Individuals with spinal cord injury (SCI) frequently exhibit low daily physical activity and reduced cardiorespiratory fitness (CRF). CRF, representing the body's ability to transport oxygen for physical work, is crucial for overall health and is positively influenced by physical activity. Following discharge from inpatient rehabilitation, physical activity levels often decline in individuals with complete SCI, potentially leading to deconditioning and increased cardiovascular disease risk. In Norway, a structured follow-up system exists for SCI patients. This study focused on ambulatory individuals with incomplete SCI, who potentially possess more intact muscle groups below the injury level allowing for higher physical fitness potential. A previous study by the same authors showed CRF improvement in this group during the initial three months post-discharge; however, the long-term maintenance of these improvements remained unclear. This study aimed to describe the changes in CRF and activity levels over the first year after inpatient rehabilitation in ambulatory individuals with incomplete SCI.

Existing literature highlights the low levels of physical activity and reduced cardiorespiratory fitness in individuals with spinal cord injury (SCI). Several studies confirm this reduced fitness level compared to able-bodied individuals. Studies also show a decline in physical activity levels post-discharge in individuals with complete SCI, leading to deconditioning and associated health risks. However, limited research specifically addresses the long-term changes in CRF and activity levels in ambulatory individuals with incomplete SCI, who may have a higher potential for improved fitness due to partially preserved lower extremity function. This gap in the literature underscores the need for a longitudinal study exploring these changes over an extended period.

This study employed a secondary analysis of data from a 12-week randomized controlled trial (RCT) investigating the effects of a home-based aerobic exercise program. Thirty ambulatory participants (25 males, 5 females; aged 18-69) with incomplete SCI, capable of walking independently in the community, were included. The RCT consisted of three groups: high-intensity interval training (HIIT), moderate-intensity training (MIT), and a control group. CRF was assessed using a maximal treadmill exercise test (measuring peak VO2, respiratory exchange ratio (RER), peak heart rate, and blood lactate), and a 6-minute walk test (6MWT). Physical activity was monitored using a SenseWear™ Pro2 Armband, measuring total daily energy expenditure (TDEE) and daily steps. Assessments were conducted at baseline (last week of inpatient rehabilitation), 3 months, and 12 months post-discharge. Due to attrition, only 20 participants completed all assessments. Linear mixed model analyses were used to examine changes in peak VO2, 6MWT distance, TDEE, and daily steps over time, considering the three intervention groups initially. Due to lack of significant between-group differences the data across groups were then pooled for further analysis.

Twenty out of the initial thirty participants completed the study. No statistically significant differences were observed between the three intervention groups (HIIT, MIT, control) regarding changes in peak VO2, 6MWT performance, or physical activity outcomes over the study period. Therefore, data from all three groups were combined for subsequent analyses. These analyses revealed significant increases in peak VO2 (both l/min and ml/kg/min), 6MWT distance, and TDEE across the 12-month follow-up. The increase in peak VO2 was most pronounced in the first 3 months, followed by a leveling off. Interestingly, while TDEE increased significantly, the increase in average daily steps did not reach statistical significance. Specifically, the mean peak VO2 increased from 2.76 l/min at baseline to 3.03 l/min at 12 months, and the 6MWT distance improved from 581 m to 672 m over the same period. The TDEE increased from 2632 kcal to 2750 kcal. The heart rate after the 6MWT was significantly higher at 12 months compared to baseline, suggesting an increase in aerobic intensity during the 6MWT.

This study demonstrates a significant improvement in CRF in ambulatory individuals with incomplete SCI during the first year after discharge from inpatient rehabilitation, as evidenced by increased peak VO2 and 6MWT performance. This improvement occurred despite a relatively small increase in overall daily activity levels. The initial substantial increase in CRF during the first three months might be attributable to the structured exercise program within the RCT, even within the control group. The subsequent plateau in CRF improvement suggests that participants continued some level of regular physical activity, maintaining the gains. The higher heart rate after the 6MWT at 12 months indicates participants were able to perform at a higher aerobic intensity. While baseline fitness levels were relatively high compared to previous studies, the participants in the study exhibited less fitness than able-bodied peers, suggesting considerable room for improvement. The observed results suggest that even minimal increases in physical activity can lead to substantial gains in CRF in this population.

Ambulatory individuals with incomplete SCI demonstrate a substantial increase in cardiorespiratory fitness during the first year after discharge from rehabilitation, despite minimal increases in overall physical activity. This highlights the importance of encouraging even moderate physical activity to optimize CRF in this population. Future research could investigate the specific types and amounts of physical activity that are most effective in maintaining or further improving CRF in the long term. Further exploration of the relationship between specific exercise interventions and changes in other parameters such as muscle strength and spasticity is also warranted.

The primary limitation of this study is the attrition rate, resulting in a smaller sample size for the 12-month follow-up. The lack of detailed information on physical activity levels beyond the structured intervention period limits a precise understanding of the relationship between activity and CRF improvement. The self-reported exercise data from the initial trial may also have been subject to recall bias. Finally, the relatively high baseline fitness levels of the participants, likely due to selective inclusion, may limit the generalizability of the findings to individuals with lower initial fitness levels.