Beyond the competition, inspiration, marketing, and nationalism surrounding the Olympic Games, physical science and biology are on elegant display: the science of biomechanics (how bodies are constructed) and kinematics (how bodies move). In this worldwide spectacle we see the remarkable heights of achievement the human body can attain and also the profound and sometimes subtle differences among individuals. The bulky shot-putter hurling a dead weight enormous distances and a sprite gymnast soaring gracefully above a narrow support beam illustrate the diversity of supreme abilities the human body can reach. But the biggest difference on display at these games is the difference between the bodies of men and women. Differences between genders are so numerous, we are often oblivious to the obvious. Women and men move differently.
An example of this often hidden difference in the way men and women's bodies move is provided in a study in the journal Gait and Posture by Valentina Graci and colleagues, working at the University of Maryland, Washington University, and St. Louis University. No sophisticated equipment was required to isolate this gender-specific difference in how our bodies move. The researchers simply asked men and women to stand on one leg and squat. Such a fundamental movement, the one-leg squat is the foundation of running, jumping, balance, and lifting. Could this simple maneuver differ that much between men and women?
The researchers videotaped the subjects performing a one-leg squat and analyzed the movements by computer. What they found was that men and women use different movement strategies in performing a single leg squat. The results are explained by structural differences in muscle and skeleton between men and women, and they also explain why women are at risk of greater knee injury and knee pain than men.
In squatting, women held their upper body (trunk) more upright than men, who leaned their upper body forward. By the laws of gravity, both men and women must keep their center of mass directly above the supporting foot to remain in balance. Both do so, but men and women do it differently. The shoulders and hip of women remain aligned vertically as they squat --forming a pillar of strength; men on the other hand, bend at the waste, forming a triangle of strength with their buttocks jutting out behind them, much the way a weight lifter hoists from a squatting position with massive barbells resting on his shoulders.
This difference means, however, that there must be other compensating actions in the joint movements of men and women in performing a squat on one leg. As they squat women flex their trunk toward the weight-bearing leg supporting them as they lower in balance straight down over their foot, but men do the opposite: flex their upper body toward the leg bearing no weight at all. Both actions keep the center of mass in the upper body balanced above the supporting leg, but men, with their buttocks slightly behind the supporting foot, move their trunk so as to use the upper and lower torso as a counter weight above the supporting leg.
This difference requires that females rotate their pelvis toward the weight-bearing limb while males rotate their pelvis in the opposite direction. As a consequence, the knees of men and women become torqued differently, exposing females to greater risk of ACL knee injuries. Women also perform the squat much faster than men (2.36 vs 3.18 sec).
All of this makes perfect sense from a biomechanical standpoint. Women keep a more erect posture during the squat because they have less upper body core strength musculature than a man. Bending the trunk, as men do, requires the vertical gravitational force vector to move farther away from the hip joint center, increasing the demand on the hip muscles for support, but decreasing demand on the knee muscles. Females must rely more on the quadriceps, the large thigh muscle, which puts the ACL at greater risk for injury. Less hip muscle strength in females than in males might also explain why females perform the squat faster.
The upper body of women also rotates in the same direction as their pelvis rotation, but men rotate their pelvis toward the non-weight-bearing limb while their upper body counter rotates in the other direction. This counter rotation of the trunk also happens normally in walking and running: our upper body twists slightly in the opposite direction to the twists of our pelvis to keep our head still. A steady head position while running and walking provides more stable visual and vestibular (balance) input to the brain. The squatting strategy women use does not help keep the head as fixed in space to maintain postural stability. However, this may not be as necessary when one is stationary as it is when one is moving. Nevertheless, "such a strategy can be more hazardous for the knee joint, which could expose females to a greater risk of knee injury and knee pain," the authors write.
While enjoying the Olympic games try spotting this and other kinematic differences between men and women athletes. The human body is a marvel and so are the sometimes subtle and unexpected differences between the sexes.
Graci, V., Van Dillen, L.R., and Salsich, G.B. (2012) Gender differences in trunk, pelvis and lower limb kinematics during a single leg squat. Gait and Posture 36, 461-466.
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