Tendon healing is the primary goal of tendinopathy therapy. However, the therapeutic response of adjacent muscles remains uninvestigated. MR-elastography (MRE) can assess muscle biomechanical properties and function. This study investigated muscle biomechanical responses in patients treated for patellar (PT) or Achilles tendinopathy (AT). A prospective longitudinal study included 20 athletes, 8 with AT (41 years ± 8; 6 men) and 12 with PT (34 years ± 10; 12 men), who underwent MRI, diffusion-tensor-imaging (DTI), and MRE before and ≥6 months post-treatment. Isotropic (ciso) and anisotropic stiffness parameters (c‖ and c⊥) in tendon-adjacent muscles were derived and analyzed between groups and longitudinally compared with clinical scores and tendon dimensions. At baseline, symptomatic legs with higher tendon dimensions and clinical symptoms exhibited lower muscle stiffness than asymptomatic legs (p<0.05). Post-treatment, AT patients showed increased ciso (4.3±8.5%) in gastrocnemius and posterior soleus (SolP; 4.9±8.6%) in treated legs but decreased SolP stiffness (-3.2±7.2%) in untreated legs (p<0.05). In PT patients, only anisotropic MRE (c‖: 5.3±15.2% and c⊥: 10.3±12.7%) in vastus lateralis showed significant increases (p<0.05) post-treatment. In treated legs of AT and PT, increased muscle stiffness post-treatment correlated negatively with tendon dimensions (p<0.05). Post-therapy, tendon-adjacent muscles exhibited increased stiffness in affected leg and decreased stiffness in contralateral leg (particularly SolP), potentially reflecting altered muscle use and loading, respectively. The observed changes in muscle stiffness may help characterize muscle biomechanical changes during rehabilitation and balanced recovery. Furthermore, anisotropic muscle stiffness was more sensitive to tendon structural changes than isotropic stiffness, warranting further validation for clinical assessment.
