Heart Rate Response and Changes in Walking Velocity during the 12-Minute Walk Test in People with Multiple Sclerosis

Background: People with multiple sclerosis (MS) commonly experience a decline in performance with prolonged walking. For example, walking velocity (WV) generally decreased during the 6-minute walk test (6MWT). In addition, heart rate (HR) and rate of oxygen consumption increases during the initial three minutes of the 6MWT after which an apparent steady-state is reached. However, the direct relationship between HR and WV during prolonged walking has not been well studied.

Objectives: To explore the relationship between change in HR (i.e. cardiovascular demand) and WV during prolonged walking in people with moderate MS-related disability.

Methods: 18 people with confirmed MS (14 women, median Disease Steps=3 (IQR=1) and Patient-Determined Disease Steps=3 (IQR 1.5)) completed a 12-minute walk test (12MWT) on an oval track with an emedded instrumented walkway (GAITRite, CIR Systems, Sparta, NJ). WV data was collected with PKMAS Gait Analysis Software (Protokinetics, Havertown, PA) and parsed into 1-minute time increments. HR was continuously measured with a Polar H7 wireless sensor (Polar Electro, Inc., Lake Success, NY) and the average HR was calculated for each 1-minute interval. Per-minute HR and WV data were first analyzed visually to identify gross patterns of change. The ratio of mean HR to mean WV was then calculated (HR/WV) for each minute and plotted over time. The Pearson correlation coefficient was calculated to examine the relationship between HR/WV and time.

Results: Visual data analysis revealed a steady rise in HR during the initial 3 minutes of the test followed by a plateau phase for the remainder. Mean WV decreased within the first two minutes to a level point that stabilized until it rose slightly during the last 2 minutes making an attenuated U-shaped pattern. These patterns are consistent with those found in previous studies of the 6MWT. There was a significant linear correlation between HR/WV and time (r= .928, p=.000).

Conclusions: Our data suggests that HR reaches a steady-state during the 12MWT and that is accompanied by a decrease in WV in people with moderate MS. This resulted in a significant increase in the HR/WV over time. The observation that a concomitant decline in HR did not accompany the decline in WV suggests that cardiovascular demand may have remained consistent at a lower work rate. 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.