Neural adaptations to long-term resistance training: evidence for the confounding effect of muscle size on the interpretation of surface electromyography

This study compared elbow flexor (EF; Experiment 1) and knee extensor (KE; Experiment 2) maximal compound action potential (M_max) amplitude between long-term resistance trained (LTRT; n=15 and n=14, 6±3 and 4±1 years of training) and untrained (UT; n=14 and n=49) men; and examined the effect of normalising electromyography (EMG) during maximal voluntary torque (MVT) production to M_max amplitude on differences between LTRT and UT. EMG was recorded from multiple sites and muscles of EF and KE, M_max was evoked with percutaneous nerve stimulation, and muscle size was assessed with ultrasonography (thickness, EF) and magnetic resonance imaging (cross-sectional area, KE). Muscle-electrode distance (MED) was measured to account for the effect of adipose tissue on EMG and M_max. LTRT displayed greater MVT (+66-71%, p<0.001), muscle size (+54-56%, p<0.001), and M_max amplitudes (+29-60%, p≤0.010) even when corrected for MED (p≤0.045). M_max was associated with the size of both muscle groups (r≥0.466, p≤0.011). Compared to UT, LTRT had higher absolute voluntary EMG amplitude for the KE (p<0.001), but not the EF (p=0.195), and these differences/similarities were maintained after correction for MED; however, M_max normalisation resulted in no differences between LTRT and UT for any muscle and/or muscle group (p≥0.652). The positive association between M_max and muscle size, and no differences when accounting for peripheral electrophysiological properties (EMG/M_max), indicates the greater absolute voluntary EMG amplitude of LTRT might be confounded by muscle morphology, rather than provide a discrete measure of central neural activity. This study therefore suggests limited agonist neural adaptation after LTRT.