Body-Borne Load Increases Peak Knee Extensor Muscle Force During a Reactive Run-to-Stop Task

Abstract

A sudden stop while running with body-borne load (i.e. run-to-stop (RTS)) reportedly causes abnormally high knee joint loads, increasing risk of injury. Performing a reactive RTS may exacerbate these joint loads, requiring greater lower limb muscle force to successfully complete the maneuver, further increasing injury risk. Currently it is not understood whether knee muscle forces increase with the addition of load during a RTS, and whether they are further exaggerated during reactive tasks. PURPOSE: The purpose of this study was to determine if peak knee extensor and flexor muscle forces increase with the addition of load during pre-planned and reactive RTS tasks. METHODS: Seven males had lower limb biomechanics recorded during a RTS task with three military-relevant load conditions: unloaded (UL; 6.2 kg), fighting load (FL; 20.0 kg) and approach load (AL; 40.0 kg). During the RTS, participants ran down a walkway and stopped their dominant limb on a force platform. Participants performed three planned and three reactive RTS maneuvers with each load configuration. OpenSim was used to estimate peak knee extensor and flexor muscle force during the stopping phase of each RTS. Subject-based means of peak muscle force were quantified and submitted to a two-way repeated measures ANOVA with alpha level 0.05. Significant interaction effects were submitted to a one-way ANOVA stratified by condition (pre-planned or reactive). RESULTS: During the reactive RTS, significantly greater peak vastus medialis (p=0.041 and p=0.013), intermedius (p=0.032 and p=0.012) and lateralis (p=0.039 and p=0.009) muscle force was evident with the FL (306.4, 355.8, 637.9 N) and AL (324.5, 382.6, 699.5 N) as compared to UL (234.2, 266.0, 485.0 N) configuration, but similar differences (p>0.05) in peak muscle force were not evident during pre-planned RTS. No significant increase (p>0.05) in peak knee flexor muscle force was evident with the addition of load, or between the pre-planned and reactive RTS. CONCLUSION: Peak knee extensor muscle force increased with the addition of load during reactive, but not pre-planned RTS. Performing a reactive RTS may require larger muscle forces to successfully complete the maneuver, resulting in greater injury risk compared to pre-planned maneuvers.