Tuesday, 27 March 2012

Fatigue and the energetic costs of walking

Epub ahead of printKempen et al.  Self-Reported Fatigue and Energy Cost During Walking Are Not Related in Patients With Multiple Sclerosis.Arch Phys Med Rehabil. 2012 Mar 20. 

OBJECTIVES:  To determine whether there is a relationship between self-reported fatigue and the energy cost of walking (ECw), and how self-reported fatigue and ECw relate to physical functioning in patients with multiple sclerosis (MS).

PARTICIPANTS: MSers (N=75) were obtained from a longitudinal study on outcome measurement and functional prognosis in early MS. Patients were included if they were able to walk for 6 minutes without being assisted by a person. The age range was between 28.0 and 69.7 years and the median Expanded Disability Status Scale was 2.5 (range, 1.0-6.5).

MAIN OUTCOME MEASURES: Self-reported fatigue was measured with the Fatigue Severity Scale, the vitality subscale of the Medical Outcomes Study 36-Item Short-Form Health Survey (SF-36), and a visual analog scale. Physical functioning was determined with the physical functioning subscale of the SF-36, fast walking speed, and comfortable walking speed. The ECw (J·kg(-1)·m(-1)) was measured with the energy cost of the walking test.

RESULTS: The relationship between ECw and latent variable fatigue had a β=-.188 (P=.236), that between ECw and physical functioning (SF-36 physical functioning) had a β=-.344 (P=.001), and that between fatigue and physical functioning had a β=-.448 (P=.000).

CONCLUSIONS: Fatigue and ECw are not related in patients with MS with mild to moderate walking problems. ECw and fatigue are independent determinants of physical functioning.


"MS-related fatigue, a percept, is not related to the energy cost of walking. A percept is a term we use as for the perception by the cortex of something in our external or internal environment. Fatigue is an important symptom and is usually a feedback mechanism to tell our bodies to take a rest. In MS this system goes awry; in MS there is a disconnect between what the brain perceives as fatigue and the energy requirements of performing a task such as walking. This confirms that fatigue is a complex symptom and will need to be tackled at multiple levels if we want to make a difference to MSers."

2 comments:

  1. I don't know if this is relevant, but this reminds me of something I recently read in another context about Timothy Noakes and the central governor theory of fatigue (http://en.wikipedia.org/wiki/Central_governor). The book I was reading (Willpower Instinct by McGonigal) said that fatigue can be deceptive. The conventional wisdom among exercise physiologists used to be that our bodies give up when they literally can't keep working. Fatigue = muscle failure = muscles run out of energy stores, i.e., they can't take in enough O2 to metabolize the energy they have or the pH of the blood becomes too acidic or alkaline, etc. Timothy Noakes (based on Archibald Hill) proposed that exercise fatigue might rather be caused by an overprotective monitor in the brain trying to prevent exhaustion. Physical exhaustion = a trick played on the body by the mind. He says fatigue is not a physical event but rather a sensation or emotion. It works like anxiety works to stop dangerous acts or disgust stops us from eating something that will make us ill. Since fatigue is an early warning sign, with motivation you can push past it.

    Here's another summary of Noakes' idea:
    " In this review, fatigue is described as a conscious sensation rather than a physiological occurrence. We suggest that the sensation of fatigue is the conscious awareness of changes in subconscious homeostatic control systems, and is derived from a temporal difference between subconscious representations of these homeostatic control systems in neural networks that are induced by changes in the level of activity. These mismatches are perceived by consciousness-producing structures in the brain as the sensation of fatigue. In this model, fatigue is a complex emotion affected by factors such as motivation and drive, other emotions such as anger and fear, and memory of prior activity. It is not clear whether the origin of the conscious sensation of fatigue is associated with particular localised brain structures, or is the result of electrophysiological synchronisation of entire brain activity."
    http://www.ncbi.nlm.nih.gov/pubmed/12656638

    This might be one way to look at fatigue as divorced from what is actually physically happening in the muscles.

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  2. Re; "This might be one way to look at fatigue as divorced from what is actually physically happening in the muscles."

    I agree with this theory as it applies to normal physiology, I am not sure if it holds true for MS-related fatigue.

    I am big fan of Tim Noakes and his book "The Lore of Running" has pride of place on my bookshelf at home.

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