LAUSR

The objective of the current study was to assess sodium (23Na) and quantitative proton (1H) parameter changes in muscle tissue with magnetic resonance imaging (MRI) after eccentric exercise and in delayed‐onset muscle soreness (DOMS). Fourteen participants (mean age: 25 ± 4 years) underwent 23Na/1H MRI of the calf muscle on a 3‐T MRI system before exercise (t0), directly after eccentric exercise (t1), and 48 h postintervention (t2). In addition to tissue sodium concentration (TSC), intracellular‐weighted sodium (ICwS) signal was acquired using a three‐dimensional density‐adapted radial projection readout with an additional inversion recovery preparation module. Phantoms containing saline solution served as references to quantify sodium concentrations. The 1H MRI protocol consisted of a T1‐weighted turbo spin echo sequence, a T2‐weighted turbo inversion recovery, as well as water T2 mapping and water T1 mapping. Additionally, blood serum creatine kinase (CK) levels were assessed at baseline and 48 h after exercise. The TSC and ICwS of exercised muscles increased significantly from t0 to t1 and decreased significantly from t1 to t2. In the soleus muscle (SM), ICwS decreased below baseline values at t2. In the tibialis anterior muscle (TA), TSC and ICwS remained at baseline levels at each measurement point. However, high‐CK participants (i.e., participants with a more than 10‐fold CK increase, n = 3) displayed different behavior, with 2‐ to 4‐fold increases in TSC values in the medial gastrocnemius muscle (MGM) at t2. 1H water T1 relaxation times increased significantly after 48 h in the MGM and SM. 1H water T2 relaxation times and muscle volume increased in the MGM at t2. Sodium MRI parameters and water relaxation times peaked at different points. Whereas water relaxation times were highest at t2, sodium MRI parameters had already returned to baseline values (or even below baseline values, for low‐CK participants) by this point. The observed changes in ion concentrations and water relaxation time parameters could enable a better understanding of the physiological processes during DOMS and muscle regeneration. In the future, this might help to optimize training and to reduce associated sports injuries.