Muscle alterations induced by electrostimulation are lower at short quadriceps femoris length

This study aimed at determining through MRI investigations, force and soreness assessments whether the modulation of muscle length is a relevant strategy for minimising neuromuscular electrical stimulation (NMES)-induced muscle damage in young healthy participants.

Comparison of 2 NMES sessions (40 isometric electrically-evoked contractions of the knee extensors) was randomly performed on 1 knee flexed at 50° (short muscle length) and the contralateral at 100° (long muscle length) in a single group of healthy participants. Indirect markers of muscle damage including changes in maximal voluntary isometric contraction (MVC) force, muscle volume and transverse relaxation time (T₂) were measured before, 2 days (D2), 4 days (D4) and 7 days (D7) after the NMES sessions in each limb of the ten participants.

Although stimulation intensity was similar during the NMES session on both limbs, significantly lower force production was recorded at long muscle length (peak at 30 ± 5% MVC force) as compared to short muscle length (peak at 61 ± 12% MVC force). In the following days, MVC force at long muscle length was decreased from D2 to D7, whereas no significant change occurred at short muscle length. Increases in muscle volume and T₂ were found at each time point in stimulated muscles at long muscle length, whereas no change was found at short muscle length.

For the same stimulation intensity, NMES-induced isometric contractions generate higher knee extension force output and result in lower muscle tissues alterations that could be related to a lower intramuscular shear strain when exercise is performed at short muscle length.