The effects of running in an exerted state on lower extremity kinematics and joint timing

Abstract

Runners rarely run to the point of maximum fatigue or exhaustion. However, no studies have investigated how the level of exertion associated with a typical running session influences running mechanics. The purpose of this study was to investigate the effects that running in an exerted state had on the kinematics and joint timing within the lower extremity of uninjured, recreational runners. Twenty runners performed a prolonged treadmill run at a self-selected pace that best represented each runner’s typical training run. The run ended based on heart rate or perceived exertion levels that represented a typical training run. Kinematics and joint timing between the foot, knee, and hip were analyzed at the beginning and end of the run. Increases were primarily observed at the end of the run for the peak angles, excursions, and peak velocities of eversion, tibial internal rotation, and knee internal rotation. No differences were observed for knee flexion, hip internal rotation, or any joint timing relationship. Based on these results, runners demonstrated subtle changes in kinematics in the exerted state, most notably for eversion. However, runners were able to maintain joint timing throughout the leg, which may have been a function of the knee. Thus, uninjured runners normally experience small alterations in kinematics when running with typical levels of exertion. It remains unknown how higher levels of exertion influence kinematics with joint timing and the association with running injuries, or how populations with running injuries respond to typical levels of exertion.

Introduction

All sustained physical activities subject the body to various levels of fatigue. This is especially evident when running, which is one of the most popular forms of exercise. Exercising in a fatigued state has been shown to increase stress, strain, shear, and impact forces within the lower extremity (Derrick et al., 2002, Donahue and Sharkey, 1999, Milgrom et al., 2007, Mizrahi et al., 2000c, Warden et al., 2006). In the case of running, the cyclical nature results in repetitive loading with increase intensity of these loads on the lower extremity over the course of the run. While these loads in isolation would not typically be considered as those above the physiologic threshold for injury, the accumulation of such loads can lead to overuse problems, which are often associated with running (Hohmann et al., 2004, Taunton et al., 2002). Muscles are greatly responsible for dissipating these dynamic loads on the lower extremity (Radin, 1986). This is primarily accomplished through the stretch-shortening cycle, which is a combination of eccentric and concentric muscle contractions (Komi, 2000). However, as runners become exerted and fatigue develops and progresses over the course of a run, the effectiveness of the protective neuromuscular mechanism of muscle diminishes (Radin, 1986) along with the tolerance to repeated stretch-shortening cycles (Hayes et al., 2004, Komi, 2000, Skof and Strojnik, 2006).

Fatigue can have considerable influence on lower extremity mechanics. With altered neuromuscular function, a reduction in the transfer of mechanical energy between eccentric and concentric muscle contractions can occur (Mizrahi et al., 2000a, Mizrahi et al., 2000b) along with slower muscle reaction times (Mizrahi et al., 2001). This creates problems when running as the ability to maintain desired angular displacements during the stance phase becomes compromised as runners become exerted (Komi, 2000). Thus, it is likely that changes in joint motion will occur over the course of a run. Previous studies that have utilized a running protocol that produces maximal effort and exhaustion at the end of a run have found that the end is associated with increases in: (1) rearfoot eversion and eversion velocity (Christina et al., 2001, Derrick et al., 2002, JiSeon, 2003, Van Gheluwe and Madsen, 1997), (2) tibial internal rotation (JiSeon, 2003), and (3) knee flexion and flexion velocity (Derrick et al., 2002, Mizrahi et al., 2000c). However, this type of running protocol does not represent the typical running session that is performed by runners on a regular basis. Considering that runners rarely run to the point of exhaustion or maximum fatigue, it would seem plausible that the development of overuse injuries would be more closely associated with the typical running session performed on a regular basis. However, no studies have actually investigated how kinematics are affected when running at a level of exertion that best represents a typical running session.

With kinematic changes occurring over the course of a run, it is likely that the coordination or joint coupling within the lower extremity will be affected as well. Joint coupling has commonly been assessed using the joint timing measure, which is the peak-to-peak synchrony between two joint motions (Bates et al., 1978). It is believed that if the joint motions that occur under eccentric control during the first half of stance do not reverse directions at the same time, the musculoskeletal system may be momentarily stressed beyond its physiological threshold. For uninjured runners in a non-exerted state, previous studies have confirmed relatively synchronous timing of rearfoot eversion, tibial internal rotation, and knee flexion, with the peaks occurring at approximately midstance (DeLeo et al., 2004, De Wit and De Clercq, 2000, Dierks and Davis, 2007, Hamill et al., 1992, McClay and Manal, 1997, Stergiou et al., 1999, van Woensel and Cavanagh, 1992). On the other hand, joint timing relationships involving knee internal rotation with either eversion or tibial internal rotation have been found to be more asynchronous, due to a relatively later time to peak knee internal rotation (McClay and Manal, 1997, Dierks and Davis, 2007). What is unknown is how these timing relationships are influenced when running in an exerted state.

A limited number of studies have investigated how running in an exerted state influences kinematics. Of these studies, all used an exhaustive running protocol rather than one that represents the exertion that is typical during a training run. Furthermore, no studies have investigated foot, knee, and hip kinematics in the same population of runners, and no studies have investigated joint timing over the course of a prolonged run. Therefore, the purpose of this study was to investigate the effect of running in an exerted state on the lower extremity kinematics and joint timing of uninjured, recreational runners. It was hypothesized that at the end of the run, increases in peaks, excursions, and velocities would be observed for the foot, knee, and hip. In addition, it was expected that joint timing between the foot, the shank, and the thigh would become more asynchronous.