Lack Of Association Between Hamstrings-to-quadriceps Co-activation And Force Control Across Varying Contraction Intensities

The interaction of agonist-antagonist musculature (co-activation) is essential for joint stability and coordinated force production. The regulation of co-activation is governed by muscular and neural mechanisms essential for efficient motor control. However, the modulation of co-activation with graded contractions is unclear, which is relevant to everyday and athletic tasks. PURPOSE: Assess the impact of knee extension contraction intensity on hamstrings-to-quadriceps co-activation and to determine its association with force steadiness. METHODS: Twelve uninjured, physically active individuals (8 females, 22.3 ± 2.7 years) performed dominant-limb knee extensions at 30, 50, and 70% of their maximum voluntary isometric contractions (MVIC) following a trapezoidal pattern with visual feedback. Quadriceps and hamstrings electromyography (EMG) were recorded to assess co-activation and EMG amplitudes. One-way repeated measures ANOVA compared co-activation and EMG amplitudes across intensities. Bivariate correlation examined associations between co-activation and force coefficient of variation (CV). RESULTS: Co-activation decreased at greater contraction intensities (30 < 50 < 70% MVIC) (Figure 1A). Quadriceps EMG amplitude increased with each intensity, while hamstrings EMG amplitude increased from 50 > 30 and 70 > 50% MVIC (Figure 1B). Co-activation did not associate with force CV at any intensity (all p > .05, r = -.36-.53). CONCLUSIONS: Reductions in hamstrings-to-quadriceps co-activation at greater contraction intensities are driven by increased quadriceps activity and suggest the neuromuscular system may optimize force production by reducing unnecessary antagonist muscle activity. The lack of association between co-activation and force CV indicate that force steadiness is maintained independently of neuromuscular strategies which may inform the assessment of knee-injured patients, who often show increased co-activation.