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SPECIAL COMMUNICATIONS: Letters to the Editor-in-Chief


Hubbard, Elizabeth A.; Motl, Robert W.

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Medicine & Science in Sports & Exercise: September 2020 - Volume 52 - Issue 9 - p 2056
doi: 10.1249/MSS.0000000000002415
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Dear Editor-in-Chief,

We thank our colleagues (1) for being interested in our recently published article (2), and we value their feedback regarding the ecological validity of our work. We compared the physiological profiles of single sessions of low-volume, high-intensity interval training (HIIT) and moderate-intensity, continuous exercise (CON) using a recumbent stepper in persons with multiple sclerosis (MS) who had mobility disability (2). Our purpose was to establish the viability of HIIT within a training protocol for improving physiological and clinically relevant outcomes in the sample population.

Ecological validity involves generalizing results from an artificial situation (e.g., research laboratory) into natural situations (e.g., physical therapy offices) and requires that researchers administer real-world tasks that people actually perform within the problem domain (3). Accordingly, we believe that choosing exercise protocols that are standard practice, meet current recommendations for the population of interest, and reduce participant burden are preferred. The standard low-volume, HIIT protocol requires 10 min of exercise performed over 20 min in a 1:1 work/rest ratio with 60 s of high intensity (i.e., the workload associated with 90% V˙O2peak) and 60 s of low intensity (i.e., active recovery at 15 W) (4). The Canadian Physical Activity Guidelines for Adults with MS recommend moderate-intensity aerobic activity for 30 min, two times per week (5). Our two exercise protocols were matched for exercise duration based on current participant-specific guidelines and minimizing time burdens as lack of time is one of the most common barriers for engaging in exercise (4,6).

Hidde et al. (1) suggest that generalizing our study findings into real-world settings requires matched exercise volume between HIIT and CON conditions. We actually noted that a limitation of our study’s design was that it did not include matched-work and suggested that future researchers use a matched-work design when comparing physiological differences between the two protocols (2). Using the ACSM’s volume recommendations for high-intensity exercise (7) and the NuStep metabolic equations provided by Hidde et al. (1), we should have suggested decreasing the total number of HIIT intervals to either 8 (i.e., 48 MET·min) or 9 (i.e., 56 MET·min) rather than 10 for a work-match with the CON session (i.e., 52 MET·min; Note: Hidde et al. incorrectly calculated the MET-minutes of the CON session at 44 MET·min instead of the actual 52 MET·min). Nonetheless, we hypothesize that a matched-work HIIT session would still yield significantly higher physiological outcomes than a CON session in this population when considering the profile of responses across time with the HIIT and CON sessions, thereby indicating an overall time efficiency of this protocol (4,8).

Nevertheless, matching for exercise volume would not have changed the overall aim of the study, which was to identify if the standard, low-volume HIIT protocol was safe, feasible, and efficacious for inducing a significant cardiorespiratory response in persons with MS who have mobility disability. Indeed, our data confirmed the safety and viability of the prescribed HIIT session for use in an intervention study for improving clinically relevant outcomes for persons with MS who have mobility disability (2).

Elizabeth A. Hubbard
Department of Kinesiology
Berry College
Mount Berry, GA
Robert W. Motl
Department of Physical Therapy
University of Alabama at Birmingham
Birmingham, AL


1. Hidde MC, Howell M, DeBord A, Leach HJ. Acute high-intensity interval exercise in multiple sclerosis with mobility disability. Med Sci Sports Exerc. 2020;52(9):2055.
2. Hubbard EA, Motl RW, Fernhall B. Acute high-intensity interval exercise in multiple sclerosis with mobility disability. Med Sci Sports Exerc. 2019;51(5):858–67.
3. Czaja SJ, Sharit J. Practically relevant research: capturing real world tasks, environments, and outcomes. Gerontologist. 2003;43(1 Suppl):9–18.
4. Gibala MJ, Little JP, MacDonald MJ, Hawley JA. Physiological adaptations to low-volume, high-intensity interval training in health and disease. J Physiol. 2012;590(5):1077–84.
5. Latimer-Cheung AE, Martin Ginis KA, Hicks AL, et al. Development of evidence-informed physical activity guidelines for adults with multiple sclerosis. Arch Phys Med Rehabil. 2013;94(9):1829–1836.e7.
6. Asano M, Duquette P, Andersen R, Lapierre Y, Mayo NE. Exercise barriers and preferences among women and men with multiple sclerosis. Disabil Rehabil. 2013;35(5):353–61.
7. American College of Sports Medicine. ACSM’s Guidelines for Exercise Testing and Prescription. 10th ed. Lippincott Williams & Wilkins. 2018. 481 p.
8. Zimmer P, Bloch W, Schenk A, et al. High-intensity interval exercise improves cognitive performance and reduces matrix metalloproteinases-2 serum levels in persons with multiple sclerosis: a randomized controlled trial. Mult Scler. 2018;24(12):1635–44.
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