The Coefficient of Propulsion: A Composite Measure of Trail Leg Force Production During Baseball Pitching

Propulsive trail leg ground reaction forces play an important role in transferring energy to the pelvis during baseball pitching. Traditionally, trail leg ground reaction forces have been reported as individual force components or resultants. However, propulsion depends on the coordinated contribution of both vertical and anterior shear forces. The coefficient of propulsion (CoPr), defined as the ratio of anterior shear to vertical force, provides a composite measure of trail leg force production during the stride phase. In this technical note, we compared how well of peak vertical force, peak anterior shear force, peak 2-D resultant force, and peak CoPr explained variance in pelvis kinetic energy (PKE) at stride foot contact. Thirty-seven collegiate pitchers threw fastballs from an instrumented mound. PKE was the sum of translational and rotational kinetic energies at stride foot contact. Linear regressions described variance explained in PKE by peak vertical, peak anterior shear and peak 2-D resultant forces, and peak CoPr. Peak CoPr explained greater variance in PKE (R2 = .32, P < .001) than peak anterior shear (R2 = .20, P = .005), peak vertical (R2 = .00, P = .920, or peak 2-D resultant forces (R2 = .01, P = .517). These findings suggest that considering the relationship between vertical and anterior shear forces may provide additional insight into trail leg propulsion during the stride phase of pitching.