The effect of foot orthotics on myoelectric fatigue in the vastus lateralis during a simulated skier’s squat

Fatigue in the legs is a problem experienced by skiers. It has been suggested that optimal orthotics may reduce muscle fatigue for a given movement task by minimising muscle activity (Med. Sci. Sports Exerc. 31 (1999) S421). The aims were to determine whether EMG would provide an independent method of analysing myoelectric fatigue in the vastus lateralis (VL) during a skier’s squat and whether orthotics could affect this fatigue response. Six skiers performed skier’s squats for as long as possible with no orthotic, low volume orthotics and high volume orthotics in their ski boots. Bipolar, active surface electrodes recorded EMG activity in the VL throughout each squat. Results for the EMG median frequency showed a significant shift in the power density spectrum towards the lower frequencies (P<0.05) at the end of the contraction, suggesting that myoelectric fatigue was occurring and was measurable using EMG. All conditions displayed a significant decrease in median frequency at the end of the contraction (P=0.001). The high volume orthotic showed a significant reduction in myoelectric fatigue, however, there was no difference in the duration of squats across the three conditions (P>0.05). Subjective and objective findings support the use of the high volume foot orthotic for skiers.