In this paper, we are presenting an alternative approach to the investigation of lower extremity coupling referred to as a dynamical systems approach. In this approach, we calculate the phase angle of each segment and joint angle. Pairing the key segment/joint motions, we use phase angles to determine the continuous relative phase and the variability of the continuous relative phase. Data from two studies illustrate the efficacy of the dynamical systems approach. Individuals who were asymptomatic, even though they may have anatomical aberrant structural problems (i.e. high Q-angle vs low Q-angle) showed no differences in the pattern of the continuous relative phase or in the variability of the continuous phase. However, differences in the variability of the continuous relative phase were apparent in comparing individuals who were symptomatic with patellofemoral pain with non-injured individuals. Patellofemoral pain individuals showed less variability in the continuous relative phase of the lower extremity couplings than did the healthy subjects. We hypothesize that the lower variability of the couplings in the symptomatic individuals indicates repeatable joint actions within a very narrow range.

Relevance:​ We claim that the traditional view of the variability of disordered movement is not tenable and suggest that there is a functional role for variability in lower extremity segment coupling during locomotion. While the methods described in this paper cannot determine a cause of the injury, they may be useful in the detection and treatment of running injuries.