M8 Dynamic Balance

Topic

For an athlete, this dynamic balance essentially consists of the ability to keep their center of mass—the same center of mass that defines the system's model—within their base of support while the entire system remains in motion.

In contrast, the condition required for balance in a purely static scenario dictates that the vertical projection of the center of mass must fall within the athlete's polygon of support.

During movement, however, there is a point of zero moment on the ground—specifically, the point where the resultant moment of all contact forces acting on the athlete cancels out; if this zero-moment point falls within the athlete's base of support, the movement is stable.

The inverted pendulum model captures the dynamics of the center of mass's fall and subsequent correction—a model that also characterizes the athlete's gait. This balance correction depends on both the athlete's sensory feedback (vestibular, visual, and proprioceptive) and their neuromotor reaction time—a factor that also defines the neuromuscular control aspect of the model.

Consequently, in high-performance sports such as gymnastics, surfing, or skiing, this model—with the center of mass positioned within the initial base of support—maintains dynamic balance through small-amplitude, high-frequency oscillations of the center of mass, ensuring it never completely leaves that base of support. The athlete's final effective ankle stiffness determines the initial response of the balance system—the very same effective stiffness that characterizes the model's behavior regarding articular cartilage under rapid loading. Consequently, the fact that this effective ankle stiffness is even greater results in an even faster initial response of the balance system.

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