Experimental muscle pain increases normalized variability of multidirectional forces during isometric contractions

Area of Science:

• Neuromuscular Physiology
• Motor Control
• Pain Research

Background:

• Muscle pain significantly alters motor strategies, impacting the ability to generate and control steady forces.
• Muscle architecture plays a crucial role in force production and stability, which can be compromised by pain.

Purpose of the Study:

• To investigate the effects of experimental muscle pain on the stability of multidirectional forces during sustained isometric contractions.
• To assess how pain influences task-related and tangential force variability and force angle during dorsiflexions, elbow flexions, knee extensions, and plantarflexions.

Main Methods:

• A within-subjects, cross-over design was employed with 15 healthy participants.
• Isometric contractions at various force levels (2.5–70% maximal voluntary force) were performed before, during, and after pain induction via hypertonic saline injection.
• Three-dimensional force output and surface electromyography (EMG) of agonist and antagonist muscles were measured.

Main Results:

• Experimental muscle pain led to increased force angle ranges in knee extensions and plantarflexions.
• Higher normalized fluctuations in both task-related and tangential forces were observed during pain.
• Mean force magnitudes and overall muscle activity levels remained largely unaffected by pain.

Conclusions:

• Muscle pain induces motor adaptations that increase multidirectional force fluctuations, impairing force steadiness.
• Despite aiming to protect the painful site, these adaptations compromise precise force control.
• The findings highlight a trade-off between protective responses and motor performance under painful conditions.