Fatigue modulates synchronous but not asynchronous soleus activation during stimulation of paralyzed muscle

Clin Neurophysiol, 2013 · DOI: 10.1016/j.clinph.2013.03.027 · Published: September 1, 2013

Simple Explanation

This study investigates how muscle fatigue affects the way paralyzed muscles are activated by electrical stimulation. The researchers looked at two types of muscle activation: synchronous (H-reflex) and asynchronous (random firing). They found that fatigue increased synchronous activation but did not change asynchronous activation, suggesting a consistent way to stress paralyzed tissues.

Study Duration

Not specified

Participants

22 individuals with complete paralysis

Evidence Level

Not specified

Key Findings

1
Fatigue enhanced the activation to the paralyzed soleus muscle, but primarily at the H-reflex latency.
2
Soleus EMG was not elevated during fatigue at latencies consistent with asynchronous activation.
3
During supra-maximal stimulation at 15 Hz, synchronous (H-reflex) activation of the soleus muscle is minimal

Research Summary

The purpose of this study is to investigate whether muscle activity from synchronous or asynchronous sources is modulated during supra-maximal electrical stimulation and muscle fatigue in humans with complete paralysis. The latency after each M-wave that showed the greatest EMG activity was 40–44 ms, consistent with that of the H-reflex. These findings support that during supra-maximal stimulation, fatigue reduces the effectiveness of the refractory period and antidromic collisions in human paralyzed muscle.

Practical Implications

Consistent Muscle Stress

Supra-maximal electrical stimulation at 15 Hz offers a consistent strategy to stress muscle and bone tissue in humans with SCI.

Clinical Protocols

Uniformity and predictability of evoked muscle contractions are important in the clinical use of electrical stimulation protocols.

Understanding Muscle Activation

The study clarifies the roles of synchronous and asynchronous muscle activation during electrical stimulation in paralyzed muscles.

Study Limitations

1
Nonlinearities in torque development were likely introduced by mechanisms peripheral to sarcolemmal propagation (M-wave).
2
The inability to measure these sources of torque nonlinearity is an inherent limitation of any electromyographic investigation of factors that contribute to force.
3
It is possible that motor neuron firings occurred at other latencies that were not measured during the 1.3 seconds between contractions.

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