Running Velocity Does Not Influence Lower Limb Mechanical Asymmetry

We examined the effect of running velocity upon magnitude and range of asymmetry in the main kinetics and kinematics of treadmill running at constant, submaximal velocities. Nine well-trained, un-injured distance runners ran, in a random order, at seven running velocities (10, 12.5, 15, 17.5, 20, 22.5, and 25 km.h(-1)) for 60 s (separated by > 90 s of rest) on an instrumented treadmill (ADAL3D-WR, Medical Development, France). Continuous measurement (1,000Hz) of spatio-temporal, horizontal force production, and spring-mass characteristics was performed and data over 10 consecutive steps (5 right and 5 leg foot contacts after similar to 50 s of running) were used for subsequent comparisons. Group mean and the range of asymmetry scores were assessed from the "symmetry angle" (SA) formulae where a score of 0%/100% indicates perfect symmetry/asymmetry. Mean SA scores for spatio-temporal variables were lower than 2%: contact time (0.6 +/- 0.1%; range: 0.4-0.7%), aerial time (1.7 +/- 0.2%; range: 1.3-2.1%) as well as step length and step frequency (0.7 +/- 0.2%; range: 0.5-0.9%). Mean loading rate (5.3 +/- 1.1%; range: 4.1-6.9%) and spring mass model [peak vertical force: 3.2 +/- 1.6% (range: 2.9-3.4%); maximal downward vertical displacement: 11.2 +/- 6.0% (range: 9.2-14.0%); leg compression: 3.6 +/- 1.9% (range: 2.9-5.6%); vertical stiffness: 8.8 +/- 1.9% (range: 7.1-11.6%); leg stiffness: 1.6 +/- 0.6% (range: 1.2-2.9%)] presented larger mean SA values. Mean SA scores ranged 1-4% for duration of braking (1.3 +/- 0.3%; range: 0.9-2.0%) and push-off (1.6 +/- 0.9%; range: 1.2-2.4%) phases, peak braking (2.4 +/- 1.1%; range: 1.6-3.6%), and push-off (1.7 +/- 0.9%; range: 1.2-2.2%) forces as well as braking (3.7 +/- 2.0%; range: 2.8-5.8%) and push-off (2.1 +/- 0.8%; range: 1.3-2.6%) impulses. However, with the exception of braking impulse (P = 0.005), there was no influence of running velocity on asymmetry scores for any of the mechanical variables studied (0.118<P<0.920). Modifying treadmill belt velocity between 10 and 25 km.h-1 induced large adjustments in most running kinetics and kinematics. However, there was no noticeable difference in group mean and the range of asymmetry values across running velocities, with the magnitude of these scores being largely dependent on the biomechanical variable of interest. Finally, the relatively large range of asymmetry between participants for some variables reinforces the importance of assessing asymmetry on an individual basis.