The Price of Force: Motor Unit Adaptation Mechanisms in Pain Versus Fatigue

Pain and fatigue are known to interfere with force production and control, although the underlying mechanisms are not fully understood. This study aimed to investigate the effects of pain and fatigue on motor neuron activity by analyzing the electrical activity of individual motor units in the vastus lateralis and rectus femoris muscles during knee extension exercises.

Electromyography decomposition was used to assess motor unit activity. Pain was induced using a 6% hypertonic saline injection, while fatigue was elicited through repeated maximal concentric/eccentric contractions. Motor units were clustered into two groups (G1 and G2) based on firing rate, recruitment threshold, and motor unit action potential (MUAP) amplitude. Maximal voluntary isometric torque production and motor unit activity were evaluated during the tasks.

Both pain and fatigue significantly reduced maximal voluntary isometric torque production. Pain increased the firing rate of G1 motor units in the rectus femoris (p = 0.033) but decreased the recruitment threshold of G2 motor units in the same muscle (p = 0.004). Conversely, fatigue decreased the firing rate of G2 motor units in the vastus lateralis (p = 0.002) while decreasing the recruitment threshold of G1 motor units in the same muscle (p = 0.022). Pain primarily affected torque production during the eccentric phase, whereas fatigue predominantly impacted the concentric phase.

Despite some shared effects on maximal isometric force, pain and fatigue are managed by distinct, complex, muscle-specific neural strategies involving the reorganization of motor unit recruitment and firing. Understanding these specific adaptations is crucial for elucidating pain- and fatigue-related motor dysfunction. However, interpretations should consider that these findings are based on acute experimental models and may not fully generalize to chronic conditions or different tasks.