Effects of stretching on the fractal dimension of rectus femoris and vastus medialis muscles

Abstract

The aim of the present study was to assess stretching induced alterations in the dynamics of the EMG signals from rectus femoris (RF) and vastus medialis (VM) muscles by using fractal dimension (FD) analysis, discriminate the contributions of the neural and the muscle related changes induced by stretching, and get an idea whether FD could be a parameter to detect the improvements of physiotherapy interventions. Twenty-two healthy subjects of both sexes were divided randomly into a control group and a stretching group. Initially, subjects were requested to produce 5 consecutive maximal voluntary isometric contractions of quadriceps muscle, each lasting 5 seconds with a resting period of 10 seconds between contractions. Then subjects in the stretching group were subjected to stretching exercise for 20 minutes while subjects in the control group remained without doing anything for 20 minutes. Afterwards, the same experiments as the first series were performed again. Force produced by quadriceps muscle and EMG signals from RF and VM were recorded. Stretching decreased significantly FD of RF from 1.3947 +/- 0.0127 to 1.3825 +/- 0.0111 (p=0.034, d=1.02) but increased FD of VM from 1.3769 +/- 0.0187 to 1.3897 +/- 0.0300 (p=0.043, d=0.51). RMS values of EMG signals from RF and VM muscles and maximal voluntary isometric force measured from quadriceps muscle did not change significantly after stretching. The results indicated that stretching altered the dynamics of RF and VM muscles oppositely; stretching decreased the complexity of the EMG signal from RF while it increased the complexity of the EMG signal from VM. The opposite dynamic responses of RF and VM muscles were interpreted by means of the electromechanical time delay results previously obtained from the same data, and it was proposed that stretching induced changes in the propagation of the motor unit action potential might explain the changes in the dynamics of RF and VM after stretching. It was also proposed that FD might be used as a discriminating feature to detect the improvements in the dynamics of RF and VM after stretching, as a physiotherapy intervention.