Response

Dear Editor-in-Chief,

We appreciate Dr Li’s interest in our paper. He has highlighted some experimental decisions that are often debated within biomechanics research; however, we disagree with his conclusion that these factors are likely to have altered our findings regarding coordination variability (⁵). The focus of Dr Li’s letter was the participants’ running velocity; therefore, we will also focus our response to this issue. Our interest was in determining if running mechanics, as measured by joint kinematics, kinetics, and coordination variability, were different in younger versus older runners. It has been well-established that kinematic and kinetic variables are altered with changes in running velocity, thus, to adequately assess differences between groups, measurements at a set speed are necessary (²).

Dr Li’s concern was that our set velocity would disguise differences in coordination variability between older and younger runners (⁵). His rationale was based on the thought that older runners would have lower coordination variability but that, by running at a greater than preferred velocity, coordination variability would increase and be similar to younger runners. However, in hindsight, we believe that our methodology was more likely to have magnified any differences in coordination variability than to mask them. From a dynamical systems perspective, we would expect to see a freezing of degrees of freedom, leading to a decrease in coordination variability, as the system becomes less flexible (⁶) (i.e., with aging; in agreement with Dr Li’s hypothesis) and with increasing task demands (³) (i.e., increasing velocity above preferred; contrary to Dr Li’s hypothesis). While little data exist investigating the relationship between running velocity and coordination variability, some support for an inverse relationship exists. Dr Li referenced the work of Jordan and colleagues (⁴), yet his interpretation of their results was misleading (⁵). Although they did find an increase in variability with changes in running velocity, this difference only occurred at slower than preferred velocities. No difference in variability was observed when runners moved at greater than preferred velocities. During walking, Chiu and Chou (¹) also found that individuals moved with significantly greater coordination variability at slower than preferred velocities than at faster than preferred velocities.

Preferred velocity is often measured in the laboratory, but how this compares to an individual’s daily training pace is unknown. Our paper included participants’ self-reported typical training pace, which may differ greatly from a preferred pace determined in a laboratory or from the range of paces that make up their training plan. While the older runners did report a slower typical training pace than the younger runners, we also have some race results for the older runners in this study. These results show that the older runners had an average race pace, in distances ranging from 3.1 to 10 miles (5–16.1 km), of 8.2 ± 0.9 minutes per mile (3.3 ± 0.3 m·s⁻¹), placing the set running velocity within one standard deviation of the runners’ average race pace.

Dr Li has brought up an interesting research question in addressing the impact of running velocity on coordination variability (⁵), but that was not the goal of the current study. A set velocity allows for direct comparison of running mechanics between groups (²). The existing evidence in the literature supports our current findings regardless of the constraints placed on participants’ velocity.

Julia Freedman Silvernail

University of Nevada, Las Vegas, NV

University of Massachusetts, Amherst, MA

Katherine Boyer

University of Massachusetts, Amherst, MA

Eric Rohr

Brooks Sports, Inc., Bothell, WA

Gert-Peter Brüggemann

German Sports University, Cologne, GERMANY

Joseph Hamill

University of Massachusetts, Amherst, MA