As stride length decreased, the runners bounced less, landed closer to their center of mass, and produced lower braking forces against the ground.
A shorter stride allowed the knee to do less work absorbing energy. A 10 percent higher-cadence stride allowed the hips also to do less work.
A shorter stride allowed the knee to bend less during stance, and decreased many hip motions. -peakperformance.runnersworld.com
Primary investigator for this study was Bryan Heidersheit, PT, Ph.D. of the University of Wisconsin's Department of Orthopedics & Rehabilitation and Biomedical Engineering. Effects of Step Rate Manipulation on Joint Mechanics during Running
According to Heidersheit, "If they increase their step rate quite subtly–by about 5 to 10 percent, which reduces their stride length by the same–then they can substantially reduce the forces to their knees. That happens primarily because they're lowering the vertical displacement of their center of gravity [their "bounce"], and hence coming down more gently. They also reduce the distance between their foot landing and their center of gravity. This reduces braking forces. The forces absorbed at the hips are also lower with shorter strides.