Changes in coordination and variability during running as a function of head stability demands.

The purpose of this study was to identify changes in segment/joint coordination and coordination variability in running with increasing head stability requirements. Fifteen strides from twelve recreational runners while running on a treadmill at their preferred speed were collected. Head stability demands were manipulated through real-time visual feedback that required head-gaze orientation to be contained within boxes of different sizes, ranging from 21 to 3 degrees of visual angle in 3-degree decrements. Coordination patterns and coordination variability were assessed between head and trunk segments, hip and knee joints, and knee and ankle joints in the three cardinal planes, respectively. Mean coupling angles and the standard deviation of the coupling angles at each individual point of the stance phase were calculated using a modified vector coding technique and circular statistics. As head stability demands increased, transverse plane head-trunk coordination was more anti-phase and showed increased head‑leading and decreased trunk‑leading patterns; for the lower extremity, there was increased in-phase and decreased anti-phase sagittal plane coordination. Increased head stability demands also resulted in an increase in coordination variability for both upper body and lower extremity couplings during the second half of the stance phase. Overall, the results provide evidence that coordinative adaptations to increasing head stability demands occur throughout the entire body: 1) through more independent control of the head relative to the trunk; 2) by increasing in-phase coordination between lower extremity joints, and 3) through increased coordination variability in the second half of stance in both upper body segmental and lower extremity joint couplings. These adaptations likely contribute to the reduction of the range of motion of the trunk in vertical direction.