Changes in cardiorespiratory fitness and activity levels over the first year after discharge in ambulatory persons with recent incomplete spinal cord injury

The study addresses whether ambulatory persons with recent incomplete spinal cord injury (SCI) can improve and/or maintain cardiorespiratory fitness (CRF) and physical activity levels during the first year after discharge from inpatient rehabilitation. The context is that individuals with SCI often have low daily physical activity and reduced CRF, which can elevate cardiovascular risk. Prior reports indicate post-discharge declines in activity for persons with complete SCI, while those with incomplete SCI—especially community walkers—may have greater potential to achieve higher fitness due to more intact musculature below the lesion. The purpose was to describe longitudinal changes in CRF and activity over 12 months following discharge in ambulatory persons with incomplete SCI.

Background literature cited indicates: persons with SCI typically exhibit low physical activity and reduced CRF; physical activity can improve CRF and health factors in SCI; after discharge, activity levels may decrease in those with complete SCI, potentially increasing cardiovascular risk. Ambulatory persons with incomplete SCI also appear inactive and less fit than able-bodied peers, though they may have greater potential for fitness gains due to preserved muscle groups. Prior work by the authors showed increased CRF but not physical activity over the first 3 months post-discharge. Cross-sectional studies in ambulatory incomplete SCI report variable fitness levels; some cohorts show lower fitness and 6MWT performance than in the present study, possibly due to differing inclusion criteria and time since injury.

Design: Secondary analysis of a randomized controlled trial (RCT) with 12-week home-based aerobic exercise intervention and follow-up at 3 and 12 months post-discharge. Groups: high-intensity interval training (HIIT), moderate-intensity training (MIT), or treatment as usual (control). Due to no between-group differences over time, groups were pooled for secondary analyses. Participants: 30 adults with incomplete SCI (25 men, 5 women; 18–69 years), community walkers, recruited during the last two weeks of inpatient rehabilitation at Sunnaas Rehabilitation Hospital (2013–2017). Inclusion: 18–70 years; traumatic or nontraumatic SCI at any lesion level; able to walk 5 minutes at 3 km/h on treadmill without assistive aids; in final phase of subacute inpatient rehabilitation. Exclusion: significant comorbidities limiting CRF (psychiatric, orthopedic, uncontrolled cardiopulmonary disease). Intended sample size was 45; recruitment ended early due to time constraints. Ten participants were lost to 12-month follow-up (lack of motivation n=6, pain n=3, comorbidity n=1); 20 completed baseline, 3-, and 12-month assessments. Interventions: MIT group trained 3 times/week, 45 minutes continuous walking/running at 70% of peak HR. HIIT group trained walking/running at 85–95% of peak HR with 3 x 3-minute recovery at 70% of peak HR. Mode: treadmill or outdoors based on CRF and constraints. Control group received no prescribed aerobic exercise but no restrictions on physical training; all participants could access community healthcare providers as needed. Procedures and Outcomes: Baseline CRF tests performed in last inpatient week: maximal graded treadmill exercise test (Woodway PPS Med) with breath-by-breath spirometry (Vmax 220) to determine peak VO₂ (primary outcome). Ancillary maximal effort criteria: respiratory exchange ratio (RER >1.15), peak HR >85% of expected (men: >220−0.88×age; women: 220−0.66×age), and blood lactate [La−] >8.0 mmol/L. Treadmill protocol: after self-selected walking speed established, incline increased 2% per minute up to 20% or until exhaustion; if still not exhausted, speed increased 0.5 km/h per minute until exhaustion. VO₂ averaged over 30 s; highest average taken as peak VO₂. Secondary outcome: 6-minute walk test (6MWT), recording distance and post-test HR (Polar M400). Physical activity monitoring: SenseWear Pro2 Armband on right upper arm for 7 consecutive days at home, measuring total daily energy expenditure (TDEE, converted to kcal by dividing kJ by 41,858) and daily step count; analyzed with SenseWear Professional 7.0. Follow-up CRF tests at 3 and 12 months post-discharge; physical activity monitoring conducted during 7 days after hospital testing. Statistical analysis: SPSS v25. Descriptive means and SDs at baseline, 3 months, and 12 months. Linear mixed models for repeated measures used to model time course of peak VO₂ (L/min and mL/kg/min), 6MWT distance, TDEE, and daily steps. Time treated as fixed repeated factor (0=baseline, 1=3 months, 2=12 months), with individual random intercept and slope; variance components covariance structure. Quadratic time term (time²) tested for nonlinearity. Model selection by Akaike Information Criterion (AIC). Between-group differences over time assessed; none were significant, so data pooled.

- Of 30 enrolled, 20 completed all assessments at baseline, 3 months, and 12 months; none reported wheelchair use during the first year after discharge. - No statistically significant between-group differences over time for peak VO₂, 6MWT, TDEE, or daily steps; groups were merged for analysis. - Peak VO₂ increased significantly over time with a nonlinear trajectory peaking at 3 months and slight decline/leveling by 12 months: • Peak VO₂ (L/min): mean ± SD baseline 2.76 ± 0.71; 3 months 3.13 ± 0.72; 12 months 3.03 ± 0.67. • Peak VO₂ (mL/kg/min): baseline 34.9 ± 9.2; 3 months 39.5 ± 8.8; 12 months 38.0 ± 9.4. • Mixed model showed significant linear time effect and significant negative time² term, indicating non-constant rate of change. - Walking capacity improved significantly: • 6MWT distance (m): baseline 581 ± 89; 3 months 658 ± 106; 12 months 672 ± 91. The ~91 m increase by 12 months is clinically meaningful. • Post-test HR increased from 135 ± 25 bpm at baseline to 148 ± 23 bpm at 12 months, suggesting higher aerobic intensity during the test. - Physical activity: • TDEE (kcal/day) increased modestly and significantly over time: baseline 2632 ± 509; 3 months 2739 ± 412; 12 months 2750 ± 443. • Daily steps showed a non-significant increase: baseline 5724 ± 2786; 3 months 5872 ± 2330; 12 months 6431 ± 2891. - Participants exhibited relatively high fitness compared with able-bodied norms, with peak VO₂ only ~10% lower at baseline and ~5% lower at 12 months than Norwegian able-bodied references.

The study demonstrates that ambulatory individuals with recent incomplete SCI can significantly improve CRF and walking capacity during the first year post-discharge, even with only minor increases in habitual activity measured by steps. The largest gains in peak VO₂ and 6MWT occurred in the first 3 months—coinciding with the structured exercise intervention period—and then plateaued, suggesting participants likely continued exercising at sufficient intensity to maintain CRF through 12 months. The dissociation between significant CRF gains and non-significant changes in steps indicates that intensity and quality of activity (e.g., structured aerobic training) may be more influential on CRF than total step count alone. The improved 6MWT performance, accompanied by higher post-test HR, supports enhanced capacity to sustain higher aerobic intensity. Compared with able-bodied references, participants had relatively high fitness levels, potentially reflecting pre-injury training habits and motivation, which may limit generalizability to the broader ambulatory SCI population. Overall, the findings address the research question by showing that post-discharge CRF can be improved and sustained with training, without large increases in total daily steps, highlighting the importance of targeted aerobic exercise in rehabilitation and follow-up.

Ambulatory persons with recent incomplete SCI significantly improved cardiorespiratory fitness and walking capacity over the first year after discharge from inpatient rehabilitation, with the most pronounced gains within the first 3 months and maintenance thereafter. Total daily energy expenditure increased modestly, while daily step counts did not change significantly, underscoring that structured, sufficiently intense aerobic exercise may drive CRF improvements more than simple activity volume. Future research should track and characterize exercise behavior between 3 and 12 months post-discharge, evaluate interventions to increase habitual physical activity alongside fitness, include broader samples to improve generalizability, and examine additional contributors (e.g., lower extremity strength, motor control, spasticity) to functional gains.

- Sample size was smaller than intended (30 enrolled vs. planned 45) and only 20 completed 12-month follow-up, which may reduce statistical power and introduce attrition bias. - No significant between-group differences and subsequent pooling limits conclusions about comparative efficacy of HIIT vs. MIT vs. usual care. - Lack of data on training behavior and exercise exposure from 3 to 12 months limits interpretation of maintenance mechanisms. - Participants likely had higher baseline fitness and motivation (e.g., pre-injury exercise habits), which may limit generalizability to the broader ambulatory incomplete SCI population. - Potential influential factors such as lower extremity muscle strength, motor control, and spasticity were not measured. - Slight body weight increase (~2.6 kg at 12 months) may have influenced relative VO₂ measures.