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Background  The m. quadriceps femoris is the strongest muscle in the human body and plays an important role in sports, activities of daily living and independence. Two older studies showed increased electromyographic (EMG) activity of the quadriceps when the dorsal extensors of the foot were pre-activated. The aim was to physiologically replicate this finding by EMG and to verify it functionally by single leg hop.    Methods  EMG activity (root mean square, RMS) was tested on the leg press at the isometric load of the individual 12-repetition-maximum (12RM) weight (on average 79.7 kg) at 45° and 90° knee flexion. Single leg hop distance was measured between the tests. Intra-individual changes between with and without dorsal foot extension were quantified and compared by standardized response means (SRM).    Results  Thirty-five healthy subjects between 21 and 57 years were included. The m. vastus medialis was activated on average to an RMS of 32.4 μV without and 53.7 μV with dorsal foot extension (SRM = 1.39, p < 0.001) at 45° knee flexion and an RMS of 124.9 μV versus 152.8 μV (SRM = 1.08,  p  < 0.001) at 90°. The corresponding data for the rectus femoris were 9.4 μV versus 18.9 μV (SRM = 0.71,  p  < 0.001) at 45° and 77.8 μV versus 135.3 μV (SRM = 0.89,  p  < 0.001) at 90°. Mean single leg hop distance was 169.8 cm without versus 178.9 cm with dorsal foot extension (SRM = 1.09,  p  < 0.001).    Conclusions  Pre-activation of dorsal foot extensors significantly increased EMG activity in the m. quadriceps femoris and single leg hop distance. It can therefore be used to improve functional quadriceps muscle performance and knee joint stability in training and rehabilitation.

Quadriceps performance under activation of foot dorsal extension in healthy volunteers: an interventional cohort study