Spatial difference can occur between activated and damaged muscle areas following electrically‐induced isometric contractions

Abstract

Key points T2 mapping combined to image registration and statistical parametric mapping analysis is a suitable methodology to accurately localize and compare the extent of both activated and damaged muscle areas. Activated muscle areas following electrically‐induced isometric contractions are superficial, but damaged regions are muscle specific and can be related to the muscle morphology and/or the relative spatial position within a muscle group leading to potential intramuscular muscle shear strain. Tissues other than active skeletal muscle fibres can be altered during unaccustomed neuromuscular electrical stimulation‐induced isometric contractions. Abstract Skeletal muscle isometric contractions induced by neuromuscular electrical stimulation (NMES) exercise can generate damage within activated muscles. This study aimed at comparing the localization and the extent of NMES‐activated muscle areas and induced damage regions using magnetic resonance imaging. Thirteen healthy subjects performed a single bout of NMES‐induced isometric contractions known to induce a decrease in maximal voluntary isometric contraction (MVC) and increase in muscle volume and transverse relaxation time (T2). All the parameters were measured before, immediately after (POST), 7 days (D7), 14 days (D14) and 21 days (D21) after the NMES session. Spatial normalization of T2 maps were performed to compare the localization of muscle activation areas and damaged muscle regions from statistical mapping analyses. A significant decrease in MVC was found at POST (−26 ± 9%) and in delayed time at D7 (−20 ± 6%) and D14 (−12 ± 5%). Although muscle activation was statistically detected through T2 increase at POST in superficial parts of the two muscles located beneath the stimulation electrodes (i.e. vastus lateralis and vastus medialis), alterations quantified in a delayed time from increased T2 were mainly located in the deep muscle region of the vastus lateralis (+57 ± 24% of mean T2) and superficial area of the vastus medialis (+24 ± 16% of mean T2) at D7 and were still observed in whole muscle at D21. The discrepancy between activated and damaged areas in the vastus lateralis implies that tissues other than active skeletal muscle fibres were altered during unaccustomed NMES‐induced isomeric contractions.

Regional changes in muscle activity do not underlie the repeated bout effect in the human gastrocnemius muscle

“…These changes in MG EMG distribution also imply that increased levels of muscle activation are needed to maintain a constant force output (ie, 60% of MVC at IB PRE) at 2H and 2D. While a conclusive explanation for this finding cannot be given, damage in muscle structural elements (eg, connective tissue, extracellular matrix), 34 changes in muscle plasmalemma, 35 and/or a long‐lasting muscle neuromuscular fatigue (central and peripheral) 20 could explain the need of an increased neural drive to the muscle after the initial bout of exercise. These factors combined might favor a process of mechanical fatigue (ie, failure), understood as the weakening of the material properties of the muscle caused by the cyclic loading of the exercise task that results in progressive and localized muscle damage.…”

“…Rather than a localized distribution of mechanical hyperalgesia, our topographical maps suggest a generalized blunt response that peaks 2D after the IB. Since significant muscle fiber damage is unlikely in the MG, 6‐8 the extensive mechanical hyperalgesia may be the result of a global phenomenon of prolonged stimulation of nociceptors, mediated by the accumulation of chemicals 36 and/or damage within the passive structural components within the muscle (eg, connective tissues, costameres) 34,37 that can occur during intense eccentric exercise. However, the homogeneity in the mechanical hyperalgesia may also be due to a limited area of muscle being covered in the PPT distribution.…”

Utility of Neuromuscular Electrical Stimulation to Preserve Quadriceps Muscle Fiber Size and Contractility After Anterior Cruciate Ligament Injuries and Reconstruction: A Randomized, Sham-Controlled, Blinded Trial

“…Indeed, one could speculate that NMES might modulate the fluid content of muscle in patients, with effects in either direction: reducing the fluid content via rhythmic contractions or increasing tissue fluid through contraction-induced muscle damage. 9 Examples of this potential bias are shown in disagreements between whole muscle and cellular muscle size estimates in studies evaluating the effects of NMES in orthopaedic and nonorthopaedic populations, 8,10,24 including the current study. Because of this potential bias, we urge caution in the interpretation of whole muscle size data and suggest single muscle fiber assessments as a more rigorous approach to evaluate the effects of NMES, particularly during the early postsurgical period.…”

Side effects of and contraindications for whole-body electro-myo-stimulation: a viewpoint

“…To increase knowledge on the potential risks and effects of WB-EMS, we propose the following research priorities: Research about muscle damage as a side effect of WB-EMS should consider current types applied, stimulation frequency and risk factors for rhabdomyolysis. Research about WB-EMS should include longitudinal muscle force measurements, the best indirect marker of muscle damage 9. Side effects of WB-EMS on other organ systems should be investigated, also by applying MRI which has been, so far, only applied in single-muscle EMS 10.…”

“…It has been shown, however, that single-muscle EMS induces muscular damage which manifests with muscle force reduction up to −20% 4 respectively 7 days afterwards 9 10. Additionally, findings from MRI suggest that tissues other than muscles, such as connective tissues and/or intracellular structures could be altered during single-muscle EMS-induced contractions 10…”