Just because a 2-joint muscle is active, does not mean it is active at both
of the joints to the same degree- for instance, at one time, the hamstrings
may act to stabilize the knee joint while extending the hip. The quadriceps
cannot be considered a complete group in all actions, because only rectus
femoris is the 2 joint muscle. Vasti act quite differently in normal gait,
much less in more dynamic and demanding sport activities.
Jill K
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