The Electromyographic Threshold in Girls and Women

Background: The electromyographic threshold (EMG(Th)) is thought to reflect increased high-threshold/type-II motor-unit (MU) recruitment and was shown higher in boys than in men. Women differ from men in muscular function. Purpose: Establish whether females' EMGTh and girls women differences are different than males'. Methods: Nineteen women (22.9 +/- 3.3yrs) and 20 girls (10.3 +/- 1.lyrs) had surface EMG recorded from the right and left vastus lateralis muscles during ramped cycle-ergometry to exhaustion. EMG root-mean-squares were averaged per pedal revolution. EMGTh was determined as the least residual sum of squares for any two regression-line data divisions, if the trace rose >= 3SD above its regression line. EMGTh was expressed as % final power-output (%Pmax) and %Pvo2(pk) power (%Pvo2(pk)) Results: EMGTh was detected in 13 (68%) of women, but only 9 (45%) of girls (p <.005) and tended to be higher in the girls (%Pmax= 88.6 +/- 7.0 vs. 83.0 +/- 6.9%, p =.080; %Pvo2(pk)= (101.6 +/- 17.6 vs. 90.6 +/- 7.8%, p =.063). When EMGTh was undetected it was assumed to occur at 100%Pmax or beyond. Consequently, EMGTh values turned significantly higher in girls than in women (94.8 +/- 7.4 vs. 88.4 +/- 9.9%Pmax, p =.026; and 103.2 +/- 11.7 vs. 95.2 +/- 9.9%Pvo2(pk), p =.028). Conclusions: During progressive exercise, girls appear to rely less on higher-threshold/type-II MUs than do women, suggesting differential muscle activation strategy.