The effect of exercise on aerobic capacity in individuals with spinal cord injury: A systematic review with meta-analysis and meta-regression
Authors: Hodgkiss Daniel D. , Bhangu Gurjeet S. , Lunny Carole , Jutzeler Catherine R. , Chiou Shin-Yi , Walter Matthias , Lucas Samuel J. E. , Krassioukov Andrei V. , Nightingale Tom E. 
Published: August 2022

Background

A low level of cardiorespiratory fitness [CRF; defined as peak oxygen uptake (V̇O2peak) or peak power output (PPO)] is a widely reported consequence of spinal cord injury (SCI) and a major risk factor associated with chronic disease. However, CRF can be modified by exercise. This systematic review with meta-analysis and meta-regression aimed to assess whether certain SCI characteristics and/or specific exercise considerations are moderators of changes in CRF.

Methods and Findings

Databases (MEDLINE, EMBASE, CENTRAL and Web of Science) were searched from inception to March 2023. A primary meta-analysis was conducted including randomised controlled trials (RCTs; exercise interventions lasting >2 weeks relative to control groups). A secondary meta-analysis pooled independent exercise interventions >2 weeks from longitudinal pre-post and RCT studies to explore whether subgroup differences in injury characteristics and/or exercise intervention parameters explained CRF changes. Further analyses included cohort, cross-sectional and observational study designs. Outcome measures of interest were absolute (AV̇O2peak) or relative V̇O2peak(RV̇O2peak), and/or PPO. Bias/quality was assessed via The Cochrane Risk of Bias 2 and the National Institute of Health Quality Assessment Tools. Certainty of the evidence was assessed using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach. Random effects models were used in all meta-analyses and meta-regressions.

Of 21,020 identified records, 120 studies comprising 29 RCTs, 67 pre-post studies, 11 cohort, 7 cross-sectional and 6 observational studies were included. The primary meta-analysis revealed significant improvements in AV̇O2peak[0.16 (0.07, 0.25) L/min], RV̇O2peak[2.9 (1.8, 3.9) mL/kg/min] and PPO [9 (5, 14) W] with exercise, relative to controls (p<0.001). Ninety-six studies (117 independent exercise interventions comprising 1,331 participants) were included in the secondary, pooled meta-analysis which demonstrated that exercise interventions significantly improve AV̇O2peak[0.22 (0.17, 0.26) L/min], RV̇O2peak[2.8 (2.2, 3.3) mL/kg/min], and PPO [11 (9, 13) W] (p<0.001). There were subgroup differences for RV̇O2peakbased on exercise modality (p=0.002) and intervention length (p=0.01), but there were no differences for AV̇O2peak. There were subgroup differences (p≤0.008) for PPO based on time since injury, neurological level of injury, exercise modality, relative exercise intensity, exercise intensity prescription method, and frequency. The meta-regression found that studies with a higher mean age of participants were associated with smaller changes in AV̇O2peakand RV̇O2peak(p<0.10). GRADE indicated a moderate level of certainty in the estimated effect for RV̇O2peak, but low levels for AV̇O2peakand PPO. This review may be limited by the small number of RCTs, which prevented a subgroup analysis within this specific study design.

Conclusions

Performing exercise >2 weeks results in significant improvements to AV̇O2peak, RV̇O2peakand PPO in individuals with SCI. Subgroup comparisons identified that exercise interventions lasting up to 12 weeks yield the greatest change in RV̇O2peak. Upper-body aerobic exercise and resistance training also appear the most effective at improving RV̇O2peakand PPO. Furthermore, acutely-injured, individuals with paraplegia, exercising at a moderate-to-vigorous intensity, prescribed via a percentage of oxygen consumption or heart rate, for ≥3 sessions/week will likely experience the greatest change in PPO. Ageing seemingly diminishes the adaptive CRF responses to exercise training in individuals with SCI.

Registration

PROSPERO: CRD42018104342

AUTHOR SUMMARY

Why was this research done?

- Individuals with spinal cord injury typically exhibit low levels of cardiorespiratory fitness. As such, these individuals are at a higher risk for the development of chronic diseases in comparison to the non-injured population.

- The current spinal cord injury-specific exercise guidelines encourage moderate-to-vigorous intensity aerobic exercise 40 minutes per week for fitness benefits or 90 minutes per week for cardiometabolic health benefits. Yet, others have suggested individuals with spinal cord injury should be achieving 150 minutes per week in line with non-injured population guidelines.

- This systematic review with meta-analysis and meta-regression aimed to identify whether specific injury characteristics (e.g., time, level or severity of injury) or exercise intervention parameters (e.g., modality, intensity, volume etc.) result in the greatest changes in cardiorespiratory fitness in individuals with spinal cord injury.

What did the researchers do and find?

- We searched for studies that investigated the effects of exercise interventions lasting longer than 2 weeks on changes in absolute and relative peak oxygen consumption and/or peak power output in individuals with spinal cord injury. In total, we included 120 studies of various study designs: 29 randomised controlled trials, 67 pre-post studies, 11 cohort comparisons, 7 cross-sectional studies and 6 observational studies.

- The greatest changes in peak power output may be achieved by individuals with acute spinal cord injury or paraplegia. Upper-body aerobic and resistance exercise were identified as the most optimal exercise modalities. Furthermore, prescribing moderate-to-vigorous intensity aerobic exercise using either a percentage of the individual’s peak heart rate or oxygen consumption, for three or more sessions per week, resulted in the greatest improvements in peak power output.

- Our findings support the minimum 40 minutes of weekly moderate-to-vigorous intensity exercise recommended by the spinal cord injury-specific exercise guidelines to significantly improve fitness. However, while not statistically significant, a two-fold greater improvement in peak power output was shown for interventions with exercise performed ≥90 min/week in comparison to ≥40 min/week. Cross-sectional comparisons also revealed that individuals with spinal cord injury performing higher levels of physical activity were associated with higher cardiorespiratory fitness.

What do these findings mean?

- Exercise interventions >2 weeks can significantly improve cardiorespiratory fitness in individuals with a spinal cord injury by a clinically meaningful change greater than one spinal cord injury adjusted metabolic equivalent (i.e., ≥2.7 mL/kg/min). A one metabolic equivalent improvement has been associated with a reduction in cardiovascular related mortality risk in non-injured individuals.

- Our findings indicate that certain participant/injury characteristics and exercise intervention parameters are moderators of the changes observed in cardiorespiratory fitness across studies. These factors should be considered in the design of future exercise interventions. Future research should consider: following spinal cord injury-specific reporting guidelines (ensuring transparency of reporting), investigating the dose-response relationship between exercise and cardiorespiratory fitness or influence of exercise intensity in this population, and consider how different injury characteristics impact the benefits of exercise on cardiorespiratory fitness.

- The main limitation of the study was the lack of randomised controlled trials (RCT) comparing changes in CRF following an exercise intervention relative to a control group. This prevented subgroup comparisons in this study design specifically and therefore we pooled pre-post and RCT exercise interventions to explore these effects.