REF-314 · Chalchat E, Gennisson JL, Penailillo L, Oger M, Malgoyre A, Charlot K, Bourrilhon C, Siracusa J & Garcia-Vicencio S, 2020 · Frontiers in Physiology, · Paper · PubMed
Changes in the Viscoelastic Properties of the Vastus Lateralis Muscle With Fatigue
After 6 × 10 maximal 5-s isometric knee extensions in 12 young men, torque fell ~37%, resting VL shear modulus fell 57.9% and viscosity rose 179%; viscosity and active stiffness were back to baseline at 20 min, resting shear modulus was not
Participants
n=12 young males (29.0 ± 4.5 years)
Protocol
Intermittent voluntary fatigue protocol: 6 sets × 10 repetitions of 5-s maximal voluntary isometric knee extensions with 5-s passive recovery between repetitions
Voluntary and evoked torque assessed before, immediately after and 20 min after exercise
VL shear modulus (μ) estimated at rest and during a ramped isometric contraction (conventional elastography); an index of active muscle stiffness = slope of shear modulus vs absolute torque
Resting viscosity (η) from shear-wave dispersion (shear-wave spectroscopy)
Key Results
Voluntary and evoked torque decreased by ~37% (P < 0.01) immediately after exercise
Resting VL shear modulus was lower at the end of the protocol (−57.9 ± 5.4%, P < 0.001); resting viscosity increased (+179.0 ± 123%, P < 0.01)
Active muscle stiffness index also decreased with fatigue (P < 0.05)
By 20 min post-fatigue, viscosity and the active stiffness index were no longer different from pre-exercise values — but the resting shear modulus still was
Mechanism
The authors: fatigue left the VL "more compliant and viscous" — greatly reduced shear modulus and greatly enhanced viscosity. Inference, not tested: the resting-modulus deficit outlasting the recovery of viscosity and active stiffness suggests that a component of the tissue's passive state recovers more slowly than its contractile/functional state; the abstract does not identify the structures involved.
Practical Application
In 12 young men, 60 maximal 5-s isometric knee extensions cut torque by ~37%, cut resting VL stiffness by 57.9% and raised viscosity by 179%; 20 min later viscosity and active stiffness had normalised while resting stiffness had not — passive tissue state lagged behind the functional measures
The heavy, sluggish feeling after a hard session has a measurable tissue correlate here: a more viscous, more compliant muscle, not a "tight" one
Inference, not tested: force-producing ability recovering before passive tissue properties would mean an athlete can be functionally ready for a second session while the muscle's resting state is still altered; this study followed recovery only to 20 min and measured no second bout
Not from this paper; the base's own practice: this is one argument for spacing two hard sessions on one day by several hours rather than back to back; the study does not test double sessions
Limitations
Isometric fatigue protocol — not endurance-specific
Only 20 min of recovery tracked — longer timeline unknown
Young non-athletes, men only, n=12
How this entry was checked and changed
Why this level
regraded 2026-09-27 from B+: uncontrolled pre-post, n=12
Content correction 2026-09-27
Added the abstract's torque result (−37%); the "double-threshold days" implication relabelled as untested inference / house practice.
Level B+ -> B (uncontrolled pre-post, n=12); Sport "cycling" -> general (isometric knee extension, not cycling).