Non-invasive estimation of muscle fibre size from high-density electromyography

Because of the biophysical relation between muscle fibre diameter and the propagationvelocity of action potentials along the muscle fibres, motor unit conduction velocity could be anon-invasive index of muscle fibre size in humans. However, the relation between motor unitconduction velocity and fibre size has been only assessed indirectly in animal models and in humanpatients with invasive intramuscular EMG recordings, or it has been mathematically derived fromcomputer simulations. By combining advanced non-invasive techniques to record motor unit activityin vivo, i.e. high-density surface EMG, with the gold standard technique for muscle tissue sampling,i.e. muscle biopsy, here we investigated the relation between the conduction velocity of populations ofmotor units identified from the biceps brachii muscle, and muscle fibre diameter. We demonstrate thepossibility of predicting muscle fibre diameter (R2 = 0.66) and cross-sectional area (R2 = 0.65) fromconduction velocity estimates with low systematic bias (∼2% and ∼4% respectively) and a relativelylow margin of individual error (∼8% and ∼16%, respectively). The proposed neuromuscular inter-face opens new perspectives in the use of high-density EMG as a non-invasive tool to estimate musclefibre size without the need of surgical biopsy sampling. The non-invasive nature of high-densitysurface EMG for the assessment of muscle fibre size may be useful in studies monitoring childdevelopment, ageing, space and exercise physiology, although the applicability and validity of theproposed methodology need to be more directly assessed in these specific populations by futurestudies.