Relative phasing

A related measure that is being used with increasing frequency by dynamical theorists to examine how limb segments are coordinated during the performance and learning of different movement skills is relative phasing. This measure is derived through a mathematical process that begins with the transformation of the phase portrait trajectories from x, y coordinates segments during a to what are referred to as polar coordinates comprised of a radius r and angIe 0. Although beyond the scope of this chapter, the reader can learn more about how to calculate this particular measure in Stergiou (2004), which describes a number of innovative methods currently being used to analyze the dynamics of human motion.

According to Kurz and Stergiou (2004), what is unique about the relative phasing measure is that it “compresses four variables (i.e., proximal and distal segments’ displacement and velocities) into one measure” (p. 99). Depending on the relative phase values obtained, a researcher can determine whether two moving segments either are in-phase, or moving in the same direction (have a relative phase value of zero degrees), or are out-of-phase, or in opposite directions (have a relative phase value of 180 degrees). Whether the relative phase value is positive or negative also tells us which limb segment being studied (distal versus proximal) is ahead of the other during performance of the movement. Relative phase analysis has been used in a number of research studies exploring the coordination and control of the neuromuscular system in both the performance and learning of many different types of movements (Barela, Whitail, Black, & Clark, 2000; IKo, Chaliis, & Newell, 2003; Lee, Swinnen, & Verschueren, 1995; van Emmerik & Wagenaar, 1996).

New measurement tools and statistical methods for exploring the dynamics of human motion continue to emerge in the motor control literature, providing a very different view of neuromuscular system function. Although only A—A diagrams, phase portraits, and relative phasing measures were discussed in this chapter, many other measurement tools and variations of those already discussed are currently being used to address important research questions in the area of motor control and learning. These nontraditional methods of analysis have been particularly useful for studying the physical dynamics of the human system from a nonlinear perspective as advocated by theorists aligned with both the dynamical and ecological approaches to motor control. Readers are encouraged to review Stergiou (2004) for a more in-depth discussion of a broad range of relatively new and innovative quantitative methods and analyses being applied to the study of human motor control.

Relative phase-angle relationship between the thigh and shank segments of the leg during the stance phase of gait. The relative phase is represented by the dashed line, and the bold lines represent the standard deviation about the mean ensemble curve. Positive slop values indicate that the shank is leading the thigh segment, whereas negative slope values indicate the thigh segment to be ahead of the trunk.