Variability of three-dimensional forces increase during experimental knee pain

Area of Science:

• Biomechanics
• Motor Control
• Pain Research

Background:

• Knee pain impacts muscle function and force generation.
• Existing research on pain's effect on force variability is limited to single-directional assessments at moderate forces.

Purpose of the Study:

• To investigate the effects of experimental knee pain on three-dimensional (3D) force variability during isometric knee extensions.
• To assess force variability across a wide range of contraction intensities (2.5-80% of maximal voluntary contraction, MVC).

Main Methods:

• Healthy subjects underwent isometric knee extensions before and after infrapatellar fat pad injections (hypertonic saline for pain, isotonic for control).
• A six-axis force sensor measured force magnitude, direction, and variability.
• Surface electromyography (sEMG) recorded quadriceps and hamstring muscle activity.
• Pain intensity was rated using a visual analogue scale (VAS).

Main Results:

• Experimental knee pain significantly increased the variability of the task-related force component at all tested force levels.
• Variability of tangential force components increased specifically at low contraction forces (≤5% MVC).
• Mean quadriceps activity decreased only at high contraction forces (80% MVC) during painful conditions.

Conclusions:

• Experimental knee pain leads to increased 3D force variability, suggesting a less efficient motor strategy.
• Increased variability at low forces may involve muscle activity reorganization not detectable by surface EMG.
• Pain relief could potentially enhance motor control and training for individuals with knee pain.