Prolonged passive static stretching-induced innervation zone shift in biceps brachii

Area of Science:

• Biomechanics
• Human Physiology
• Electromyography

Background:

• Passive static stretching is a common practice in sports and rehabilitation.
• Understanding its effects on muscle properties, such as innervation zone (IZ) location, is crucial for accurate physiological assessments.
• Surface electromyography (EMG) is widely used to evaluate muscle activation, but its reliability can be influenced by changes in muscle anatomy.

Purpose of the Study:

• To investigate the impact of repeated, prolonged passive static stretching on the innervation zone (IZ) location of the human biceps brachii muscle.
• To assess changes in isometric strength following the stretching intervention.
• To determine if the IZ shift is dependent on contraction intensity.

Main Methods:

• Eleven male participants underwent a stretching protocol involving 12 sets of 100-second passive stretches on their biceps brachii.
• Isometric strength was measured via maximal voluntary contractions (MVCs) before and after stretching.
• Innervation zone (IZ) location was identified using multi-channel surface EMG recordings from a linear electrode array during submaximal and maximal contractions.

Main Results:

• A significant average loss of 10% in isometric strength was observed post-stretching (p<0.001).
• An average proximal shift of approximately 4.5 mm in the innervation zone (IZ) was recorded.
• The observed IZ shift was consistent across different contraction intensities (30%, 50%, and 70% MVC).

Conclusions:

• Repeated and prolonged passive static stretching can induce a significant proximal shift in the biceps brachii innervation zone (IZ).
• This IZ displacement, likely due to muscle-tendon unit elongation, can impact the accuracy of surface EMG measurements.
• Researchers and clinicians should exercise caution when interpreting EMG data following stretching interventions due to potential IZ contamination.