Electrically-stimulated contractions

Electrically-stimulated contractions are not a hugely common topic of discussion in the fitness industry, but they are in occasional use. Importantly, there are certain features of electrically-stimulated contractions that can teach us about how muscle physiology works.

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#1. Electrically-stimulated, submaximal isometric contractions at moderate muscle lengths can cause muscle damage

It is frequently assumed that muscle damage can only happen after training with eccentric contractions or isometric contractions at long muscle lengths because these are the types of exercise that involve forcefully stretching muscle fibers. This follows from the assumption that muscle damage is caused by mechanical tension. Yet, research into electrically-stimulated contractions shows that this cannot possibly be the case. When electrical stimulation is used, muscle damage can occur after very low-force isometric contractions at moderate muscle lengths. This tells us that muscle fibers are not damaged by physically pulling them apart.

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Indeed, there is good evidence that muscle damage is actually caused by a simple biochemical process that results from calcium ion accumulation in fast twitch muscle fibers. Electrical stimulation likely allows more fast twitch fibers to be activated earlier than in voluntary contractions because the fibers are activated simply by virtue of their proximity to the stimulating electrode instead of where they are in sequence according to Henneman's size principle (read more).

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#2. Post-workout muscle damage is greater after electrically-stimulated contractions but fatigue is not

A number of studies have now shown that post-workout muscle damage is greater after eccentric training workouts that involve involuntary (electrically-stimulated) contractions in comparison with those that involve voluntary contractions. Yet, the amount of post-workout fatigue is not greater after involuntary contractions. 

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The presence of greater muscle damage after involuntary contractions compared to after voluntary contractions has been confirmed by multiple measurements, including direct assessments of sarcomere disruptions by electron microscope. This finding indicates that muscle damage plays a smaller role in producing fatigue after a workout than excitation-contraction coupling failure and CNS fatigue.

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Indeed, there is good evidence that excitation-contraction coupling failure is actually the primary mechanism of post-workout fatigue and it is certainly the most important fatigue mechanism in the first few hours (read more).

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#3. Electrically-stimulated contractions cause a repeated bout effect

Although there is some evidence for the repeated bout effect crossing over between limbs of the body, the presence of a pronounced repeated bout effect after training with involuntary contractions tells us that local muscular adaptations must contribute to the repeated bout effect phenomenon.

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Indeed, there is good evidence that peripheral adaptations like sarcomerogenesis and rapid muscle fiber type shifts are implicated in the repeated bout effect since they reduce the accumulation of calcium ions inside the cytoplasm (read more).

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#4. Electrically-stimulated contractions cause post-workout CNS fatigue

It is frequently assumed that post-workout CNS fatigue is caused by workouts that involve high levels of central motor command. However, post-workout CNS fatigue can also be caused by involuntary contractions, which invalidates this hypothesis.

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Indeed, there is good evidence that the inflammatory response that occurs after muscle-damaging exercise is the primary factor that is responsible for the CNS fatigue that occurs during the post-workout period (read more).

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Conclusions

Research into involuntary (electrically-stimulated) contractions can provide us with valuable insights. Firstly, it has shown that involuntary, submaximal isometric contractions at moderate muscle lengths can cause muscle damage. This tells us that muscle damage is not caused by forcefully pulling muscle fibers apart (indeed, it is instead caused by calcium ion accumulation). Secondly, it has shown that muscle damage (but not fatigue) is greater after involuntary contractions than after voluntary efforts. This suggests that muscle damage likely plays a smaller role in causing fatigue after workouts compared to other fatigue mechanisms (indeed, excitation-contraction coupling failure is likely the most important). Thirdly, it has shown that involuntary contractions cause a repeated bout effect. This tells us that the repeated bout effect probably involves peripheral, muscular adaptations (indeed, it is likely caused by an increased ability to handle calcium ion accumulation). Fourthly, it has shown that involuntary contractions cause post-workout central nervous system (CNS) fatigue. This tells us that post-workout CNS fatigue cannot be related to the production of high levels of central motor command during the workout (indeed, it is likely caused by the inflammatory response to muscle damage causing an increase in the perception of effort in the brain).​