Fibromyalgia fatigue shows up in the muscles, not just the mind

Fibromyalgia and chronic fatigue syndrome are defined largely by symptoms: pain, profound tiredness and poor tolerance of exertion. A long-standing question is whether these conditions come with objectively measurable differences in the heart, lungs and muscles, or whether standard physiological tests find little out of the ordinary. The answer matters because perceived effort and actual capacity are easily conflated, and because people with these diagnoses are often encouraged to be more active. Establishing what the accumulated evidence shows about fitness and muscle function in these groups, compared with healthy people, is the first step towards understanding what is actually limiting them.

The researchers swept six databases, including PubMed, Medline, CINAHL, AMED, the Cochrane trials register and PEDro, from their inception to June 2022. They included observational and interventional studies that reported baseline measurements of cardiorespiratory fitness or neuromuscular function in adults aged 18 or older diagnosed with fibromyalgia or chronic fatigue syndrome, provided results were also given for healthy controls. In total 99 studies met the criteria, covering 9853 participants: 5808 patients and 4405 healthy controls. Study quality was assessed with a recognised appraisal tool, and the results were pooled statistically across studies.

People with fibromyalgia or chronic fatigue syndrome showed lower cardiorespiratory fitness than healthy controls across VO2max, anaerobic threshold and peak lactate. Their neuromuscular function was also poorer, spanning maximal voluntary contraction, fatigability, voluntary activation, muscle volume and muscle mass, and they reported higher perceived exertion. The differences were moderate to large across the board rather than marginal. The authors also noted indications of dysregulated interactions between nerves and muscles: a heightened perception of effort, a reduced ability to activate the musculature that is available during exercise, and reduced tolerance of exercise overall.

This pooled analysis confirms, across a large and varied literature, what many smaller studies had suggested individually: the reduced capacity in these conditions is consistent and objectively measurable, not confined to self-report. What it cannot settle is direction. Because it draws on baseline comparisons rather than experiments, it cannot say whether illness mechanisms impair the muscles and cardiovascular system directly, or whether reduced activity over years produces deconditioning, or both. It also leaves open whether training can shift these measures in patients, and whether fibromyalgia and chronic fatigue syndrome differ from each other in ways this combined analysis obscures.

For readers living with either condition, this is a reason to understand exercise intolerance as something with measurable physiological correlates: lower aerobic capacity, weaker maximal strength, less complete muscle activation and a genuinely heightened sense of effort. For readers who know someone with these diagnoses, it is a reason to take their reported limits seriously rather than reading them as low motivation. What it does not do is tell anyone what caused those differences or how to reverse them, and it offers no verdict on any particular exercise approach.