Background Coactivation of the hamstrings and quadriceps has been identified as the most effective stabilising pattern of the knee. The risk of knee injury is increased in landing compared to running and may be associated with a different muscular response to the loading pattern.
Objective The aim of this study was to compare coactivation patterns of the knee flexors and extensors during two common athletic activities.
Design Kinetic and EMG analysis.
Setting Competitive athletes.
Participants Eight young adult athletes (5 male/3 female).
Interventions Data were collected from two running interventions (2.5 and 3.0 m/s) and expected landings from a height of 73 cm.
Main outcome measurements Muscular activation of selected quadriceps and hamstrings muscles was determined using normalised RMS of surface EMG. The relative amount of knee flexor and extensor activity was determined using a coactivation ratio (CCR, Sum RMS Hams/Sum RMS Quads). For the running trials, the stance phase was normalised and mean muscle activity calculated during three intervals: weight acceptance (WA); middle stance (MS); and late stance (LS). Landing trials were divided into two phases: initial contact (IC, the first 100 ms after touchdown) and eccentric (EC, from 100 ms until maximal knee flexion).
Results The CCR was significantly greater (p<0.001) during running (WA=0.56, MS=0.49) than landing (IC=0.20, EC=0.32). Muscular activity observed during landing greatly favoured the quadriceps while hamstring activity was highest during LS in running. There was a trend for an increase in CCR with running velocity, however only during LS was CCR significantly higher (p<0.05) (2.5 m/s=1.67, 3 m/s=2.14).
Conclusion Quadriceps activity was observed to be much higher than the hamstrings in landing than running and this has been hypothesised as a mechanism of anterior cruciate ligament injury. The Quadriceps contract eccentrically to control landing and a low hamstring contribution may affect joint stability and loading patterns.
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