Eccentric contraction causes excessive mechanical strain on muscle fibres and produces so-called Exercise Induced Muscle Damage (EIMD). EIMD causes a reduction in skeletal muscle force production but the effect of EIMD on muscle mechanical properties is still poorly understood. The only previous Magnetic Resonance Elastography (MRE) study of EIMD was in 8 subjects who performed moderate exercise affecting the lower leg. Region of Interest (ROI) analysis revealed a significant increase (21%) in the storage modulus (G′) of gastrocnemius muscle but no significant change was reported in soleus [1]. We used Super-Resolution (SR) MRE [2] to extend an initial ROI analysis of data obtained in a previous study [3] so as to obtain detailed measurements of changes in thigh muscle viscoelastic properties caused by EIMD induced using a well established muscle damage protocol. SR-MRE is made possible by analysing Multi-frequency MRE (MMRE) images in a manner whereby multiple low-resolution images of the same scene are interpolated and fused to create a single, high-resolution image.Twenty healthy male subjects (mean age 24.1± 4.3yrs) were recruited. For each subject a work target was set based on the peak values of concentric and eccentric force that could be generated by the quadriceps muscle measured using a BioDex isokinetic dynamometer. Subsequently, to produce EIMD each subject performed 12 sets of eccentric contractions until the established work target was achieved and the effect on Maximum Voluntary Contraction (MVC) was measured at baseline, and at 2, 3, 5 and 7 days after the muscle damage protocol. MR data were acquired before and 2 days after the muscle damage protocol using a 3T Siemens Verio MRI system (Siemens Medical Systems, Erlangen, Germany) equipped with a 32 channel receiver coil. In particular, a spin echo EPI sequence was used to acquire 3D multi-frequency MRE data with acquisition parameters, TE = 1600 ms, TR = 54 ms and 8 phase offsets. For each frequency of 25, 37.5, 50 and 62.5 Hz acquisition time was 1 min 6 sec. MRE data analysis was performed using Multi-Frequency Dual Elastovisco Inversion (MDEV) running in the Elastography Software Pipeline (ESP) software [2]. High resolution images of the Magnitude of the Complex Modulus |G*| and Phase Angle of the Complex Modulus f were obtained for each subject, and image co-registration techniques were used to produce images of the average value of, and changes in, |G*| and Φ [4].

Eighteen of the 20 subjects successfully completed the muscle damage protocol. Two days after the muscle damage protocol, MVC was significantly reduced by an average of 27% (p<0.001) and pain was significantly increased by an average of 220% (p<0.01). High-resolution images of the average value, and changes in, |G*| and Φ in the 18 subjects with EIMD are shown in Figure 1.

The high-resolution images of |G*| and f show fine detail and with the boundaries between individual muscles clearly demarcated. The images offer the opportunity to study the precise patterns of damage in individual subjects and group average effects after image co-registration. Rectus Femoris (RF) and Vastus Intermedius (VI) muscle groups showed significant increase in |G*| and with no change in Φ.

This study has shown that muscle tissue is well suited to study using SR-MRE and that the site of muscle damage can be clearly identified suggesting potential useful clinical applications for the technique. The Super Resolution multi-parameter (i.e. |G*| and Φ.) measurements of muscle viscoelastic properties can potentially provide insight regarding the physiological mechanisms underlying tissue damage in EIMD and their functional effect.