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Station 4: The Foot
Station 4: The Foot
The foot consists of three types of bones: the tarsals (heel and ankle), the metatarsals (midfoot), and the phalanges (toes). Since bipedalism uses one leg at a time for propulsion, the weight of the entire body passes through each foot during locomotion. Therefore, bipedal adaptations to the foot reflect the increased demands of weight-bearing and balance, in addition to how weight is transferred along the foot during locomotion.
The feet of quadrupeds are adapted to more flat-footed walking with weight bearing and transmission equal among feet and hands.
Movement
Movement
When comparing the foot anatomy of quadrupedal apes and bipedal hominins, there are many differences because of different ways in which weight is transmitted along the feet during movement. Foot movement in both quadrupeds and bipeds includes three things: where the foot hits the ground, how and where the weight transmits along the bottom of the foot, and where the push-off propulsion comes from on the foot.
In quadrupeds the foot is placed flat on the substrate, the weight rolls forward along the edge of the foot, and the push-off is from the center of the toes. In a bipedal stride, the heel strikes first, and body weight is transmitted along the outside of the foot, then internally across the ball of the foot, and finally push-off is made by the big toe (“toe off”). Foot morphology reflects adaptations to the differential demands of movement and stability in locomotion.
Foot morphology
Foot morphology
As the first foot bone to contact the ground during bipedal locomotion, the calcaneus (heel bone) is robust to provide stability and absorb the high forces encountered during heel strike. In comparison, quadrupeds have a more gracile calcaneus since the entire foot hits the ground flat during locomotion, and weight bearing is equally transmitted among all four limbs.
Although all quadrupeds have transverse arches in their feet, bipeds have an additional longitudinal arch. These arches act as shock-absorbers to accommodate weight bearing during locomotion.
In bipedal locomotion, the ‘toe off’ is the main propulsive force. This is reflected in adducted (non-opposable) hallux (big toe) that is much larger than the other toes to assist in propulsion and increase stability in the foot. In contrast, quadrupeds have abducted (opposable) big toes which allow more flexibility in the foot to grasp onto trees during locomotion.
Long lateral toes (toes other than the hallux) would be too energetically costly for bipedal locomotion, so the phalanges are adapted to be short and straight. This also helps with stability and weight bearing demands on the feet. In contrast, quadrupeds have long, curved, phalanges to assist with grasping branches during arboreal locomotion.
Comparative Anatomy
Comparative Anatomy
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