Muscle injuries are frequently observed in sports, accounting for 10% to 55% of all sustained injuries and often hinder athletes from participation in training or competition (Jarvinen et al., 2013). One of the most common reasons for impaired muscle performance and complaints in sports is delayed onset muscle soreness (DOMS), which results from high-load eccentric muscle contractions or unaccustomed exercises (Hotfiel et al., 2018a). According to the „Munich Consensus Statement “, DOMS is classified as an overexertion-functional muscle disorder type Ib (Mueller-Wohlfahrt et al., 2013). It is associated with damage to skeletal muscle, involving protein degradation and ultrastructural changes (Ulbricht et al., 2015), loss of myofibrillar integrity with Z-band streaming and disruption, increased muscle proteins and enzymes in the bloodstream (Peake et al., 2005), inflammatory processes (Lewis et al., 2012) and changes in microcirculation and vascular permeability (Yanagisawa et al., 2015). DOMS is associated with impaired muscular force capacities and increased soreness, tissue stiffness and swelling, and with altered biomechanics to the adjacent joints (Hotfiel et al., 2018a). The signs and symptoms of DOMS typically begin 6–12 hours after exercise, progressively increase until they reach peak pain levels at 48–72 hours (Pearcey et al., 2015, Valle et al., 2013). Timely diagnosis and exclusion of higher-grade structural damage play essential roles for an appropriate treatment of low-grade muscle injury and the recovery from exercise-induced muscle damage (EIMD) (Hotfiel et al., 2018a). Clinical and laboratory examination in patients with suspected muscle injury are often non-specific regarding the precise extent and severity of the injury (Guermazi et al., 2017, Hotfiel et al., 2018a). Hence, imaging of muscle tissue is essential for providing a correct diagnosis (Draghi et al., 2013, Ekstrand et al., 2016). B-mode ultrasound has been used for approximately three decades to diagnose muscle injuries in sports (Guermazi et al., 2017, Heiss et al., 2023a, Hotfiel et al., 2018a). In low-grade muscle damage such as DOMS, conventional ultrasound is often limited as the affected muscles often appear normal (Guermazi et al., 2017, Heiss et al., 2023a, Hotfiel et al., 2018a). For this reason, magnetic resonance imaging (MRI) has been reported as the preferred imaging modality providing detailed image analysis and characterization of DOMS (Guermazi et al., 2017, Heiss et al., 2023a, Hotfiel et al., 2018a). DOMS can be visualized on MRI as intramuscular edema with high signal changes, affecting one or several muscles (Guermazi et al., 2017, Heiss et al., 2023a, Hotfiel et al., 2018a). However, in case of EIMD leading to DOMS, MRI performed directly after strenuous exercise may reveal negative results as the signal intensity of edema commonly begins to increase during the inflammatory response after 24 hours to 72 hours (Hotfiel et al., 2018a). An imaging approach to show changes in EIMD even earlier may be helpful in training management or for selecting appropriate therapeutic strategies that can modify the known inflammatory cascade in professional sports (Heiss et al., 2019, Hotfiel et al., 2018a). Ultrahigh-field MRI at 7 Tesla (T) may overcome these limitations due to superior visualization of small anatomical structures compared to lower field-strength MRI (Juras et al., 2019). Besides morphologic imaging, magnetic resonance methods such as T1ρ, T2, and T2* mapping, and susceptibility weighted imaging (SWI) substantially benefit from ultrahigh field scanning (Haacke et al., 2009, Juras et al., 2019). T2 mapping has been used excessively for the quantification of intramuscular fluid concentrations in patients following specific exercise protocols for the assessment of muscle injuries and for the quantitative evaluation of muscle tissue in patients with hereditary muscle dystrophies (Heiss et al., 2018a, Hotfiel et al., 2018b, Kim et al., 2010). SWI is an MRI sequence sensitive to compounds that distort the local magnetic field (Haller et al., 2021). It is routinely used for imaging of the central nervous system e.g., for the detection of hemorrhage/blood products within the brain (Haller et al., 2021). SWI is scarcely applied on the musculoskeletal system, however, it can be used to detect hemorrhages or calcium deposits, providing additional information beyond morphological characteristics observed in musculoskeletal lesions (Martin-Noguerol et al., 2021).

We hypothesized that 7 T MRI would allow the direct visualization of morphological alterations in muscle tissue and the detection of micro-bleedings and changes in intramuscular fluid concentration immediately after EIMD that leads to DOMS. Thus, the purpose of this study was to investigate the muscles of the lower leg after induction of DOMS using high-resolution imaging at 7 T MRI, including T2 mapping and SWI.