Myoelectric activation differences in vastus lateralis and vastus medialis obliquus muscles of young and elderly active male subjects.

Aims: The aim of the study was to assess quadriceps’ myoelectric activation and manifestation of fatigue in a group of healthy active elderly males compared to a control group of healthy active young males. Methods: fourteen elderly (65-77 years) and 12 young males (21-26 years) performed three knee position tasks aimed to maintain 60deg angle (0deg= full extension) at 30%, 50% and 70% of knee extension maximal voluntary contraction (MVC). Initial values and rate of change of average rectified value (ARV), mean spectral frequency (MNF), and muscle conduction velocity (CV) were estimated recording surface EMG signals (sEMG) from vastus lateralis (VL) and vastus medialis obliquus (VMO) muscles of dominant limb, using eight electrodes arrays. Friedman tests were used to compare sEMG variables among the three contraction levels, whereas Mann Whitney U tests were used to compare the groups. Results: In VMO: Friedman and post hoc tests found significant increase of CV at 50% and 70% with respect to 30% (p=0.001) in both elderly and young groups. Mann Whitney tests did not find differences between groups. In VL: Friedman and post hoc tests found significant increase of CV at 50% and 70% with respect to 30% (p=0.001) in both elderly and young groups. Mann Whitney tests found greater CV in young with respect to elderly at 50% (p=0.04) and 70% (p=0.01). No differences in rate of change of CV and MNF were found between groups and muscles. Conclusion: The increase of CV with contraction levels is related to an increase of neuromuscular recruitment in both group. The observed increase from 30% to 50% MVC and from 50% to 70% confirmed the possibility to detect small differences in force output (20% MVC) based on such a sEMG variable. VMO CVs were found similar for the two groups, whereas VL greater CVs in young could be explained by a greater type II motor unit availability with respect to elderly. Indeed elderly were demonstrated to show shifting toward type I motor unit due to age (1). Differences in motor unit recruitment in VL probably were not enough wide to determine an effect in fatigue manifestation. Indeed, rate of change of sEMG variables failed to detect differences in myoelectric fatigability between the two groups probably since, as previous demonstrated (2), constant physical exercise could counteract type II fibres ipotrophy usually observed in sedentary elderly. References: 1) Merletti R et al. (2002) Effect of age on muscle functions investigated with surface electromyography. Muscle Nerve 25(1):65-76. 2) Casale R et al. (2003) Can continuous physical training counteract aging effect on myoelectric fatigue? A surface electromyography study application. Arch Phys Med Rehabil 84:513-7.