In most normal subjects, however, there is a concentric power burst
to K1 - let's call it K0. It can be
seen in the normative data recorded by most gait laboratories:
The knee moment at this time is flexor, indicating that the source of the concentric power is the knee flexors (hamstrings). This seems rather paradoxical, since during loading response the knee is traditionally regarded as being potentially unstable without quadriceps action - hamstrings activity would seem to be totally inappropriate at this time. Notice that the ground reaction vector is briefly vertical during this time (which causes it to pass anterior to the knee and so produce an external extensor moment), before rapidly turning posterior - a phenomenon known as the heel strike transient (Whittle, 1999). Electromyography also confirms hamstring activity at this time (Winter, 1991; Hof et al, 2002).
Winter DA (1983) Energy generation and absortion at the ankle and knee during fast, natural and slow cadences. Clin. Orthop. Rel. Res. 197: 147-154.
Winter DA (1991) The biomechanics and motor control of human gait: normal, elderly and pathological. University of Waterloo Press, Ontario, Canada.
Whittle MW. (1999) Generation and attenuation of transient
impulsive forces beneath the foot: a review. Gait and Posture 10, 264-275.
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