Runners increase speed by taking longer strides and by taking them more often, and the balance between the two changes across the speed range. The reason involves a limit on how quickly a leg can be swung forward.
Speed is stride length multiplied by step rate
Running velocity is the product of how far the body travels per step and how many steps are taken per second. Any increase in speed must come from one or both.
At low and moderate speeds, most of the increase comes from stride length. Step rate rises only modestly across a wide range of easy and moderate paces.
Near maximum speed the pattern changes, because stride length reaches a limit set by leg length and by how much force can be applied to the ground. Beyond that point step rate carries the remaining increase.
Ground contact time falls as speed rises
Faster running means less time with the foot on the ground, since the body passes over the support leg more quickly. Contact time shortens steadily with velocity.
Less contact time means force must be applied more rapidly to produce the same impulse. This is why sprinting depends heavily on how quickly force can be developed rather than on maximum strength alone.
The shortening of contact time accounts for a substantial part of the increase in step rate, since the total step cycle includes both contact and flight.
Swing time has a floor
Repositioning the leg forward takes a minimum period that changes little across speeds, because it depends on limb mass and on how fast the hip flexors can act.
Measured swing times are remarkably similar between moderate running and sprinting in the same individual, which means the reduction in step time comes almost entirely from contact.
Once contact time approaches its minimum, further speed requires more force per contact rather than a faster turnover, which is the limit sprinters work against.
Leg length changes the individual pattern
Taller runners cover more ground per stride at the same step rate, so their cadence at a given pace is typically lower than a shorter runner's.
This is why a single recommended cadence figure fits some runners and not others, and why comparisons between individuals are less informative than changes within one runner. Self-selected cadence is usually close to metabolically optimal.
Cadence manipulation changes loading
Increasing step rate at a fixed speed shortens stride, which brings the foot down closer beneath the body and reduces the load at the knee per step.
The load does not disappear; it redistributes toward the ankle and is spread over more steps. Whether that helps depends on which tissue is the problem, which is a question for a clinician. The change also raises the metabolic cost slightly.
