Effect of muscle length on maximum evoked torque, discomfort, contraction fatigue, and strength adaptations during electrical stimulation in adult populations: A systematic review

PLoS ONE, 2024 · DOI: https://doi.org/10.1371/journal.pone.0304205 · Published: June 10, 2024

Simple Explanation

This study looks at how muscle length affects the results of neuromuscular electrical stimulation (NMES), which is used to improve muscle function. Muscle length influences how well muscles generate force. The review looks at the best joint angle for NMES to work best, considering things like torque, discomfort, fatigue, and strength gains. The researchers looked at many studies to find the best muscle length during NMES for different outcomes. They focused on healthy and clinical adult populations and analyzed the available data to determine the ideal conditions for NMES interventions. The review suggests that using the optimal muscle length during NMES can lead to better force production, potentially helping to preserve or gain muscle strength and mass while reducing discomfort. Understanding these principles is important for prescribing NMES effectively.

Study Duration

Not specified

Participants

448 healthy young participants (age: 19–40 years), six participants with spinal cord injuries, and 15 healthy older participants

Evidence Level

Systematic Review

Key Findings

1
Optimal quadriceps muscle length (50˚ - 70˚ of knee flexion) results in greater evoked torque during nerve stimulation compared to very short, short, and long muscle lengths.
2
The shortest quadriceps muscle length generated the highest perceived discomfort for a given current amplitude.
3
Strength gains were greater for a protocol at the optimal muscle length than for short muscle length.

Research Summary

This systematic review investigated the impact of muscle length on neuromuscular electrical stimulation (NMES) outcomes, including evoked torque, discomfort, contraction fatigue, and strength adaptations. Meta-analyses revealed that optimal quadriceps muscle length (50˚-70˚ knee flexion) resulted in greater evoked torque during nerve stimulation compared to shorter or longer muscle lengths. Shorter muscle lengths were associated with increased discomfort. The review concludes that optimal muscle length should be considered the primary choice during NMES interventions to promote higher force production and muscle preservation/gain, while also potentially reducing discomfort.

Practical Implications

Optimal Muscle Length in NMES

Prioritize optimal muscle length (e.g., 50-70 degrees of knee flexion for quadriceps) during NMES interventions to maximize force production and potentially improve muscle preservation/gain.

Discomfort Management

Be cautious when using shorter muscle lengths during NMES, as they may lead to increased discomfort. Monitor patient feedback and adjust parameters accordingly.

Individualized NMES Programs

Consider individual factors, such as joint range of motion and specific training goals (e.g., muscle-tendon unit properties), when determining the appropriate muscle length for NMES training.

Study Limitations

1
The scope of the investigation was confined to evoked torque, contraction fatigue, discomfort, and chronic adaptation measures.
2
Considerable variability across evaluations and clinical heterogeneity among studies precluded us from conducting more meta-analyses.
3
The search primarily targeted English-language journals, potentially overlooking studies in non-English publications and regional databases.

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