Background Appropriate levels of muscle strength, muscle stiffness (MS) and musculoarticular stiffness (MAS) are important factors linked to athletic performance. Alterations in knee joint mechanical behavior induced by neuromuscular fatigue may have implications for injury susceptibility.

Objective To investigate the acute effects of fatigue on peak torque of the knee joint extensor musculature, MS of the vastus lateralis and MAS of the knee joint in young male athletes.

Design Experimental study.

Setting University biomechanics laboratory.

Participants 27 male recreational athletes.

Interventions A 15-minute cycle ergometer fatiguing protocol.

Main outcome measurements Peak torque of the knee joint extensor musculature was assessed on a leg-extension dynamometer, whilst MS of the vastus lateralis was measured using a myometer (Müomeetria AS, Tallinn, Estonia) in both relaxed and contracted conditions. MAS was evaluated by the free oscillation technique on a leg-extension dynamometer. These measures were performed before and after the fatiguing intervention.

Results A significant decrease in peak torque was observed from pre-fatigue (208.2±47.1 Nm) to post-fatigue (189.3±52.5 Nm) (P<.01). Absolute MS was significantly increased from pre-fatigue (362.7±49.4 Nm⁻¹ – relaxed; 493.6±71.2 Nm⁻¹ – contracted) to post-fatigue (454.3±82.8 Nm⁻¹ – relaxed; 551.3±89.7 Nm⁻¹ – contracted) (P<.01). Normalized MS (MS/load supported) was significantly increased from pre-fatigue (26.4±6.9 Nm⁻¹·kg⁻¹) to post-fatigue (230.8±6.5 Nm⁻¹·kg⁻¹) (P<.01). MAS was decreased significantly from pre-fatigue (1518.8±502.6 Nm⁻¹) to post-fatigue (1421.6±445.7 Nm⁻¹) (P<.05). However, for normalized MAS (MAS/load supported) no significant difference was observed (P=.06).

Conclusions It is hypothesized that the observed increase in MS may be a compensatory mechanism to offset the fatigue induced decrease in peak torque and MAS, with the aim of preserving knee joint stability.