ANTHROPOMETRY - Biomechanics and Motor Control of Human Movement - page 103

of the knee, the rectus femoris is a combined hip flexor and knee extensor,
and the gastrocnemius are knee flexors and ankle plantarflexors. The fiber
length of many of these muscles may be insufficient to allow a complete
range of movement of both joints involved. Elftman (1966) has suggested
that many normal movements require lengthening at one joint simultaneously
with shortening at the other. Consider the action of the rectus femoris, for
example, during early swing in running. This muscle shortens as a result of hip
flexion and lengthens at the knee as the leg swings backward in preparation
for swing. The tension in the rectus femoris simultaneously creates a flexor
hip moment (positive work) and an extensor knee moment to decelerate the
swinging leg (negative work) and start accelerating it forward. In this way,
the net change in muscle length is reduced compared with two equivalent
single-joint muscles, and excessive positive and negative work within the
muscle can be reduced. A double-joint muscle could even be totally isometric
in such situations and would effectively be transferring energy from the leg
to the pelvis in the example just described. In running during the critical
push-off phase, when the plantarflexors are generating energy at a high rate,
the knee is continuing to extend. Thus, the gastrocnemii may be essentially
isometric (they may appear to be shortening at the distal end and lengthening
at the proximal end). Similarly, toward the end of swing in running, the knee
is rapidly extending while the hip has reached full flexion and is beginning
to reverse (i.e., it has an extensor velocity). Thus, the hamstrings appear to
be rapidly lengthening at the distal end and shortening at the proximal end,
with the net result that they may be lengthening at a slower rate than a single
joint would.
It is also critical to understand the role of the major biarticulate muscles
of the lower limb during stance phase of walking or running. Figure 4.10
shows the gastrocnemii, hamstrings, and rectus femoris and their moment-arm
lengths at their respective proximal and distal ends. The hamstrings have a
5-cm moment-arm at the ankle and 3.5-cm moment-arm at the knee. Thus,
when they are active during stance, their contribution to the ankle exten-
sor moment is about 50% greater than their contribution to the knee flexor
moment. The net effect of these two contributions is to cause the leg to
rotate posteriorly and prevent the knee from collapsing. The hamstrings, with
the exception of the short head of the biceps femoris, have moment-arms of
6 - 7 cm at the hip but only 3.5 cm at the knee. Thus, when these muscles are
active during stance, their contribution to hip extension is about twice their
contribution to knee flexion. The net effect of these two actions is to cause the
thigh to rotate posteriorly and prevent the knee from collapsing. Finally, the
rectus femoris is the only biarticulate muscle of the large quadriceps group,
and its moment-arm at the hip is slightly larger than at the knee. However,
the quadriceps activate as a group, and because the uniarticulate quadriceps
comprise 84% of the PCA of the quadriceps (see Table 4.3), the dominant
action is knee extension. Thus, the net effect of the major biarticulate mus-
cles of the lower limb is extension at all three joints, and therefore they