Effect of Running Intensity on Leg Force Asymmetry and its Relationship to Internal Load Biomarkers

Higher leg force asymmetry had been linked to increased energy expenditure during long-distance running. Muscle fatigue would be expected to influence leg force asymmetry during the effort due to its influence on coordination, but previous studies failed to show this effect. Considering the influence of exercise intensity on fatigue, leg force asymmetry progression may be intensity-dependent. This study aims to investigate the effect of running intensity on the leg force asymmetry in the vertical and horizontal orientations and their relationship to a physiological response. Nine recreational long-distance runners were submitted to three test sessions interspaced by 48–72 hours on an instrumented non-motorized treadmill, consisting of an incremental test and three constant intensity efforts, below RCP, at RCP and above VO2max ($38.1 \pm 10.9, 59.8 \pm 14.7$, and $126.7 \pm 11.8$ % of the VO2max, respectively). Results shows a significant effect of intensity on leg force asymmetry in the horizontal orientation ($p = 0.04; \eta \mathrm{p}^{2} = 0.167$), but not in the vertical orientation ($p = 0.14; \eta \mathrm{p}^{2} = 0.067$). Leg asymmetries do not tend to progress along with the effort, irrespective of force orientation or the intensities included here ($p > 0.97; \eta \mathrm{p}^{2} < 0.011$ for time). However, the association between leg force asymmetry and biomarkers of physiological stress may be intensity-dependent, with the intensity at RCP showing the stronger coefficient of determination between the studied variables ($\mathrm{R}^{2}$ between 0.52 and 0.93) compared to the other intensities ($\mathrm{R}^{2}$ between 0.08 to 0.16).