Every now and then, a study challenges our deeply held beliefs about rehabilitation. Recently, a randomized controlled trial published in the Journal of Physiotherapy (2024) suggested that electrical stimulation (ES) combined with strength training has negligible effects on the strength of very weak muscles in individuals with recent spinal cord injuries (SCI). This conclusion, while provocative, appears to contradict decades of research supporting ES as a tool for neurorehabilitation.

So, how should we interpret these findings? And more importantly, what can this teach us about the necessity of critical analysis in research?

A Result That Defies Consensus?

Previous studies have painted a much more optimistic picture. For instance:

• Progressive resistance training combined with ES has been shown to increase isometric strength in SCI patients by as much as 14 Nm (Harvey et al., 2010).
• Even in weak muscles, ES-driven protocols have demonstrated modest but measurable improvements in functional outcomes.

In contrast, the new study found a between-group difference of just 0.7 points on a 13-point manual muscle test—well below their predetermined threshold for clinical significance. Why such a divergence?

Scrutinizing the Methodology

The beauty—and burden—of science lies in the details. Here are some critical considerations that might explain the disconnect:

1️⃣ Training Protocols: Were They Sufficient?

The intervention protocol involved:

• 8 weeks of training, with participants performing 3 supervised sessions per week (24 sessions in total).
• 120 repetitions per session for the target muscle, divided between voluntary contractions combined with ES and ES-driven contractions alone.

At first glance, this might seem adequate, but let’s think critically:

• Frequency and Duration: Eight weeks is a relatively short time to produce structural changes in severely atrophied or weak muscles, especially in SCI populations. Studies with longer protocols (e.g., 12-16 weeks) have shown greater improvements in strength.
• Intensity of Stimulus:The ES parameters used (50 Hz frequency, 300 ms pulse width) are standard but may not have been sufficient to fully activate the very weak muscle fibers. Weak muscles often require greater pulse widths to recruit a broader range of motor units, especially those that have undergone denervation or partial denervation.Increasing the pulse width can enhance the recruitment of deep, smaller motor units critical for initiating muscle contraction in very weak muscles. Without this adjustment, the intervention may have been biased toward activating stronger, superficial fibers, leaving the weaker fibers under-stimulated.Additionally, the study relied on fixed ES parameters across participants, yet responses to stimulation vary significantly. Tailoring the settings (e.g., adjusting pulse width, amplitude, or duty cycle) to each individual’s muscle properties could have yielded more robust results.
• Fatigue and Recovery: Each session included 120 repetitions, divided between voluntary and ES-driven phases. This is a significant workload, particularly for individuals with very weak muscles. If proper recovery wasn’t achieved between sets or sessions, participants could have experienced early fatigue, undermining their performance.

In rehabilitation science, dose and specificity matter. A higher frequency of sessions, longer intervention duration, or individualized ES parameters might have unlocked the potential of the protocol. Unfortunately, the chosen design may have been too conservative for the studied population.

2️⃣ Outcome Measures: Did They Capture the Right Changes?

The primary outcome measure, a modified 13-point manual muscle test (MMT), was practical and familiar to clinicians, but it raises questions about sensitivity:

• Detection Limitations: MMT is known to be reliable, but it measures strength in broad increments. It might miss smaller, yet clinically meaningful, changes in force production or neuromuscular control.
• Alternative Tools: Tools like isokinetic or handheld dynamometry provide more precise, quantitative measurements of muscle strength. These tools could have captured subtle gains in force output that MMT could not detect.
• Functional Relevance: Strength alone may not tell the whole story. Functional tests like gait speed, wheelchair propulsion, or activities of daily living might have provided a better picture of whether the intervention had practical benefits.

Moreover, the secondary outcomes relied heavily on subjective perceptions (e.g., self-reported change in strength and function). While valuable, these measures are prone to bias, especially in unblinded participants who might either overestimate or underestimate their progress based on their expectations.

3️⃣ Contextual Considerations

• Natural Recovery: Participants were less than six months post-injury, a period of significant spontaneous recovery. Distinguishing the effects of the intervention from natural healing processes would require a more extended baseline observation period or additional control measures.
• Control Group Overlap: Participants in the control group were allowed up to two strength training sessions per week for the same muscle groups. This overlap in treatment might have minimized the differences between groups.

The Takeaway: Critical Thinking in Rehabilitation Science

The authors concluded that ES has negligible effects, but their findings are laden with caveats:

• Confidence intervals that suggest the possibility of clinically meaningful effects (upper bounds at 2.1 points).
• A focus on the hardest-to-treat population (Grade 1 and 2 strength) without considering whether the intervention was sufficiently robust.

Rather than accept the findings as definitive, we should ask: What could have been done differently?

• A longer and more specific intervention?
• More sensitive tools?
• A comparison to natural recovery alone?

This isn’t just about electrical stimulation. It’s about understanding how methodological decisions shape scientific outcomes and, ultimately, clinical practice.

A Call to the Scientific Community

As scientists, clinicians, and thinkers, our job is to approach research with both respect and skepticism. Respect for the effort and rigor that goes into trials like this, and skepticism to ask: Does this really reflect the clinical reality?

Let’s not lose sight of the larger picture: Evidence is built incrementally, and every study is a stepping stone. Even «negative» findings push us closer to the truth—so long as we remain critical, curious, and committed to uncovering what works.

Discussion Point: What’s your perspective on the use of electrical stimulation in SCI rehabilitation? Are we overlooking key methodological issues, or does this study mark a turning point in our understanding?

Based on the publication «Strength training with electrical stimulation has no or little effect on the very weak muscles of patients with spinal cord injury: a randomised trial»: https://www.sciencedirect.com/science/article/pii/S1836955324001176

José López Sánchez

CEO @ Centro Europeo de Neurociencias | Intensive Therapy Specialist