Heterogenous quadriceps femoris inter-head but consistent intra-head rigidity changes after voluntary eccentric exercise-induced muscle damage.

OBJECTIVES: Maximal voluntary eccentric contractions can generate damage within the activated skeletal muscles. Intramuscular tissue alterations have been reported within the quadriceps femoris. This study aimed at comparing the quadriceps femoris inter-head and intra-head changes in resting mechanical properties with shear wave elastography. METHOD: Nine healthy males performed 120 maximal isokinetic eccentric contractions of the knee extensors at 60°/s, designed to induce a decrease in isometric maximal voluntary contraction (iMVC) torque and to generate muscle soreness. Resting shear wave velocity (SWV, an index of tissue rigidity) was assessed in both proximal and distal parts of the vastus lateralis (VL), vastus medialis (VM), vastus intermedius (VI) and rectus femoris (RF). All parameters were measured before, immediately after (POST), one day (D1), two days (D2) and three days (D3) after exercise-induced muscle damage. RESULTS: Muscle soreness was significantly increased at POST, D1 and peaked at D2. iMVC torque was significantly diminished at POST ( – 25%) with a slight recovery at D2 ( – 19%) but was still reduced at D3 ( – 14%). Increases in SWV were found at POST in the VL, VM, VI, and RF (i.e., from + 14% to + 31% at long muscle length) but remained homogeneous across the different regions within each head. Only the increase in RF resting SWV was significantly prolonged until D3 (+ 8%). CONCLUSIONS: A specific increase in the rigidity of the bi-articular head of the quadriceps femoris at rest was observed, without any significant differences between proximal and distal parts. Further investigations near the myotendinous junction are needed to clarify the potential involvement in musculotendinous injuries.