Changes to balance dynamics following a high-intensity run are associated with future injury occurrence in recreational runners.

Background: Fatigue is associated with increased injury risk along with changes in balance control and task performance. Musculoskeletal injury rates in runners are high and often result from an inability to adapt to the demands of exercise and a breakdown in the interaction among different biological systems. This study aimed to investigate whether changes in balance dynamics during a single-leg squat task following a high-intensity run could distinguish groups of recreational runners who did and did not sustain a running-related injury within 6 months. Methods: Thirty-one healthy recreational runners completed 60 s of single-leg squat before and after a high-intensity run. Six months after the assessment, this cohort was separated into two groups of 13 matched individuals with one group reporting injury within this period and the other not. Task performance was assessed by the number of repetitions, cycle time, amplitude, and speed. To evaluate balance dynamics, the regularity and temporal correlation structure of the center of mass (CoM) displacements in the transverse plane was analyzed. The interaction between groups (injury, non-injured) and time (pre, post) was assessed through a two-way ANOVA. Additionally, a one-way ANOVA investigated the percent change difference of each group across time. Results: The injured group presented more regular (reduced entropy; 15.6%) and diffusive (increased short-term persistence correlation; 5.6%) CoM displacements after a high-intensity run. No changes were observed in the non-injured group. The within-subject percent change was more sensitive in demonstrating the effects of fatigue and distinguishing the groups, compared to group absolute values. No differences were observed in task performance. Discussion: Runners who were injured in the future demonstrate changes in balance dynamics compared to runners who remain injury-free after fatigue. The single-leg squat test adopted appears to be a potential screening protocol that provides valuable information about balance dynamics for identifying a diminished ability to respond to training and exercise.

Dynamic visual acuity during asymmetric walking.

Necessary for effective ambulation, head stability affords optimal conditions for the perception of visual information during dynamic tasks. This maintenance of head-in-space equilibrium is achieved, in part, by the attenuation of the high frequency impact shock resulting from ground contact. While a great deal of experimentation has been done on the matter during steady state locomotion, little is known about how locomotor asymmetry might affect head stability or dynamic visual acuity. In this study, fifteen participants walked on a split-belt treadmill while verbally reporting the orientation of a randomized Landolt-C optotype that was projected at heel strike. Participants were exposed to baseline, adaptation, and washout conditions, as characterized by belt speed ratios of 1:1, 1:3, and 1:1, respectively. Step length asymmetry, shock attenuation, high and low frequency head signal power, and dynamic visual acuity were averaged across the first and last fifty strides of each condition. Across the first fifty strides, step length asymmetry was significantly greater during adaptation than during baseline (p < 0.001; d = 2.442), and shock attenuation was significantly lower during adaptation than during baseline (p = 0.041; d = -0.679). High frequency head signal power was significantly greater during adaptation than during baseline (p < 0.001; d = -1.227), indicating a reduction in head stability. While dynamic visual acuity was not significantly lower during adaptation than during baseline (p = 0.052), a moderate effect size suggests a decrease in the measure between the two conditions (d = 0.653). Across the last fifty strides, many of the decrements observed between the baseline and adaptation conditions were greatly reduced. The results of this study indicate that the locomotor asymmetry imposed by the split-belt treadmill during early adaptation might lead to moderate decrements in shock attenuation, head stability, and dynamic visual acuity. Moreover, the relative reduction in magnitude of these decrements across the last fifty strides underscores the adaptive nature of the locomotor and visuomotor systems.