E6 Center of Mass and Body Segments

Topic

The human body is generally modeled as a system of rigid segments articulated with one another. The system's center of mass—a key element of this model—is the point where the resultant gravitational force acts on the entire body; consequently, its trajectory determines the athlete's overall movement.

During running, however, this center of mass follows an undulating vertical path with each stride; this vertical oscillation serves as an indicator of the runner's locomotor efficiency. A wider vertical oscillation of the center of mass during each stride results in greater gravitational work being performed on that center of mass, as defined by the model.

The moment of inertia of each of the athlete's body segments—calculated relative to both its individual center of mass and the athlete's body axes—determines the segment's resistance to rotation. Steiner's theorem allows for the calculation of this moment of inertia relative to any axis other than the one passing through the segment's individual center of mass.

In rotational movements—such as a somersault or a jump with a twist—the conservation of the body's total angular momentum explains the acceleration of the rotation that occurs when the athlete reduces their total moment of inertia by drawing their body segments closer to the center of mass. Bringing these segments closer to the center of mass further reduces the athlete's moment of inertia, thereby accelerating the rotation. Consequently, this model—based on the human body as a system of articulated rigid segments—allows a standard segmental model (such as De Leva’s or Winter’s) to assign a specific mass and center-of-mass position to each of the athlete's body segments as a fraction of their total body weight; this enables the calculation of both the overall center of mass and the final moment of inertia for the entire body by aggregating these individual segments—the very same segmental composition established by another model within this set based on segmental kinematics.

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