How effective is electrical stimulation in strengthening skeletal muscle? What stimulation parameters appear to be most effective?
Electrical stimulation has become a vital component of rehabilitation strategies to improve skeletal muscle strength after a spinal cord injury, stroke, or other neurological injury. We have a wide variety of clients who use electrical stimulation in various forms, whether as an FES Cycling system or as an adjunct to other forms of exercise. Using electrical energy in clinical applications is not new (it's origins lie in the mid 19th Century), but the subtleties are still poorly understood.
We can apply electrical energy to the body via the skin, which produces physiological effects on the motor and sensory nerves within the muscle tissue. When this is done "artfully", we can create some beneficial therapeutic effects with relatively low risk. Specifically, we want to make muscles contract and relax to increase their strength using electrical stimulation.
Have you wondered how effective it is? How does the nature of the stimulation affect its effectiveness? This article covers those questions.
We most often discuss the forms of electrical stimulation we use with our clients, who typically have neurological injuries such as spinal cord injuries. In this article, we go a bit wider in scope and consider findings from physiological research, clinical trials, and meta-analyses to assess the effectiveness of neuromuscular electrical stimulation (NMES). We also explore the stimulation parameters necessary to gain maximum strength in properly innervated muscles.
We don't cover denervated muscle in this article but will do so in a later article. We also restrict our discussion to transcutaneous electrical stimulation (applied through the skin via skin surface electrodes rather than implanted electrodes).
Mechanisms of NMES-Induced Strength Adaptations
Neuromuscular Recruitment Patterns
As we stated above, electrical stimulation can create physiological and therapeutic effects. We will examine the form of that energy shortly, but we should recognise that as the intensity of energy increases, we will necessarily activate both sensory and motor nerves. Indeed, when sensation is intact, we will produce pain if the intensity is too high.
The nerve activation order depends on the diameter of the nerve fibres and how close the nerves are to the stimulation source. We have various nerve fibre types roughly classified as myelinated and unmyelinated. The nerve fibres responsible for muscle contraction are the myelinated, A-Alpha types, with the largest diameter (around 12 - 20 micrometres).
These nerves have a "resting potential", meaning their normal state is to be charged and ready to fire when triggered. The nerves will be triggered if the membrane potential exceeds a threshold value. This triggering produces an “Action Potential” or nerve impulse, a temporary change of state of the nerve membrane.
This triggering can arise via the typical path of the central nervous system or from outside the body via NMES.
NMES bypasses central motor commands by directly depolarising motor axons, recruiting muscle fibres synchronously and superficially. NMES causes an action potential to propagate along the nerves and produce a muscle contraction via recruited muscle fibres. Muscle fibres have only an on or off state and have various properties affecting their speed of contraction and endurance, which could be described as "fast-twitch" and "slow-twitch". This on-or-off behaviour of fibres is sometimes called the "All or None" principle.
The contraction strength is linked to the number of recruited and active fibres and, when using NMES, to the pattern of applied energy. The shape of a delivered electrical pulse will have an effect, and as we can see, nerve membranes respond most effectively to a rapid change in applied energy.
This contrasts with voluntary contractions, which follow the orderly recruitment of slow-twitch to fast-twitch fibres. Despite the unphysiological stimulation pattern, NMES can induce hypertrophy (muscle bulking) and enhance force generation through repeated activation of high-threshold motor units. For example, Petrofsky et al. demonstrated that increasing current from 20 mA to 60 mA elevated quadriceps force output from 15% to 75% at maximal voluntary contractions, highlighting the dose-response relationship between intensity and muscle activation.
Central Nervous System Modulation
Sensory feedback from NMES activates spinal and supraspinal pathways, increasing corticospinal excitability. Transcranial magnetic stimulation (TMS) studies reveal enhanced motor-evoked potentials in stimulated muscles, particularly after high-frequency NMES (100 Hz). Mang et al. reported a 45-56% increase in MEP amplitudes following 40 minutes of NMES, with broader effects in leg muscles compared to arm muscles, suggesting limb-specific neuroplasticity. These central adaptations complement peripheral strength gains, contributing to functional improvements in mobility and endurance.
Critical Stimulation Parameters
Many patterns of stimulation have been used over the years. Perhaps the most common waveform is the biphasic rectangular pulse. As the name suggests, this waveform is charge-balanced and has a negative and positive pulse form. The fact that no direct current should be applied ensures net zero charge delivery to the tissue, which prevents tissue damage from charge accumulation and reduces the risk of electrochemical burns or irritation. This should be more comfortable for patients compared to monophasic stimulation. There is no need to be concerned about which electrode is the anode and which is the cathode, which is necessary with some other waveform types.
As every engineer knows, such a waveform can be accurately characterised by three things: amplitude, pulse width, and frequency. Increasing any or all of these parameters will increase the intensity of stimulation but will do so with different effects on strength of contraction, muscle fibre type recruitment and fibre fatigue. The stimulators we use at Anatomical Concepts are usually current-controlled, so the pulse amplitude reflects the applied current.
Higher amplitudes (e.g., 60 mA plus) generate greater force by recruiting more motor units, though pain tolerance will limit many clinical applications. Stevens-Lapsley et al. observed that NMES at maximal tolerable intensities post-total knee arthroplasty correlated with quadriceps strength recovery (at 3.5 weeks). However, excessive intensity may induce antidromic motor axon blockades, reducing central nervous system engagement. Balancing intensity to evoke strong contractions without discomfort remains key for patients with preserved stimulation.
Frequency and Pulse Duration
Optimal frequencies range from 20 Hz (for fatigue resistance) to 100 Hz (for maximal force). Wide-pulse high-frequency NMES (for example, 100 Hz, 1 ms pulses) elicits reflexive motor unit recruitment via persistent inward currents, enhancing plantar flexor force by up to 80% MVC in responders. Pulse durations of 350-500 μs are ideal for innervated muscles, as shorter durations require higher current amplitudes to achieve similar effects. For example, Gorgey et al. demonstrated that 450 μs pulses generated 30% greater torque than 150 μs pulses in spinal cord injury patients.
In FES Cycling applications, we tend to start clients with 35Hz frequency and pulse widths of 250-350 μs, with current levels sufficient to produce a strong tetanic contraction.
Duty Cycle and Session Duration
The pattern of muscle fibre recruitment induced by NMES, can produce rapid onset of fatigue. Intermittent stimulation (e.g., 10 s on, 20 s off) preserves force output by allowing metabolic recovery. A 1:3 or 1:4 duty cycle is fairly standard, though it is adjustable based on patient tolerance. Prolonged sessions (e.g., 6 weeks of daily 15-contraction NMES) yield cumulative strength gains, as shown in post-TKA (total knee arthroplasty) cohorts.
Electrode Configuration
Electrode size and placement critically influence current density. Larger electrodes (≥ 200 cm²) disperse current across broad muscle areas, while smaller electrodes concentrate stimulation for focal activation. Larger electrodes may be beneficial for comfort when clients have preserved sensation but may induce activation of neighbouring muscle groups which may not be desirable for functional reasons. Positioning electrodes over motor points—identified via stimulation mapping enhances torque production by 20-30% compared to manufacturer-recommended sites.
For instance, in one study, NMES applied to the tibialis anterior via motor-point electrodes increased oxygen utilization and blood flow, facilitating hypertrophy.
Clinical Applications and Outcomes Post-Surgical Rehabilitation
In TKA patients, NMES at 50-100 Hz and 400-450 μs pulses restored quadriceps strength to 85% of pre-surgical levels within 6 weeks. Strength gains paralleled improvements in walking endurance (6-minute walk distance: 7.5%), underscoring NMESʼs functional impact.
Chronic Conditions
For moderate-to-severe COPD, NMES (30-50 Hz, 350-500 μs) improved quadriceps strength (standardized mean difference: 1.12) and 6-minute walk distance (+51.5 meters). These adaptations likely stem from enhanced oxidative capacity and reduced catabolic signalling (e.g., lower Atrogin-1/MuRF1 expression).
Pediatric Neurorehabilitation
In children with cerebral palsy, 8 weeks of daily NMES (30 Hz, 200-350 μs) increased ankle dorsiflexor volume by 15% and strength by 20%, with effects persisting for 6 weeks post-intervention. Functional gains were use-dependent, emphasising the need for consistent stimulation.
Comparative Efficacy Across Muscle Groups
Plantar flexors exhibit superior responsiveness to NMES compared to elbow flexors, attributable to their higher proportion of slow-twitch fibres and role in posture. Wide-pulse high-frequency (WPHF) NMES (1 ms, 100 Hz) evoked 45% greater extra force in plantar flexors versus elbow flexors, with sustained EMG activity indicating prolonged PIC activation. Knee extensors showed intermediate responses, suggesting muscle-specific parameter optimisation.
Limitations and Adverse Effects
Fatigue remains a challenge, particularly at frequencies greater than 50 Hz. Rhabdomyolysis risk could necessitate supervised use of multi-muscle NMES systems. Discomfort scores (visual analog scale: 4-6/10) limit tolerance in 10-15% of patients.
Conclusion
NMES is a potent modality for strengthening innervated skeletal muscle when parameters are tailored to individual needs. Key recommendations include:
Intensity: Maximise within patient tolerance to produce a tetanic contraction(e.g., 60 mA for quadriceps).
Frequency: Higher frequencies, 50-100 Hz for strength; 20-30 Hz for endurance.
Pulse Duration: 350-500 μs for optimal motor unit recruitment.
Duty Cycle: 1:3 on:off ratio to balance fatigue and adaptation for strength buikding.
Electrode Placement: Some experimentation may be need to identify the best placement. For patients working at home, some simple guidance as to the most effective placement should be given to ensure consistent application.
Future research could utilise personalised NMES protocols leveraging real-time biofeedback to automatically optimise parameter adjustment. Clinicians must weigh peripheral and central neuromuscular effects to maximise functional outcomes in diverse populations.
Key Findings and Summary
FES Cycling is a commonly used method of using NMES for muscle strengthening and fitness at home following a spinal cord injury
NMES is a robust modality for improving muscle strength, particularly in various clinical populations. Its effectiveness hinges on thoughtful parameter selection, including intensity, frequency, pulse duration, duty cycle, and electrode placement. In some activities such as FES Cycling, the muscles are stimulated in sync with the movement of the pedals so this dictates the duty cycle. In general strengthening applications, the duty cycle should be chosen to reduce fatigue. Duty cycles (e.g., 1:3 or 1:4 on:off ratios) help to mitigate fatigue.
Higher stimulation intensities (e.g., 60 mA) elicit greater muscular contractions (up to 75% maximal voluntary contraction), while frequencies of 20 - 100 Hz and pulse durations of 350 - 500 μs optimise motor unit recruitment. Electrode positioning over motor points enhances torque production. Cross-education effects—strength gains in contralateral limbs—and improvements in non-targeted muscles underscore NMESʼs systemic neuromuscular benefits.
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