By Ines Lacarne
This is a summary of a paper published by researchers at the Toronto Rehab Institute. Original article: Moore C.D., et al (2015). Lower-extremity muscle atrophy and fat infiltration after chronic spinal cord injury. Journal of Musculoskeletal and Neuronal Interactions. 15(1):32-41. Find the original article here.
Chronic spinal cord injury (SCI) is associated with higher risk for a number of complications, such as cardiovascular disease and type II diabetes. Changes in muscle physiology and body composition (percentage of fat, bone, water and muscle in our bodies) are linked to these clinical outcomes, and SCI affects both muscle physiology and body composition.
This study looked at two measures of changes in muscle physiology and body composition in the legs of SCI patients: muscle atrophy and fat infiltration. Muscle atrophy is a decrease in muscle mass, typically due to underuse; fat infiltration refers to deposition of fat tissue within skeletal muscle (the majority of muscle tissue, which powers movement of the skeleton such as walking). Skeletal muscles help maintain metabolic and bone health. Fat tissue deposits in skeletal muscle are linked to chronic inflammation, glucose intolerance, and decreased strength and mobility.
Reductions in muscle size and increases in fat infiltration have been reported in small-scale studies after SCI, however there is a need for studies that include larger and more diverse samples of individuals living with SCI to understand factors related to these muscle changes. Therefore, this study aimed to look at levels of muscle atrophy and fat infiltration in a sample of chronic SCI patients with various impairments.
A sample of 70 participants with SCI (and age, height, and gender matched able-bodied controls) was used. All were at least 2 years post-injury, and varied in severity and level of impairment: 45 had motor-complete spinal cord injury (no motor or sensory function is preserved), 25 had motor-incomplete spinal cord injury (some motor and sensory function preserved), of which 14 used a wheelchair for ambulation.
Researchers took measurements of calf muscle area and density (a measure of fat content) via CT scans (specialized X-rays that use a series of images taken from different angles to create cross-sectional images, or slices) and participants were assessed for the following characteristics:
- Body mass and waist circumference
- Frequency and severity of spasticity (muscle spasms)
- Physical activity
- Wheelchair use
- Duration of SCI
- Age at the time of SCI
- Level of SCI
- Voluntary Calf-muscle function
The authors then looked at the relationships between these different characteristics and muscle status.
As expected, individuals with SCI had smaller calf muscles and more fat infiltration of muscle compared to able-bodied controls.
Muscle area was strongly linked to body mass after complete and incomplete SCI. Wheelchair use, having a lower body mass, being paraplegic, having less voluntary muscle function, experiencing less muscle spasms, and less vigorous physical activity participation, were associated with smaller calf muscles.
Having a larger waist circumference, wheelchair use, having less voluntary muscle function, having less spasticity, performing fewer minutes of daily vigorous physical activity, and having an older age, were associated with lower muscle density.
What does this mean for individuals living with SCI?
As well as characterizing some of the physiological changes that occur after spinal cord injury, these findings can also contribute to the application of therapies and treatments to help preserve muscle health. Having looked at a variety of SCI-related characteristics can help inform who should use these therapies.
From the results of this study, it appears changes in muscle area and density are heavily linked to factors related to either muscle function or body size/composition.
Physical activity and involuntary spasticity were associated with improved muscle health (decreased fat infiltration and increased muscle size), which emphasizes the importance of exercise and muscle activation.
Muscle changes were associated with wheelchair use: this suggests that maintaining muscle function through upright ambulation (in incomplete SCI with some preserved motor function) may prevent muscle loss and secondary metabolic complications as a result. Participation in physical activity supplemented with electrical stimulation therapy may thus be beneficial for patients without voluntary muscle contraction.
With regards to body size and composition, results of the study suggest that maintaining a healthy body composition may help prevent metabolic complications. Waist circumference is commonly used as an indicator of risk for obesity. In the studied participants, leg muscle density was associated with higher waist circumference; therefore this measurement could potentially be used to predict metabolic health outcomes after chronic SCI in the future.