Acute effects of resistance exercise on the muscle architecture and tissue organization of the maximal isometrically contracted and uncontracted gastrocnemius medialis muscle

This study aimed to investigate the acute effects of resistance exercise on muscle architecture and tissue organization at rest and during isometric maximum voluntary contraction (iMVC) of the gastrocnemius medialis muscle. In 16 healthy adults (25 ± 3 y), ultrasound scans were captured before (pre), immediately after (post), and 15 min after (post_15) single-leg calf raises during rest and iMVCs. Measurements included muscle thickness (MT), pennation angle (PA), and fascicle length (FL). Spatial frequency analysis (SFA) assessed tissue organization (e.g. peak spatial frequency radius (PSFR), quality factor (Q6)). MT increased significantly from pre to post (+5.7%) and post_15 (+5.6%) at rest. During iMVC, MT and FL increased significantly from pre to post (MT: +8.1%, FL: +14.9%) and post_15 (MT: +5.2%, FL: +10.5%), while PA decreased significantly from pre to post_15 (-10.2%). PSFR and Q6 decreased significantly from pre to post (-7.0-8.2%) and increased from post to post_15 (+10.3-10.6%) at rest, with no significant changes during iMVC. Resistance exercise led to altered muscle architecture for more than 15 min, while SFA parameters reverted to baseline within 15 min. Thus, SFA potentially enables early differentiation of potential physiological alterations in muscle micro-morphology.