Do cognitive and physical fatigue tasks enhance pain, cognitive fatigue, and physical fatigue in people with fibromyalgia?

Fibromyalgia affects 4-10% of the US population affecting 3.4% of women and 0.5% of men in the US (1). The 1990 American College of Rheumatology diagnostic criteria for fibromyalgia includes chronic widespread pain on both sides of the body and above and below the waist as well as pain in 11 of 18 specified sites (tender points) with digital palpation (2). Fatigue is an extremely common symptom in fibromyalgia with up to 100% of people with fibromyalgia reporting fatigue and is greater in severity compared to other arthritic conditions (3). Fatigue, as described clinically, is a subjective experience and may have both physical and cognitive components that are related yet distinct (4). While the pain associated with fibromyalgia contributes to significantly reduced function, the relationship between pain, fatigue and function is currently not well understood. Muscle fatigue has been previously measured in people with fibromyalgia using a variety of techniques. In general prior studies have examined static contractions of a single muscle in the upper extremity or the lower extremity, or a bicycle exercise task. Voluntary muscle strength and endurance are decreased in people with fibromyalgia (5-9). When compared to sedentary controls, the majority of studies show no differences in peripheral or central fatigue indices for a variety of muscle types (5, 9, 10). However, when compared to healthy controls, one study showed altered motor recruitment during voluntary contraction of the biceps muscle in people with fibromyalgia (11), and two showed greater superimposed twitches during quadriceps contraction in people with fibromyalgia (8, 12). On the other hand, people with fibromyalgia rate their perceived fatigue and exertion significantly higher before and during exercise tasks (bicycle task or single muscle contractions) compared to healthy controls (6, 10). People with fibromyalgia typically describe fatigue as an overall feeling of tiredness or exhaustion, fatigue while completing functional tasks (e.g. folding laundry, drying hair or getting dressed), decreased attention, sleepiness, or feeling of heaviness. Thus, examination of perceived physical fatigue in response to a functional task is multifactorial and may be distinctly different from responses to a fatiguing exercise task using a single muscle, or direct measures of muscle fatigue mechanisms. Perceived cognitive fatigue is distinctly different from perceived physical fatigue. Perceived cognitive fatigue is a subjective self-report of cognitive fatigue often measured using a visual analog scale. Perceived cognitive fatigue is different from cognitive dyscognition (13) occurring when there is decreased performance during an acute task requiring sustained mental effort. Cognitive performance is measured by verbal fluency, memory, concentration, automatic processing (13, 14), and is commonly decreased in people with fibromyalgia (15). ‘Fibro fog” is a subjective report of cognitive difficulties such as mental confusion, memory difficulties, memory decline or speech difficulties (13, 15, 16) and could reflect both cognitive fatigue and/or cognitive performance. Performing both a physical and a cognitive fatigue task simultaneously exacerbates both physical and cognitive performance when done as individual tasks in people with neurological diseases, i.e. stroke, multiple sclerosis and Parkinson's (17, 18). However, it is unclear if a decrease in performance is observed in people with fibromyalgia during a dual fatigue task, and if there are alterations in cognitive fatigue in people with fibromyalgia. The primary objective of this study was to determine if pain, perceived cognitive fatigue, and perceived physical fatigue were enhanced in participants with fibromyalgia compared to healthy controls during a cognitive fatigue task, a physical fatigue task and a dual fatigue task. We tested the following hypotheses: 1) perceived cognitive and physical fatigue are enhanced during a cognitive fatigue task, physical fatigue task and dual fatigue task in people with fibromyalgia compared to healthy controls and 2) physical and/or cognitive fatigue tasks will result in greater pain and worse function in people with fibromyalgia compared to healthy controls.