Lepley, A. S., Bahhur, N. W., Murray, A. M., & Pietrosimone, B. G. (2013). Contributions of neural excitability and voluntary activation to quadriceps muscle strength. Medicine & Science in Sports & Exercise, 45(5), Supplement abstract number 1542.

This study evaluated if spinal-reflexive excitability, corticospinal excitability, and voluntary activation predict quadriceps strength. Ss (M = 12; F = 8) had quadriceps strength assessed via maximum voluntary isometric contractions performed at 90° of knee flexion. Spinal-reflexive excitability was assessed using Hoffmann reflexes normalized to maximal muscle responses. Corticospinal excitability was evaluated with active motor thresholds, which were elicited by stimulating specific areas on the motor cortex using Transcranial Magnetic Stimulation during a standardized contraction at 5% of maximum voluntary isometric contractions. Voluntary activation was measured via a superimposed burst technique and the central activation ratio. A hierarchal multiple Linear Regression model was used to determine the ability of the three factors to predict maximum voluntary isometric contraction. The change in R2 to the model from the addition of each predictor variable was also analyzed.

The overall multiple regression model significantly predicted 45% of the variance in maximum voluntary isometric contractions. The strongest variable loading was the Hoffmann reflexes normalized to maximal muscle responses which accounted for 24% of the variance in maximum voluntary isometric contractions. The subsequent addition of active motor thresholds accounted for a further significant increase of 16% in the prediction capability of the model. The central activation ratio contributed an insignificant increase of 5% to the prediction capability of the model, and was therefore removed.

Implication. Neural excitability predicted nearly half of the variance in quadriceps strength providing evidence that neural pathways and stimulation are essential in maintaining and increasing quadriceps strength.

To increase neural drive and excitability, exercises should be performed with maximum explosive effort with an emphasis on speed of movement rather than loads that slow movement execution.

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