Muscle strength, voluntary activation, twitch properties, & endurance in patients with fibromyalgia (FM )

Previous studies have shown decreased voluntary muscle strength and

endurance in patients with fibromyalgia. The aim of this study

was to determine to what extent this is due to lack of

exertion. The twitch interpolation technique was used to

determine the degree of central activation and estimate the

“true” quadriceps muscle strength in patients with

fibromyalgia and age and sex matched controls.


hereafter performed an endurance test consisting of repetitive

contractions at 50% of estimated “true” muscle strength of

four seconds duration followed by a six second rest until

exhaustion, or maximally for 40 minutes. Twitch decline and

increases in mean rectified EMG were used as objective markers

of fatigue. The estimated “true” muscle strength was 82 (SD

26) Nm in 20 patients with fibromyalgia compared with 133 Nm

(SD 28) Nm in the 21 controls (p < 0.001). The "true" muscle

strength per cm2 midthigh cross sectional area was lower 0.50

(SD 0.15) Nm/cm2 in the patients compared with 0.74 (SD 0.15)

Nm/cm2 in the controls (p < 0.001). The decline over time in

twitch sizes was similar in the two groups. The mean rectified

EMG signal at a fixed force level of 50% of “true” muscle

strength increased similarly in the two groups. Relaxation

rates and contraction rates also increased equally in the two


In conclusion, a reduction of the estimated muscle

strength per area unit of about 35% was found in the patients

with fibromyalgia. This might be secondary to physical

inactivity or neuroendocrine factors. No differences in

changes in the neurophysiological indices associated with

fatigue were found between the two groups.

Norregaard J, Bulow PM, Danneskiold-Samsoe B

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