Influence of Step Rate on Running Mechanics

Introduction

The number of runners in the United States has grown exponentially over the last several decades, now with over 35 million people running for exercise or sport. With the rise in the number of individuals running both recreationally and competitively, a corresponding rise in running-related injuries has occurred as well. Many running-related injuries are suggested to result from biomechanical faults in an individual's gait pattern. Patellofemoral pain, for example, may result from increased hip adduction during stance phase, while tibial stress injuries have been suggested to result from excessive impact peaks and loading rates.

With increasing utilization in recent years, gait retraining has become a common approach to address biomechanical faults with the goal of reducing injury risk and facilitating recovery from injury. Common targets of gait retraining include footstrike pattern, impact loading, proximal mechanics, and lower limb stiffness. Many of the retraining methods targeting these mechanics require multiple visits over several weeks during which the runner receives visual and audio feedback to facilitate the desired changes.

Perhaps one of the more straightforward methods of gait retraining is modification of step rate, also known as step frequency modification. Step rate modification involves determining a runner's preferred step rate (i.e., the number of steps per minute) at a particular speed and instructing the runner to increase their step rate by a certain percentage of their preferred. When running speed is held constant, small increases in step rate (5%–10%) can result in notable biomechanical and neuromuscular changes that can reduce injury risk and aid in recovery among injured individuals. This chapter will detail the biomechanical, neuromuscular, and metabolic changes resulting from step rate modification, as well as provide recommendations for successful implementation into clinical practice.

Before detailing the variety of changes expected from step rate modification, it is important to note that changes following step rate modification are primarily reported, both in the literature and in this chapter, relative to the “preferred” (self-selected) step rate condition. This is essential for summarizing findings not only across all runners within a study but also across studies. With information reported relative to preferred, the changes described in this chapter can be generalized to a wide variety of running populations.

Biomechanical Changes

The most obvious biomechanical change observed when step rate is increased and speed is held constant is a decrease in stride length. This change occurs because running speed is a function of step rate and stride length. As step rate is increased, stride length must decrease proportionally to maintain a constant speed. If speed is not held constant, an increase in step rate will simply result in an increase in speed and not produce the desired biomechanical changes. Thus, it is imperative that speed be held constant when step rate modification is performed. As a result of an increase in step rate and a simultaneous decrease in stride length, there are a number of corresponding changes in a runner's gait mechanics including changes in ground reaction forces (GRFs) and lower limb kinematics and kinetics.

Changes in Kinematics

Step rate modification can elicit numerous changes in lower limb biomechanics throughout stance and swing phase. While changes at the knee are most prominent with step rate modification, changes across the hip, knee, and ankle in both the sagittal and frontal planes are also observed.

Initial Contact

Initial contact occurs at the instant the foot strikes the ground. As step rate is increased, the runner's leg position at initial contact changes to accommodate the shorter stride length. Specifically, the horizontal distance between the body's center of mass (COM) and heel position at initial contact decreases. This is an important benefit as it can correct a familiar running fault commonly referred to as “overstriding,” i.e., landing with the foot too far in front of the hip and knee. This improved position at initial contact (reduced overstriding) is achieved by the tibia assuming a more vertical position and an increased knee flexion angle. This more flexed landing posture enables impact forces to be dissipated more easily, which could facilitate recovery from injury and reduce future injury risk.

Change in foot orientation at initial contact is less predictable with step rate modification. On average, the foot inclination angle (i.e., the sagittal plane angle between the foot and the ground) is reduced as step rate increases. However, this effect is primarily observed among rearfoot strike runners, resulting in a less steep orientation of the foot relative to horizontal at initial contact ( Fig. 8.1 ).

Fig. 8.1, Primary kinematic characteristics at preferred, +10% (blue), and −10% (red) step rates. Relative to the preferred step rate condition, vertical center of mass (COM) displacement, sagittal plane COM to heel distance, step length, and foot inclination angle are all reduced with a 10% increase in step rate relative to preferred. Correspondingly, all characteristics increase when step rate decreases by 10% of preferred.

Midstance

The peak knee flexion angle during the stance phase of running occurs at midstance, the moment when the body's COM is directly over the center of pressure. As step rate is increased, peak knee flexion decreases. This is due, in part, to less time being spent on the ground, often described as decreased contact time or stance phase duration. Likewise, because peak knee flexion angle is reduced, the vertical position of the body's COM does not drop as low. These changes suggest that overall lower limb stiffness increases with step rate, which may be beneficial for both performance and injury reduction.

When considering frontal plane mechanics, increasing step rate above one's preferred primarily results in changes in two variables: peak hip adduction and contralateral pelvic drop, or downward movement of the contralateral innominate relative to the stable innominate of the stance limb. Peak hip adduction, which typically occurs near midstance, is reduced with increased step rate. Similarly, pelvic drop decreases as step rate increases. Another frontal plane variable associated with a variety of running-related injuries is base of gait, which describes the mediolateral distance between a runner's heel and COM. Base of gait is commonly termed “crossover” when the heel lands contralateral to a runner's COM position in the frontal plane. While the effect of increased step rate on base of gait and crossover has not yet been studied, the reductions in hip adduction and pelvic drop noted at increased step rates suggest that base of gait may increase (the heel moving ipsilateral and farther away from the COM) as step rate increases. As all of these variables (hip adduction, pelvic drop, and base of gait) have been associated with common running-related injuries such as patellofemoral pain, iliotibial band strain, and tibial stress injuries, increasing step rate holds promise in preventing and treating these conditions.

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