head and trunk movements was the most linear. We define this as an optimal range of walking velocity with regard to head-trunk coordination. T he coordination of head and trunk movement was less coherent at walking vel ocities below 1.2 m/s and above 1.8 m/s. These results suggest that two mec hanisms are utilized to maintain a stable head fixation distance over the o ptimal range of walking velocities. The relative contribution of each mecha nism to head orientation depends on the frequency of head movement and cons equently on walking velocity. From consideration of the frequency character istics of the compensatory head pitch, we infer that compensatory head pitc h movements may be produced predominantly by the angular vestibulocollic re flex (aVCR) at low walking speeds and by the linear vestibulocollic reflex (lVCR) at the higher speeds.