Neuromuscular versus Mechanical Stretch-induced Changes in Contralateral versus Ipsilateral Muscle

Area of Science:

• Neuromuscular Physiology
• Biomechanics
• Exercise Science

Background:

• The effects of passive stretching on muscle force production are well-documented.
• However, the specific neuromuscular and mechanical factors contributing to force reduction in both the stretched muscle (SM) and the homologous contralateral muscle (CM) remain incompletely understood.

Purpose of the Study:

• To investigate the neuromuscular and mechanical factors responsible for stretch-induced force reduction in both the stretched muscle (SM) and the homologous contralateral muscle (CM).
• To determine if the contralateral muscle also experiences a reduction in force following passive stretching of the ipsilateral limb.

Main Methods:

• Twenty-one participants underwent unilateral knee extensors passive stretching.
• Measurements of maximum voluntary contraction (MVC), peak force (pF), and voluntary activation (VA) were taken before, immediately after, and at 5 and 10 minutes post-stretching for both CM and SM.
• Electromyographic (EMG) and mechanomyographic (MMG) root mean square (RMS), M-wave, and electromechanical delay (EMD) were assessed during MVC to analyze neuromuscular and mechanical responses.

Main Results:

• The contralateral muscle (CM) exhibited an immediate decrease in MVC, voluntary activation (VA), EMG RMS, and MMG RMS, along with an increased time delay between EMG and MMG (Δt EMG-MMG), with all measures returning to baseline within 5 minutes.
• The stretched muscle (SM) showed a significant reduction in MVC, pF, VA, and EMG RMS, accompanied by an increase in MMG RMS and total electromechanical delay (EMD), effects which persisted for at least 10 minutes post-stretching.
• No changes in the M-wave were observed in either muscle group, indicating that the observed force reductions were not due to alterations in motoneuron excitability or muscle fiber conduction velocity.

Conclusions:

• Contralateral force reduction following passive stretching appears to be primarily mediated by a decrease in central motor drive.
• In contrast, force reduction in the stretched muscle is attributed to a combination of central inhibition and altered mechanical properties.
• These findings highlight distinct neuromuscular and mechanical adaptations occurring in the ipsilateral and contralateral limbs after passive stretching.