Multiple Sclerosis (MS) is a progressive disorder that affects the brain and spinal cord. In MS, the body’s own immune system directs an abnormal response against the central nervous system, damaging and destroying myelin (the sheath that surrounds nerves) and nerve fibers. This damage disrupts nerve signals, causing the symptoms of MS, which vary from one person to another.
Walking limitations are one of the key components of disability in patients with MS. Previous clinical studies have shown that during a 6-minute walk test (6MWT), walking velocity (WV) typically decreases. It has also been shown that during the initial three minutes of the test, heart rate (HR) and the rate of oxygen consumption increase, reaching consistent rates for the remaining 3 minutes. Less studied is the relationship between heart rate and walking velocity during prolonged walking.
Heart Rate Response and Changes in Walking Velocity during the 12-Minute Walk Test in People with Multiple Sclerosis, a joint study between Rutgers University School of Health Professions (New Jersey) and Thomas Jefferson University (Pennsylvania), explored the relationship between the change in cardiovascular demand and Walking Velocity in 18 people with moderate MS symptoms.
Each patient completed a 12 Minute Walk Test (12MWT) on an electronic walkway, using Protokinetics’ PKMAS Gait Analysis software to collect data which was then parsed into 12 1-minute increments. During the 12MWT, the patients’ heart rates were continuously measured, and the average heart rate was calculated for each 1-minute interval. Pearson correlation coefficients were used to evaluate the relationship between heart rate and time, walking velocity and time, and the ratio of mean heart rate to mean walking velocity (HR:WV) and time.
In visual analysis of the parsed heart rate and walking velocity data, a steady rise was noted in heart rate during the initial 3 minutes of the 12MWT, followed by a consistent heart rate for the rest of the test. Visual analysis of walking velocity data revealed a U-pattern: during the first 2 minutes of the test, walking velocity decreased, then stabilized, then increased slightly during the last 2 minutes. This U-shaped result was similar to previous studies of MS patients in a 6MWT.
Then the ratio of mean heart rate to mean walking velocity was calculated for each minute and plotted over time. A significant linear correlation was found between HR and time and HR:WV and time.
Results:
The study’s data suggests that in people with moderate MS, heart rate reaches a consistent rate during the 12MWT, accompanied by a decrease in walking volume. This resulted in a significant positive correlation of HR:WV over time. Additionally, an accompanying decline in heart rate did not accompany the decline in walking velocity which suggests that cardiovascular demand may have remained consistent at a lower walking rate.
The study’s authors conclude: “The negative effect of fatigue on physical performance in people with MS is well-known; however, this data suggests that prolonged walking activity may also increase cardiovascular demand. Further study is warranted to fully evaluate the effect of fatigue-related changes on the metabolic cost of walking using objective physiologic measurements (e.g. VO2 max). A clearer understanding of these effects will enable rehabilitation professionals to better target the multiple contributors to the decline in performance during prolonged activities observed in people with MS.”