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Here are the Sagittal plane Joint moments in Normal Walking from right heel-strike to the next right heel-strike. Positive moments are flexor (i.e. the flexor muscle is active) while negative moments are extensor (extensor muscle active). The vertical line indicates Toe-off, at which point the movie has been paused. There's a QuickTime video here if you prefer.
If these moments are plotted on a 3D graph, where the hip moment is represented on the vertical (magenta) axis, the knee on the cyan axis, and the ankle on the yellow axis, the following shape results:
Notice that whilst the stance phase (red balls) has no obvious shape to it, the swing phase moments (green balls) are lined up in a straight line, suggesting that they are linearly correlated. Actually, of course, the ankle moment is zero in swing, so the correlation is really between knee and hip joints, but a 2D graph is boring to look at! It does make the linear correlation easier to see, though, especially if the sign of the hip moment is reversed:
Notice that the linearity begins shortly after toe-off and continues throughout swing until heel-strike.
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