Electromyogram changes during sustained contraction after resistance training in women in their 3rd and 8th decades

The present study aimed at investigating the neuromuscular adaptations to 6 weeks of resistance training in women in their third (6 experimental, 8 co ntrols) and eighth decades (8 experimental, 8 controls). The surface electr omyogram (sEMG) was measured from the biceps brachii muscle during constant -force isometric contractions lasting 12 s at 80% of maximal voluntary cont raction (MVC). All the signals were analysed adopting in the time domain th e root mean square (RMS) as a measure of amplitude and in the frequency dom ain the median frequency (MDF) of the power spectrum. Quantitative analysis was performed from the 3rd to the 6th second, to describe the early phase of the contraction ("Early"), starting from point at which 80% of the MVC w as reached, and from the 9th to the 12th second, to describe the last part of the constant-force sustained contraction ("Late"). After training, the M VC increased by 22.4% in the young (P < 0.0001) and by 13.4% in the older ( P < 0.05) women. The "Early" RMS increased by 60.4% with respect to the pre -training condition in the young: (P < 0.01) but not in the older women. In contrast, the "Late" RMS increased by 46.7% in the older (P < 0.05) but no t in the young women. The MDF remained unchanged in both groups. These resu lts indicate that young and older women showed different training-induced a daptation of the motor unit (MU) activation pattern, in order to keep a con stant level of force during a sustained isometric contraction at 80% of MVC .