Intramuscular pressure and surface EMG in voluntary ankle dorsal flexion: Influence of elastic compressive stockings

Intramuscular pressure (IMP) is of major importance in blood flow and is often taken as a good estimate of muscular tension. However, its measurement remains invasive. The aims of the present work were: (1) to re-examine the possibility of evaluating IMP and muscular tension changes by means of surface electromyographic recordings, and (2) to clarify the influence of elastic compressive stockings (ECS). Surface EMG of muscles tibialis anterior (TA), soleus, gastrocnemius, and IMP from the anterior tibial compartment (ATC), deep posterior compartment (DPC), superficial posterior compartment (SPC) of the right leg, were simultaneously recorded in nine healthy subjects. Subjects performed series of voluntary concentric TA contractions (right ankle dorsal flexions) and TA isometric contractions, with or without elastic ECS, in a decubitus posture. Rest IMP mean values, measured over 60 s, ranged between 12.3 and 26.6 mmHg, i.e. in the range or slightly higher than those reported in the literature. When ECS were applied, mean IMP increase was 6.4 mmHg in ATC, 8.7 mmHg in DPC and 21.0 mmHg in SPC, while the corresponding EMG amplitude decreased. In ankle dorsal flexion movements, instantaneous values of TA-EMG amplitudes were linearly correlated to ATC-IMP instantaneous values, over the whole of the EMG rising part of every movement. When ECS were applied, the relationships between TA-EMG amplitude and ATC-IMP amplitude remained linear but where shifted towards higher IMP, in agreement with the increase in rest IMP. Because of antagonist co-contractions, IMP from DPC and SPC were also linearly correlated with ATC-IMP but with low coefficients of proportionality. As in TA concentric contractions, TA-EMG amplitudes were linearly correlated to ATC-IMP instantaneous values in isometric contractions, but the slopes of the latter were always greater. This result is explained by the relationship between muscle tension and shortening velocity. Al the results showed that: (1) instantaneous changes in surface EMG amplitude may provide a good estimate of IMP changes during the rising part of isometric, but also of concentric voluntary contractions; (2) elastic compressive stockings do not impair subjects relaxation capacity but actually increase the ratio IMP/muscle activation. As a consequence, ECS may actually increase the venous return during voluntary contractions.