Person:

Lieberman, Daniel

Although the earliest known hominins were apparently upright bipeds, there has been mixed evidence whether particular species of hominins including those in the genus Australopithecus walked with relatively extended hips, knees and ankles like modern humans, or with more flexed lower limb joints like apes when bipedal. Here we demonstrate in chimpanzees and humans a highly predictable and sensitive relationship between the orientation of the ankle joint during loading and the principal orientation of trabecular bone struts in the distal tibia that function to withstand compressive forces within the joint. Analyses of the orientation of these struts using microCT scans in a sample of fossil tibiae from the site of Sterkfontein, of which two are assigned to Australopithecus africanus, indicate that these hominins primarily loaded their ankles in a relatively extended posture like modern humans and unlike chimpanzees. In other respects, however, trabecular properties in Au africanus are distinctive, with values that mostly fall between those of chimpanzees and humans. These results indicate that Au. africanus, like Homo, walked with an efficient, extended lower limb.

The human gluteus maximus is a distinctive muscle in terms of size, anatomy and function compared to apes and other non-human primates. Here we employ electromyographic and kinematic analyses of human subjects to test the hypothesis that the human gluteus maximus plays a more important role in running than walking. The results indicate that the gluteus maximus is mostly quiescent with low levels of activity during level and uphill walking, but increases substantially in activity and alters its timing with respect to speed during running. The major functions of the gluteus maximus during running are to control flexion of the trunk on the stanceside and to decelerate the swing leg; contractions of the stance-side gluteus maximus may also help to control flexion of the hip and to extend the thigh. Evidence for when the gluteus maximus became enlarged in human evolution is equivocal, but the muscle’s minimal functional role during walking supports the hypothesis that enlargement of the gluteus maximus was likely important in the evolution of hominid running capabilities.

Humans habitually swing their arms in phase with the contralateral leg during walking and running. This arm motion is generally thought to counteract the torque about the body’s vertical axis (i.e. yaw moment) that is generated by the legs as they swing with each step. Thus it has been argued that the motion of the arms is a tuned, habitual, active response that is critical for maintaining stability during human locomotion, especially running. In this study, we investigated whether arm swing is in fact an active behavior, or is instead a passive response that follows solely as a consequence of our anatomical design. Human subjects walked and ran on a treadmill under different arm- and leg-weighting conditions, and without armswing, while kinematic and surface EMG data were recorded. A modeling study was also performed to determine the inherent effect of leg swing on arm movement in a human-like biped. Results of both studies suggest that arm swing is largely a passive response, and is not entirely an active, tuned behavior. Arm swing may therefore be an emergent property of human bipedalism, with the arms acting largely as passive damping mechanisms that decrease whole-body yawing.

We investigated the control and function of arm swing in human walking and running to test the hypothesis that the arms act as passive mass dampers powered by movement of the lower body, rather than being actively driven by the shoulder muscles. We measured locomotor cost, deltoid muscle activity and kinematics in 10 healthy adult subjects while walking and running on a treadmill in three experimental conditions: control; no arms (arms folded across the chest); and arm weights (weights worn at the elbow). Decreasing and increasing the moment of inertia of the upper body in no arms and arm weights conditions, respectively, had corresponding effects on head yaw and on the phase differences between shoulder and pelvis rotation, consistent with the view of arms as mass dampers. Angular acceleration of the shoulders and arm increased with torsion of the trunk and shoulder, respectively, but angular acceleration of the shoulders was not inversely related to angular acceleration of the pelvis or arm. Restricting arm swing in no arms trials had no effect on locomotor cost. Anterior and posterior portions of the deltoid contracted simultaneously rather than firing alternately to drive the arm. These results support a passive arm swing hypothesis for upper body movement during human walking and running, in which the trunk and shoulders act primarily as elastic linkages between the pelvis, shoulder girdle and arms, the arms act as passive mass dampers which reduce torso and head rotation, and upper body movement is primarily powered by lower body movement.

Although bipedal locomotion is a hominin synapomorphy, disagreements persist about whether early hominin bipeds were capable of fully extended limb posture, or used a bent-knee, bent-hip gait. Several recent studies have used the orientation of trabecular bone in limb joints to infer posteral differences during bipedal locomotion between early bipeds and later Homo. There analyses depend on the assumption that the orientation of the trabeculae in joint corresponds to the orientation of compressive forces that are transmitted through the joints. However, the hypothesis that trabecular struts will differ in orientation because of differences in the orientation of loads they experience during growth have not been tested. This study experimentally tests the hypothesis that there is a quantifiable relationship between the orientations of trabeculae and joint posture. The experiment included 16 guinea fowl (Numida melegris): 6 extended-posture runners, 6 flexed-posture runners, and 4 sedentary controls. The exercised animals ran 6 days per week at 1.9 mph for 15 minutes, on either a flat treadmill or a treadmill inclined to 20°. Kinematic and ground reaction force data collected as the birds moved on horizontal and inclined substrates confirm that the degree of flexion at the knee at toe-off is 10° greater when moving up inclines relative to level running. Micro-CT scans were analyzed using image analysis software to relate this difference to trabecular and subchondral bone morphology within the distal femoral epiphysis, including subchondral bone thickness, and trabecular orientation, number, thickness, volume, and connectivity.

While recent attempts have been made to link trabecular orientation to gait and posture in extinct hominids, there have been few controlled experiments to test the hypothesis that there is a predictable functional relationship between the orientation of trabecular struts within a point and the orientation of loads applied to the joint. We tested the hypothesis (often termed Wolff’s Law) by comparing the strut orientation in the spongiosa of the distal femur in two groups of immature guinea fowl that were exercised on treadmills (10 min/day, for 60 days) at 0º versus 20º inclines. Kinematic and force plate analysis found that, a peak ground reaction force, the posture of the knee in the parasagittal plane was approximately 16º more flexed in the 20º incline group. Strut orientation was analyzed from micro-CT scans of the joints using a radon transform analysis that quantifies the orientation of peak trabecular density (OPTD). As predicted by Wolff’s Law, the OPTD was approximately 18º more flexed relative to the long axis of the femur in the incline versus horizontal birds (p<0.05). These results demonstrate a predictable relationship between the orientations of trabecular struts and compressive loads applied to a joint during growth, supporting Wolff’s Law. This technique may be useful for determining gait and posture in fossil species such as Australopithecus afarensis.

The shape of the human female pelvis is thought to reflect an evolutionary trade-off between two competing demands: a pelvis wide enough to permit the birth of large-brained infants, and narrow enough for efficient bipedal locomotion. This trade-off, known as the obstetrical dilemma, is invoked to explain the relative difficulty of human childbirth and differences in locomotor performance between men and women. The basis for the obstetrical dilemma is a standard static biomechanical model that predicts wider pelves in females increase the metabolic cost of locomotion by decreasing the effective mechanical advantage of the hip abductor muscles for pelvic stabilization during the single-leg support phase of walking and running, requiring these muscles to produce more force. Here we experimentally test this model against a more accurate dynamic model of hip abductor mechanics in men and women. The results show that pelvic width does not predict hip abductor mechanics or locomotor cost in either women or men, and that women and men are equally efficient at both walking and running. Since a wider birth canal does not increase a woman’s locomotor cost, and because selection for successful birthing must be strong, other factors affecting maternal pelvic and fetal size should be investigated in order to help explain the prevalence of birth complications caused by a neonate too large to fit through the birth canal.

One of the most distinctive features of humans relative to other apes is a greatly expanded gluteus maximus. We examined the role of this muscle in walking and running humans to test the hypothesis that the derived expansion of the gluteus maximus may be related to various musculoskeletal specifications for endurance running. During a walk, the trunk is relatively vertical, positioning the upper body’s center of gravity over the hip joint: during a run, the trunk is more forwardly inclined, with the upper body’s center of gravity well in front of the hip joint. This inclination causes the trunk to have an inertial tendency to pitch forward at foot strike. Although the gluteus is well know to be a hip extensor, its contraction will also counteract pitching of the trunk when the leg is on the ground. The hypothesis was tested using EMG and kinematic analysis of human subjects during walking and running under various conditions. The results indicate that the gluteus maximus contracts bilaterally at foot strike during running but not walking. On the stance side, the gluteus maximus functions to stabilize the trunk against its inertial tendency to pitch at foot strike. On the swing side, the gluteus maximus may contract to help decelerate the leg prior to foot strike. Presence of an enlarged surface of attachment for this muscle in Homo erectus suggests that the expansion of this muscle may have played an influential role in early human endurance running capabilities.