The objective of this study was to characterize the influence of step rate on lower extremity biomechanics during running at a constant speed, with an emphasis on the change in mechanical energy absorbed at the hip, knee and ankle. In partial support of our hypothesis, we observed a substantial reduction in energy absorption at the knee and hip when step rate was increased above preferred. Our findings demonstrate that subtle changes in step rate can reduce the energy absorption required of the lower extremity joints, which may prove beneficial in the prevention and treatment of running injuries.
The decreased energy absorption observed at the knee and hip as step rate increased is likely due primarily to the corresponding change in step length and lower extremity posture at initial contact (10
). Indeed, when step length and step rate were manipulated independent of each other, energy absorption was observed only if step length decreased (26
). Because subjects in our study ran at their preferred speed for all conditions, an increase in step rate necessitated a proportional decrease in step length. As such, the heel was located more underneath the COM at initial contact with an accompanying decrease in the braking impulse. Similarly, peak knee flexion during stance and the COM vertical excursion were observed to decrease as step rate increased, suggestive of greater lower extremity stiffness (15
). Of note, many of the biomechanical changes we found when step rate increased are similar to those observed when running barefoot or with minimalist footwear (12
Our findings regarding energy absorption were comparable to those of Derrick et al. (11
), despite calculating the negative work over different portions of the stance phase. Specifically, Derrick et al (11
) was interested in the impact phase, defined as the initial ~20% of stance, while we considered the entire LR, representing the initial ~42% of stance. As a result, our absolute energy absorption values are greater; however, the relative change between conditions and joints is comparable.
While systematic kinematic and kinetic alterations were observed across the step rate conditions, the knee joint appeared to be most sensitive to changes in step rate. In particular, only the knee displayed significant changes in energy absorption between all step rate conditions, with a 20% decrease observed when preferred step rate was only increased by 5%. When combined with the significant reduction (18%) in energy generation at the knee during the same step rate condition, it is clear that a substantial decrease in mechanical work performed at the knee occurs with as little as a 5% increase in step rate.
Despite the clear reduction in the magnitude of knee joint loading when step rate is increased, the corresponding increase in the number of steps required for a given distance (i.e. loading cycles) may offset any potential benefit to injury reduction. That is, the cumulative loading incurred by the lower extremity may be the same for a given running distance. However, running with shorter stride lengths has been suggested to reduce the risk of a tibial stress fracture, despite the greater number of loading cycles (14
). Thus, it appears that the benefits of reducing the magnitude of loading outweigh the detriments of increased loading cycles. Whether this same injury-reducing benefit is realized for other common running-related injuries (e.g., anterior knee pain, iliotibial band syndrome) has yet to be determined.
The reduced energy absorption at the hip and knee when running with an increased step rate may prove useful as an adjunct to current rehabilitation strategies for running injuries involving these joints and associated tissues. That is, injured runners could be instructed using a metronome to increase their step rate while maintaining the same speed. The associated reduction in loading may enable injured individuals to continue running without aggravating symptoms, while receiving care for their injuries. Similarly, utilizing an increased step rate may prove beneficial following injury recovery as part of a progressive return to running. Recent work has demonstrated that runners can be taught to modify their gait to reduce impact loading and that this modification can be maintained at a 1-month follow-up (6
). The effectiveness of such strategies in reducing symptoms, facilitating injury recovery, and promoting a return to full running performance, however, remains unknown.
Excessive hip motion during running, specifically adduction and internal rotation, has been associated with anterior knee pain and iliotibial band syndrome (16
). Our findings indicate that a 5–10% increase in step rate can significantly reduce peak hip adduction during the LR. Interestingly, an associated reduction in the hip abduction and internal rotation moments was not realized until step rate was increased by 10%. Regardless, it appears that running with a step rate greater than preferred reduces the biomechanical demands incurred by the hip in the frontal and transverse planes of motion, and therefore may be useful in the clinical management of running injuries involving the hip. However, it is uncertain whether injured individuals display the same biomechanical changes to step rate manipulation, or if existing symptoms or impairments would interfere.
Because preferred step rate and length are closely aligned with minimizing metabolic energy cost (2
), modifying an individual’s step rate may have a metabolic consequence. For example, subjects in the current study reported a greater RPE when step rate increased 10% above preferred. However, given the novelty of the modified step rate conditions to the subjects, we believe that this increase in perceived effort may be reflective of increased attentional focus (5
), rather than an actual metabolic response (21
). Indeed, increasing one’s step rate to 10% above preferred has demonstrated no significant increase in oxygen consumption or heart rate (18
). Further, the reduction in peak knee flexion observed at the higher step rate conditions in the current study has been associated with an improved economy (2
Certain limitations with the present study should be considered when interpreting its findings. Despite subjects receiving adequate time to achieve the prescribed step rate, it is uncertain whether the observed biomechanical changes persist beyond the short-term. As one becomes more experienced running at a faster step rate, further biomechanical changes may occur. The step rate was determined through visual inspection and may have been prone to measurement error; however, post-hoc analysis of the force plate data confirmed the accuracy of the step rate assessments. Further, our step rate modification protocol was conducted on a treadmill, potentially limiting its generalizability to overground running. However, our findings are consistent with those performed overground (11
). Finally, due to a limited number of markers during our experimental capture, non-sagittal kinematics and kinetics at the knee and ankle were not calculated. Given the clinical relevance of these additional degrees of freedom, their inclusion in future studies is warranted.
In conclusion, our findings indicate that a substantial reduction in energy absorption occurs at the hip and knee when step rate is increased to 10% above preferred with a constant running speed, while a 5% increase appears to reduce the total work performed by the knee. Thus, the reduction in joint loading via step rate manipulation may have distinct benefits in the treatment and prevention of common running-related injuries involving the knee and hip.