Spatial decrease in muscle activation induced by acute passive static stretching: a possible neuromuscular compartmentalization of the gastrocnemius muscle

Abstract

Recently, spatial adaptation of muscle excitability during sustained muscle voluntary contractions (MVC) up to fatigue has been reported, suggesting a functional muscle compartmentalization. Acute passive stretching (PS) can induce a reduction in surface electromyography (sEMG) amplitude; however, it is not clear whether PS may decrease motor-unit activity heterogeneously, similar to what occurs with fatigue. Therefore, the aim of the study was to evaluate possible heterogeneity in muscle activation following PS intervention in the gastrocnemius muscle. Thirty healthy men (age: 24±2 yrs., stature: 1.78±0.07 m, body mass: 77±6 kg) underwent a 5-min PS intervention (5 x 45s-on/15s-off cycles). Before (PRE), immediately after (POST) and after 5 (POST5) and 10 min (POST10), participants performed MVCs of the plantar-flexor muscles. Spatial muscle activation was recorded by means of high-density sEMG (HD-sEMG) in the gastrocnemius medialis and lateralis of the stretched and of the contralateral, non-stretched limb. The HD-sEMG root mean square (RMS) and its centroid were calculated. During stretching, the fascicle length and pennation angle (ultrasound approach) were measured for the proximal and distal portion of both muscles, together with the perceived level of discomfort (0-10 visual analogue scale). HD-sEMG RMS was reduced in POST, POST5 and POST10 in both gastrocnemius medialis (p<0.05 in all) and lateralis (p<0.05 in all) of the stretched limb, while the centroid shifted toward a more caudal direction (p<0.05). Interestingly, the same behaviour was found in the contralateral limb in POST for both muscles (both p<0.05). No differences were found in control. During stretching, no differences were found in fascicle length and pennation angle between the proximal and distal portion, whereas a higher discomfort was perceived in the distal portion of both muscles (p<0.05 in all). The uneven spatial decrease in muscle activation following acute PS suggests differences in neuromuscular organization of the gastrocnemius muscle. Possibly, a different discomfort perception between the proximal and distal portion could have induced a different inhibitory effect. The study was funded by a grant of the Universitnà degli Studi di Milano This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.