Here's a comment from reader Philly Guy on my posting about whether there's a g factor of athleticism:
I don’t think “athleticism” is just a byword for black. What is meant by athleticism (as opposed to athletic ability, which is a much broader and more nebulous concept) is the ability to move one’s body quickly (in the sense of acceleration, change of direction, and top speed) and powerfully. This is largely a function biomechanics and musculo-tendon physiology and chemistry. Differences in physique and physiology clearly give blacks an advantage over whites in this respect and the word “athleticism” is applied to them much more often because they are much more likely to possess a high degree of “athleticism.”
Re physique, blacks and whites, when matched for SES variables, are virtually identical in average height, however, their physique and body proportions are much different. Blacks have shorter torsos, broader shoulders, smaller ribcages, narrower hips, and longer limbs (with comparative elongation of the distal segments). Fit samples of blacks and whites appear to be about the same weight on average (or perhaps blacks are marginally heavier), but fit blacks have on average less body fat, particularly on their limbs. It follows that blacks have a greater fat free body mass (i.e., bone, muscle and viscera) than whites. The smaller ribcage and narrower pelvis would indicate less viscera weight and indeed, blacks have been found to have substantially more total body musculature and denser, heavier bones than whites. This greater musculature is also distributed differently. Blacks show considerably more musculature in their glutes and thighs than whites, which gives a substantial advantage in locomotion. Blacks have also been shown to have on average greater cross sectional thickness in their limb musculature than whites, with the difference much greater in the proximal than distal limb segments. Naturally higher average testosterone levels and a greater number of testosterone receptors probably contribute to the leaner more heavily muscled physique of blacks compared to whites.
The average physiological differences give blacks a substantial advantage in athleticism. Narrower hips mean that locomotive force is applied closer to the center of the body’s mass, leading to greater biomechanical efficiency by putting a greater portion of the force generated into forward locomotion and causing less to be wasted in rotational force on the body (not to mention the smaller adjustments that have to be constantly made while running or walking to move the center of gravity towards the leg in contact with the ground). Longer limbs provide greater reach, stride length (both forward and lateral), and longer acceleration paths, both for imparting velocity to thrown objects and for accelerating one’s own body for jumping. The problem with long limbs is that they move the center of mass farther from the pivot point (i.e., the joint), causing greater torque and requiring more force to accelerate the limb or change its direction. However, here again black physique provides them with the best of both worlds. The heavy musculature concentrated in the hips and in the proximal segments keeps most of the mass close to the pivot point and provides a lot of force, and the relatively long, thin distal segments don’t create that much torque while still giving the benefits of long reach, stride and acceleration paths. Blacks lower average body fat and viscera weight from comparatively short torsos with small ribcages provide a greater power to weight ratio and less top-heaviness, with a greater portion of their mass being located in the musculature of the hips and thighs, which provide locomotive power.
Muscle and tendon physiology also contribute to black “athleticism.” It is well publicized that blacks have a higher percentage of fast twitch muscle fibers and higher levels of glycotic and phosphogenic enzyme activity levels, when diet and activity levels are controlled for, which would provide an advantage in explosive, short-duration muscular exertions. Additionally, blacks have comparatively long tendons and short muscle bellies compared to total muscle-tendon length compared to whites. When doing repetitive movements like running, the muscle and tendon of the opposing muscle group to contracting muscles are stretched, which stores energy like an elastic band, which is then added to the backstroke as the opposing muscles contract back. (Try taking an appendage such as a finger and stretching it back and notice how it snaps back to its resting position when you release it to see this stored energy effect.) Tendons are more elastic than muscles and contribute more to this effect. Blacks’ comparatively longer tendons would give them comparatively greater energy storage and retransfer while running, causing them to spend less energy and be more efficient. It is also established that blacks have greater muscle elasticity as well as proportionally longer tendons (their tendon elasticity is about the same as whites).