Comparison of the Aerobic and Peripheral Benefits Bestowed by Isometric Functional Electrical Stimulation and Functional

Abstract

Objective: To examine the acute and long-term changes of isometric functional electrical stimulation (FES) on aerobic capacity and limb function in individuals with lower limb paralysis, and to compare these effects to those of other FES modalities.

Data Source: Eight electronic databases: MEDLINE, EMBASE, Scopus, Cochrane Library, CINAHL, PsycINFO, SPORTDiscus, and Web of Science, encompassing a diverse collection of articles from the earliest available to the present, and concluded on September 20, 2022.

Study selection: The study selection focused on identifying research involving individuals with lower limb paralysis due to conditions such as spinal cord injury, multiple sclerosis, cerebral palsy, hereditary spastic paraplegia, and stroke. Eligible studies employed functional electrical stimulation (FES) or FES cycling interventions, with primary outcomes including measures of aerobic fitness (e.g., oxygen consumption), cardiovascular outcomes (e.g., cardiac output), and peripheral benefits (e.g., muscle mass, blood flow, bone mineral density). Secondary outcomes included cardiorespiratory parameters such as VCO₂. Studies conducted in hospitals, clinics, academic institutions, laboratories, or home-based settings were included, encompassing various designs such as randomised controlled trials, non-randomised clinical trials, cohort studies, and case series.

Data extraction: From 4177 articles, 82 were included and 20 articles were included for meta-analysis. The Kmet, Lee & Cook Checklist was used to assess study quality.

Data Synthesis: The synthesis of data revealed that the comparison between FES exercise subgroups for oxygen consumption showed a total mean difference in VO₂ of 0.42 [95% CI 0.34–0.49] (13 studies, n=129), indicating a statistically significant transient increase from rest (Z = 11.15, p < 0 xss=removed>.

Conclusion: FES cycling demonstrates a significant positive impact on aerobic capacity, cardiovascular outcomes, and lower limb health in individuals with lower limb paralysis, as indicated by improved oxygen consumption and muscle composition. However, the benefits are accompanied by challenges such as muscle fatigue during FES-induced exercise. Isometric FES offers comparable benefits in oxygen consumption and cardiovascular outcomes. Despite these promising findings, the evidence remains inconclusive regarding long-term benefits, particularly for isometric FES, and further research is necessary to better understand its potential applications and limitations.

Keywords: Functional Electrical Stimulation, Lower limb paralysis, Cardiovascular outcomes, Musculoskeletal Composition

Background

Lower limb paralysis, often resulting from spinal cord injuries (SCI) or other central neurological disorders, has a profound impact an individual's physical, emotional, psychological, social, occupational, financial, recreational, and existential well-being (Rimmer 2006, Alizadeh, Dyck et al. 2019). Paralysis affects both the upper and lower extremities, impairing functions such as bowel and bladder control, breathing, and sensation (Peckham and Kilgore 2013), These conditions can lead to reduced fitness, diminished quality of life, and an increased risk of cardiovascular and metabolic diseases.

Functional Electrical Stimulation (FES) has been shown to mitigate some of adverse effects of paralysis, improving independence and quality of life. FES delivers high-voltage electrical pulses, delivered through gel-electrodes placed on the skin, to elicit skeletal muscle contractions (Marquez-Chin and Popovic 2020). This therapeutic intervention has demonstrated effectiveness in enhancing muscle and bone health, blood circulation, tissue healing, and pain management in individuals with SCI (Dudley, Castro et al. 1999, Atkins and Bickel 2021).

FES cycling has been extensively studied for its benefits in improving cardiorespiratory fitness (Davis, Hamzaid et al. 2008) and muscle strength (Hasnan, Ektas et al. 2013, Sadowsky, Hammond et al. 2013, Gorgey, Poarch et al. 2014, Kuhn, Leichtfried et al. 2014). The primary advantage of FES cycling is that it enable exercise for individuals with paralysis, helping them engage in active rehab despite neuromuscular impairments (Peng, Chen et al. 2011). However, significant gaps remain in understanding its broader, physiological effects, particularly the influence of exercise intensity on these outcomes. For instance, Fornusek and Davis (2008) report that pedalling cadence (15, 30, or 50 rpm) may not significantly influence aerobic fitness improvements; however, lower-cadence FES cycling appears to provide superior muscle strength gains (Fornusek and Davis 2008). Despite its widespread application, the interaction between FES cycling intensity and physiological adaptations remains insufficiently explored. Addressing these gaps requires further investigation into key exercise parameters, such as intensity, stimulation settings, and intervention protocols, to optimize therapeutic outcomes. While FES cycling remains the most extensively studied modality, a more comprehensive examination of its effects particularly on aerobic responses and other physiological factors is paramount to establish robust conclusions.

Isometric FES, in contrast to dynamic FES cycling, is an alternative modality that induces muscle contractions without joint movement, leading to static muscle activation (Fornusek, Gwinn et al. 2014, Ibitoye, Estigoni et al. 2014). This method has been shown to elicit significant physiological responses, including enhanced muscle strength, improved blood circulation, and increased metabolic demand (Dudley-Javoroski and Shields 2008, Griffin, Decker et al. 2009). However, the extent to which isometric FES replicates the aerobic and peripheral benefits of FES cycling remains unclear.

Oxygen consumption (VO₂) during FES exercise is influenced by multiple stimulation parameters, such as pulse width, amplitude, and the number of muscles engaged, which directly impact muscle activation and exercise intensity (Brurok, Tørhaug et al. 2013, Gorgey, Poarch et al. 2014). Fornusek et al. (2014) hypothesized that VO₂ during isometric FES and FES cycling would be comparable under equivalent conditions, including matching duration, stimulation parameters, and muscle groups (Fornusek, Gwinn et al. 2014). However, this hypothesis remains unverified in current analyses. Instead, existing literature predominantly focuses on FES cycling, given its strong association with aerobic adaptations. A more systematic investigation into exercise intensity, VO₂ dynamics, and peripheral outcomes across different FES modalities is essential to deepen our understanding of their physiological mechanisms and broader clinical applications.

This systematic review aimed to compare the physiological effects of isometric FES cycle to those of other FES modes, specifically FES cycling, in individuals with lower limb paralysis. The focus was on both acute and long-term aerobic and peripheral responses associated with these interventions. By synthesising existing evidence, the review sought to identify research gaps, evaluate the effectiveness of different FES modalities, and provide insights into their clinical and rehabilitative applications. Through this, the study aims to contribute to the development of optimised FES protocols for improving cardiorespiratory fitness and lower limb health.

Methods

This systematic review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement (Page, McKenzie et al. 2021). The study was registered in PROSPERO, the International Prospective Register of Systematic Reviews CRD42022351768.

Information sources

Academic databases searched were MEDLINE (Ovid), EMBASE, Scopus, Cochrane Library, CINAHL, PsycINFO, SPORTDiscus, and Web of Science using database-specific subject headings and text terms. Databases were searched from the earliest record to September 20, 2022. Searches were limited to the English language and human articles. Reference lists of included studies and any previous systematic reviews were checked for relevant references.

Search strategy

A health sciences librarian developed the search strategy and performed the literature searches in the eight databases. The MEDLINE strategy was developed with input from the project team and peer-reviewed by another researcher (Appendix 1). After the initial MEDLINE strategy was finalized, it was adapted for use in the other databases. See Appendix 1 for keywords used in the search.

Study eligibility criteria

Participants: People with lower limb paralysis from neuromuscular conditions. Specifically –

spinal cord injury, multiple sclerosis, cerebral palsy, hereditary spastic paraplegia and stroke.

Interventions: Studies utilizing FES cycling.

Comparator/Control: No comparator

Outcome measures

The primary outcomes of interest included measures of long-term cardiorespiratory fitness, short-term oxygen consumption (VO₂), and peripheral benefits indicated by measures of muscle mass such as muscle volume, thigh girth, or circumference. Additionally, secondary outcomes of interest encompassed assessments of cardiorespiratory benefits, including variables such as cardiac output (CO). Furthermore, peripheral benefits of lower limbs such as blood flow, and improvement in bone mineral density were also considered as secondary outcome measures.

Settings: Hospital, multicentre clinics, academic or research institutions, laboratory, clinics, and home-based.

Study designs: Studies including randomized control trials, non-randomized clinical trials, cross-sectional, cohort intervention, before and after studies, case studies and case series were included. Systematic and narrative reviews, conferences, and abstracts were excluded.

Selection process

To organize the complete search results from the eight different databases, the studies were imported from their respective databases to Endnote and duplicates were removed. From there, the references were imported into Covidence, where additional duplicates were removed. Two authors (PD, SM) independently screened the title and abstracts of relevant papers, and any discrepancies were discussed and resolved through consensus by a third senior author (CF). Next, full-text reviews were performed independently by two reviewers (PD, SM). A final agreement for text screening was performed by two reviewers (PD, CF).

Data extraction

A data extraction form was designed to extract data from eligible studies. Data were extracted from each study by one reviewer, another reviewer (SM) checked and confirmed the accuracy of the data with disagreements discussed with a third senior author (CF).

Data synthesis and statistical analysis

Data were synthesised with qualitative and quantitative analysis. Meta-analysis was undertaken with studies that included change from at rest to the end of the FES exercise. The primary outcomes were changes in VO₂, and muscle mass from baseline to post-intervention and measured as mean (+) and standard deviation (SD) (+). If the measurements were not reported directly as mean or SD, the percentage change was calculated from baseline to post-intervention period. Studies with standard error (SE) were converted to SD for consistency of the results.

For statistical analysis on RevMan software, the random effect model was used to provide a relatively conservative estimate with a 95% confidence interval (CI) of the pooled per cent changes (Higgins, Thompson et al. 2003). The heterogeneity across studies was tested by I² and Cochran’s Q test. A P value <0>2>50% was regarded as the existence of significant heterogeneity (Higgins, Thompson et al. 2003). Heterogeneity was assessed using Cochran’s Q test (considering heterogeneous results to have P < 0>2 index. Fixed-effect models were used in analyses if the P value was greater than 0.1 and the I² was less than 50%; otherwise, random-effect models were used. P values less than 0.05 were considered significant.

We performed a subgroup analysis to gain a deeper understanding of the impact of various factors on the outcomes related to FES modes and training. Specifically, we focused on two important aspects: the duration of the intervention and the training frequency of FES modes and training. Duration was based on whether the condition was acute or chronic/training. Acute meant the short-term response of the participants to different modes or interventions whereas chronic/training involved a number of training sessions. We categorized the studies into two groups: "acute" and "chronic/training."

Acute: This category included studies that investigated the immediate responses of participants to different FES modes or interventions. Specifically, it examined how individuals responded to FES during or directly after its application, focusing on outcomes such as muscle contraction, pain reduction (when measured through self-reported pain scales or physiological markers), or other relevant effects observed within a short duration.

Chronic/Training: In contrast, the chronic/training category included studies that involved multiple FES training sessions over an extended period. This aspect was crucial for understanding the long-term effects of FES interventions, particularly in scenarios where participants underwent repeated training sessions. We analysed how FES interventions impacted individuals after several sessions, which could range from days to weeks or even months.

Methodological data quality assessment

Two reviewers (PD, CF) independently evaluated the methodological quality of the included studies using the evaluation criteria proposed by Kmet et al. (Kmet, Cook et al. 2004) known as the "Standard Quality Assessment Criteria for Evaluating Primary Research Papers from a Variety of Fields." This tool was chosen for its comprehensive coverage of methodological quality and the risk of bias in both qualitative and quantitative studies. However, in our study, we used checklist for assessing the quality of quantitative studies. The checklist consisted of 14 items, with a scoring system of “yes” = 2, “partial” = 1, and “no” = 0 for each category. Categories that were not applicable to a specific study design were marked "n/a" and excluded from the summary score calculation (Appendix 2). The summary score was used to objectively quantify the quality of each study, based on the agreement score between the two independent reviewers, with a maximum score of 1.0 (100%). Studies with stronger design and more accurate data received higher scores. Importantly, no studies were excluded from the review based on quality. In cases of any discrepancies in scoring and rating, the authors resolved them through consensus.

Results

Study Characteristics: A total of 4,177 records were identified through database searching and other sources. After the removal of duplicates, 3893 studies were screened based on the predetermined inclusion and exclusion criteria and 192 studies were retrieved for full-text screening. Upon thorough examination of these 192 studies, 82 articles were ultimately selected for both qualitative synthesis and quantitative analysis (Figure 1. Flowchart of study selection procedure (PRISMA)). The 82 included studies encompassed a total of 1020 subjects with spinal cord injury and other neurological disorders such as stroke and MS (706 males and 178 females, six studies did not report gender information). The age of participants ranged from 18 to 67 years; 3 studies did not report age. Among these subjects, seven had tetraplegia and 23 had paraplegia. Patients’ characteristics and FES characteristics are available in Table 1 and Table 2 respectively (supplementary files).

Out of the 82 studies, 97 % (80/83) involved participants with SCI. The majority focussed on individuals with paraplegia, accounting for 11 (14%). Only two studies examined participants with tetraplegia. Additionally, 15 (19%) studies incorporated a mixed population of paraplegic, tetraplegic, or quadriplegic individuals. However, 65% of the studies did not mention the reasons for injury within the SCI patient group.

Assessment Requirements Summary

Objective:
The assessment required a systematic review and meta-analysis examining the effects of isometric functional electrical stimulation (FES) on aerobic capacity, cardiovascular outcomes, and lower limb health in individuals with lower limb paralysis, compared to other FES modalities.

Key Pointers to Cover:

1. Clear articulation of the study objective and research questions.

2. Comprehensive literature search strategy across multiple electronic databases (MEDLINE, EMBASE, Scopus, Cochrane, CINAHL, PsycINFO, SPORTDiscus, Web of Science).

3. Inclusion and exclusion criteria for studies (participants, interventions, comparators, outcomes, study designs, and settings).

4. Data extraction and methodological quality assessment using a recognized checklist (Kmet, Lee & Cook).

5. Data synthesis, including both qualitative and quantitative analysis (meta-analysis) and statistical considerations such as heterogeneity (I⊃2;), confidence intervals, and subgroup analyses (acute vs chronic interventions).

6. Clear presentation of results, including study characteristics, participant demographics, and outcomes related to VO₂, cardiac output, muscle mass, and other lower limb health markers.

7. Critical discussion of findings, limitations, and clinical implications.

8. Proper referencing and adherence to academic standards for systematic reviews.

Approach Guided by Academic Mentor

Step 1: Understanding the Assessment Task

• The mentor helped the student interpret the requirements: comparing isometric FES to FES cycling, focusing on aerobic and peripheral outcomes.

• Key outcomes and variables were highlighted (VO₂, cardiac output, muscle composition).

Step 2: Literature Search Strategy

• The mentor guided the student to systematically search eight electronic databases.

• Search terms, inclusion/exclusion criteria, and database-specific strategies were clarified.

• Emphasis was placed on documenting the search process for transparency (PRISMA flowchart).

Step 3: Screening and Selection of Studies

• The mentor explained the stepwise screening: title/abstract review → full-text review → final selection.

• Duplicate removal using EndNote and Covidence was demonstrated.

• Consensus processes for resolving disagreements between reviewers were outlined.

Step 4: Data Extraction

• The mentor showed how to design a data extraction form capturing key study characteristics and outcomes.

• Dual review was emphasized to ensure accuracy, with a senior reviewer resolving discrepancies.

Step 5: Quality Assessment

• The Kmet, Lee & Cook Checklist was introduced, scoring each study to assess methodological quality.

• The student learned to document scores and interpret quality without excluding studies based on quality alone.

Step 6: Data Synthesis and Statistical Analysis

• Mentor explained how to conduct meta-analysis using RevMan software.

• Random-effects vs fixed-effects models, confidence intervals, heterogeneity (I⊃2;), and significance thresholds were clarified.

• Subgroup analysis for acute vs chronic interventions was demonstrated.

Step 7: Reporting Results

• The mentor guided the student to present participant demographics, intervention characteristics, and outcome data clearly.

• Emphasis on interpreting both quantitative (VO₂ changes, muscle mass) and qualitative findings.

Step 8: Discussion and Conclusion

• Mentor coached the student to critically evaluate findings, discuss limitations, and suggest clinical implications.

• The student was guided to link findings back to the research question and highlight gaps for future research.

Outcome and Learning Objectives Covered

Outcome Achieved:

• A complete, structured systematic review and meta-analysis covering 82 studies, including qualitative synthesis and meta-analysis of 20 studies.

• Clear comparison between isometric FES and FES cycling on aerobic capacity, cardiovascular function, and lower limb health.

• PRISMA flowchart, data tables, and critical appraisal included.

Learning Objectives Covered

1. Apply systematic review methodology in health sciences research.

2. Conduct comprehensive literature searches across multiple databases.

3. Critically appraise the methodological quality of research studies.

4. Extract, synthesize, and analyze quantitative and qualitative data.

5. Interpret statistical results, including meta-analysis and heterogeneity.

6. Discuss clinical implications and identify research gaps.

7. Communicate complex research findings clearly and professionally.

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