Prescribing appropriate exercise intensity is crucial for maximizing training adaptations and improving physical and mental health. While methods like percentage of peak oxygen uptake (%VO2peak) and heart rate (%HRpeak) are used, they suffer from inter-individual variability. Metabolic thresholds (LT1, LT2, etc.) offer an alternative, but their assessment is invasive, time-consuming, and costly. Ratings of perceived exertion (RPE) provide a simpler, easily implemented alternative. Previous research suggests RPE at LT1 is independent of age, sex, training status, and exercise mode in able-bodied individuals. However, this hasn't been extensively studied in individuals with spinal cord injury (SCI), and different RPE scales (Borg's 6-20 and CR10) lack direct comparison. This study aimed to compare Borg's RPE scale and CR10 in AB participants during upper and lower body exercise and in participants with SCI during upper body exercise, investigating RPE at LT1 and LT2 across groups and exercise modes.

The literature highlights the importance of appropriate exercise intensity prescription for diverse populations, emphasizing the limitations of using %VO2peak and %HRpeak due to inter-individual variability. Metabolic thresholds, while offering a more precise approach, present challenges due to invasiveness, time constraints, and cost. RPE scales, particularly Borg's 6-20 and CR10, emerge as user-friendly alternatives, with prior research suggesting independence from various factors in able-bodied individuals. However, research on RPE in SCI populations, and especially the comparative validity of different RPE scales in this context, remains limited. The need for a direct comparison between Borg's RPE scale and CR10 in SCI individuals, particularly during upper body exercise, is emphasized to aid in the interchangeable use of these scales for exercise prescription.

Twenty-four participants (8 AB, 8 PARA, 8 TETRA) completed graded exercise tests. AB performed cycle and handcycle ergometry; PARA performed handcycle ergometry; and TETRA performed wheelchair propulsion. Oxygen uptake (VO2), blood lactate concentration ([BLa]), heart rate (HR), and RPE (Borg's 6-20 and CR10) were measured. LT1 was identified as the intersection of log-[BLa] and log-VO2 plots; LT2 was defined as 1.5 mmol/L above LT1. RPE on both scales were individually fit against [BLa] using a quadratic function. Peak exercise responses and RPE at LT1 and LT2 were compared between groups using ANOVA. The relationship between RPE on both scales was analyzed using curve analysis (linear, quadratic, exponential, and power functions) with a multilevel model to account for within-subject dependency.

Peak VO2 was significantly higher in AB-CYC than in other groups. A quadratic model best represented the relationship between Borg's RPE scale and CR10 across all groups and exercise modes (R²: 0.965–0.970, P < 0.005). The quadratic coefficients were similar to those found in previous studies comparing these scales in AB adults. While absolute and relative VO2 at LT1 and LT2 were significantly higher in AB-CYC compared to other groups, there was no significant difference in RPE at LT1 or LT2 between groups on either scale (Borg's or CR10). Transformed values for CR10 based on Borg's RPE are provided in Table 4 of the paper.

The strong association between Borg's RPE scale and CR10, independent of exercise mode or participant group, supports their interchangeable use. The similar quadratic coefficients observed in this study and previous research reinforce the consistent relationship between the two scales. The lack of group differences in RPE at LT1 and LT2 suggests that RPE might serve as a reliable indicator of exercise intensity, regardless of exercise mode or level of SCI. However, the significant inter-individual variation in VO2 at these thresholds highlights the need for individualized exercise prescription based on RPE, rather than relying on group averages.

This study demonstrates a strong, consistent relationship between Borg's RPE scale and CR10 across various exercise modes and participant groups, including those with SCI. This supports their interchangeable use in exercise intensity prescription. While RPE at lactate thresholds appears independent of exercise mode and SCI level, substantial inter-individual variability necessitates individualized exercise prescription based on RPE. Future studies should explore the reliability and responsiveness of RPE across a wider range of SCI severities and training levels to further refine its use in clinical practice.

The relatively small sample size might limit the generalizability of the findings. The study focused on specific exercise modes (cycle ergometry, handcycle ergometry, wheelchair propulsion), and the findings may not be directly transferable to other activities. The cross-sectional design limits conclusions regarding causal relationships between RPE and physiological responses. Finally, the study did not include individuals with complete SCI (AIS A), potentially influencing the results.