Aerobic exercise benefits adults with spinal cord injury (SCI), improving physical and mental health. Existing guidelines recommend "moderate to vigorous" intensity but lack specific physiological thresholds, hindering practical application and research. Guidelines for non-disabled individuals define intensity zones using %VO2max, %HRmax, %VO2R, %HRR, and RPE, but these thresholds are not applicable to the SCI population due to physiological differences caused by SCI. An alternative approach focuses on exercise intensity domains (moderate, heavy, severe) based on similar VO2 and blood lactate responses. This study aimed to investigate and critique potential methods for prescribing aerobic exercise intensity in adults with SCI, addressing the lack of clarity in current guidelines and the limitations of fixed percentage approaches demonstrated in non-disabled populations.

Current guidelines for adults with SCI recommend aerobic exercise at a "moderate to vigorous" intensity, but this lacks the precision needed for effective prescription. Studies in non-disabled individuals have shown the limitations of using fixed percentages of VO2max or HRmax for prescribing exercise intensity, as this approach does not guarantee a uniform intensity distribution among individuals. Furthermore, individual %VO2R:%HRR relationships may deviate from linear trajectories, questioning the appropriateness of these metrics for individual-level prescription. The use of exercise intensity domains (moderate, heavy, severe) based on lactate thresholds provides an alternative approach.

This retrospective cohort study analyzed data from 134 athletes (98 male, 36 female) categorized into PARA (n=47), TETRA (n=20), and NON-SCI (n=67) groups. Participants underwent individualized submaximal step tests followed by graded exercise tests (GXT) to exhaustion using handcycling, arm crank ergometry, or wheelchair propulsion, depending on their sport. Oxygen uptake (VO2), heart rate (HR), blood lactate concentration ([BLa]), and ratings of perceived exertion (RPE) were measured. Lactate thresholds (LT1 and LT2) were determined using log-VO2 vs. log-[BLa] plots. Multilevel models were used to analyze the relationship between RPE, %VO2peak, and %HRpeak. One-way ANOVA with Bonferroni post-hoc correction was used to compare physiological responses at LT1 and LT2 between groups. The percentage of participants in each intensity domain (moderate, heavy, severe) was calculated at 5% intervals from 35% to 95% VO2peak and %HRpeak.

The associations between RPE and both %VO2peak and %HRpeak were not significantly affected by sex or exercise mode. However, at LT1 and LT2, %VO2peak and %HRpeak were significantly greater in TETRA compared to PARA and NON-SCI (P<0.05). The variability in %VO2peak and %HRpeak at lactate thresholds resulted in large variability in domain distribution at fixed %VO2peak and %HRpeak. There was no significant difference in RPE between groups at LT1 and LT2. The analysis revealed that no single fixed %VO2peak or %HRpeak resulted in all participants being within the same intensity domain across all three groups. For example, at various fixed percentages between 55-70% VO2peak in the PARA group and 60-70% VO2peak in the TETRA group, participants were spread across moderate, heavy and severe domains.

The findings confirm that using fixed %VO2peak or %HRpeak for exercise prescription in adults with SCI does not guarantee uniform intensity across individuals, supporting similar findings in non-disabled populations. The variability in domain-specific distribution appears even greater in the SCI population. The study supports the move away from using fixed percentages of VO2peak and HRpeak for exercise prescription in favour of methods that lead to participants exercising within the same intensity domain. While lactate thresholds (LT1 and LT2) could be used, the use of LT2 in this study had limitations. The study suggests RPE may be a useful tool, as RPE at LT1 and LT2 did not differ significantly between groups. However, the inter-individual variability in RPE values points to the importance of individualization in exercise prescription.

This study demonstrates that using a "moderate to vigorous" intensity range or fixed percentages of VO2peak and HRpeak for exercise prescription in adults with SCI is inappropriate due to significant inter-individual variation in physiological responses. Future research should focus on individualized intensity prescription based on intensity domains, considering factors such as lactate thresholds and CP/CS to ensure homogenous intensity distribution. However, balancing the need for individualized prescription with the simplicity required for population-level guidelines remains a challenge.

The study population consisted of competitive athletes, potentially limiting the generalizability of the findings to sedentary or less active individuals with SCI. The use of LT2 to identify the transition between heavy and severe exercise intensity, instead of critical power, is also a limitation. Furthermore, the use of various methods of identifying lactate thresholds may also affect the generalisability of these findings. Lastly, individualisation of prescription poses challenges when creating simple population-level recommendations.