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Modifications
for upright posture and human musculature. Muscles of the
lower limb
The
major muscular changes directly associated with the shift
to bipedal locomotion are seen inthe lower limb. The obvious
skeletal changes are in the length of the hind limb, the
developmentof the heel, and the change in the shape of the
knee joint so that its surface is flat and not evenly rounded.
The hind limbs of modern humans are proportional to body
size, while the hind limbs of the apes are relatively short
for their body size. The changes that occur in the bones
of the pelvis are not all directly related to the shift
in locomotion, but they are a consequence of it. Bipedality,
by freeing the hands from primary involvement with support
and locomotion, enabled the development of manual dexterity,
and thus the manufacture and use of tools, whichhas been
linked to the development in human ancestors of language
and other intellectual capacities. The result is a substantially
enlarged brain. Large brains clearly affect the form of
the skull, and thus the musculature of the head and neck.
A larger brain also has a direct effect on the pelvis because
of the need for a wide pelvic inlet and outlet for the birth
of relatively large-brained young. The larger pelvic cavity
means that the hip joints have to be farther apart. Consequently,
the hip joints are subjected to considerable forces when
weight is taken on one leg, as it has to be in walking and
running.
To
counteract this, the muscles (gluteus minimus and gluteus
medius) that are used by the chimpanzee to push the leg
back (hip extensors) have shifted in modern humans in relation
to the hip joint so that they now act as abductors to balance
the trunk on the weight-bearing leg during walking. Part
of a third climbing muscle (gluteus maximus) also assists
in abduction as well as in maintaining the knee in extension
during weight bearing. The gluteal muscles are also responsible
for much of the rotation of the hip that has to accompany
walking. When the right leg is swung forward and the right
foot touches the ground, the hip joint of the same side
externally rotates whereas that of the opposite side undergoes
a similar amount of internal rotation. Both these movements
are made possible by rearrangements of the muscles crossing
the hip.
The
bones of the trunk and the lower limb are so arranged in
modern humans that to stand upright requires a minimum of
muscle activity. Some muscles, however, are essential to
maintaining balance, and the extensors of the knee have
been rearranged and realigned, as have the muscles of the
calf.
The
foot is often, but erroneously, considered to be a poor
relation of the hand. Although the toes in modern humans
are normally incapable of useful independent movement, the
flexor muscles of the big toe (hallux) are developed to
provide the final push-off in the walking cycle. Muscles
of all three compartments of the modern human lower leg
contribute to making the foot a stable platform, which nonetheless
can adapt to walking over rough and sloping ground.
