The electromyographic threshold in boys and men

Abstract Background Children have been shown to have higher lactate (LaTh) and ventilatory (VeTh) thresholds than adults, which might be explained by lower levels of type-II motor-unit (MU) recruitment. However, the electromyographic threshold (EMGTh), regarded as indicating the onset of accelerated type-II MU recruitment, has been investigated only in adults. Purpose To compare the relative exercise intensity at which the EMGTh occurs in boys versus men. Methods Participants were 21 men (23.4 ± 4.1 years) and 23 boys (11.1 ± 1.1 years), with similar habitual physical activity and peak oxygen consumption (VO2pk) (49.7 ± 5.5 vs. 50.1 ± 7.4 ml kg−1 min−1, respectively). Ramped cycle ergometry was conducted to volitional exhaustion with surface EMG recorded from the right and left vastus lateralis muscles throughout the test (~10 min). The composite right–left EMG root mean square (EMGRMS) was then calculated per pedal revolution. The EMGTh was then determined as the exercise intensity at the point of least residual sum of squares for any two regression line divisions of the EMGRMS plot. Results EMGTh was detected in 20/21 of the men (95.2 %) and only in 18/23 of the boys (78.3 %). The boys’ EMGTh was significantly higher than the men’s (86.4 ± 9.6 vs. 79.7 ± 10.0 % of peak power output at exhaustion; p < 0.05). The pattern was similar when EMGTh was expressed as percentage of VO2pk. Conclusions The boys’ higher EMGTh suggests delayed and hence lesser utilization of type-II MUs in progressive exercise, compared with men. The boys–men EMGTh differences were of similar magnitude as those shown for LaTh and VeTh, further suggesting a common underlying factor.

Myosin Regulatory Light Chain Phosphorylation and Its Effect on the Contractile Economy of Mouse Fast Muscle

Activated by elevations in myoplasmic calcium concentration, myosin light chain kinase (skMLCK) phosphorylates the regulatory light chains (RLCs) of fast muscle myosin. This covalent modification potentiates force production, but requires an investment of ATP. Our objective was to investigate the effect of RLC phosphorylation on the contractile economy (mechanical output:metabolic input) of fast twitch skeletal muscle. Extensor digitorum longus muscles isolated from Wildtype and skMLCK-/- mice mounted in vitro (25°C) were subjected to repetitive low-frequency stimulation (10Hz,15s) known to cause activation of skMLCK, and staircase potentiation of force. With a 3-fold increase in RLC phosphate content, Wildtype generated 44% more force than skMLCK-/- muscles over the stimulation period (P = .002), without an accompanied increase in energy cost (P = .449). Overall, the contractile economy of Wildtype muscles, with an intact RLC phosphorylation mechanism, was 73% greater than skMLCK /- muscles (P = .043), demonstrating an important physiological function of skMLCK during repetitive contractile activity.