Functional Fitness Deficits in Women with Fibromyalgia Relative to Later-Life Independence Standards: Contributions of Sedentary Time and Physical Activity

Background: Maintaining adequate physical fitness is essential for preserving functional independence with ageing. Women with fibromyalgia often exhibit reduced fitness levels, and daily sedentary time (ST) and physical activity (PA) may be important contributors to physical fitness. Objectives: To examine (1) the proportion of women with fibromyalgia meeting fitness standards for later-life independence, (2) associations of ST and PA with fitness, and (3) how reallocating ST to PA relates to fitness. Methods: 407 women with fibromyalgia (51.9 ± 8.0 years) participated. Fitness (Senior Fitness Test and handgrip strength) was compared with standards for women aged 60–64 as indicators of later-life independence. ST and PA were measured using triaxial accelerometry. Linear and isotemporal substitution models examined associations and estimated the theoretical associations of replacing ST with light, moderate, or vigorous PA in relation to physical fitness. Results: 13.3% of participants met cardiorespiratory, 7.9% lower-body strength, 29.9% upper-body endurance, and 4.4% agility standards. ST and PA were associated with fitness (all p < 0.05; adj R2 < 0.06). Reallocating 30 min of ST to light PA was associated with higher handgrip strength (B = 0.3); with moderate PA, cardiorespiratory fitness (B = 9.0); and with vigorous PA, upper-body muscular endurance (B = 11.1) and cardiorespiratory fitness (B = 104.9), all p < 0.05. Conclusions: Overall, only a small proportion of women met established fitness reference standards for later-life independence. ST was negatively and PA positively associated with fitness, although these associations explained little of its variability. In isotemporal substitution models, estimated replacement of ST with PA was associated with better fitness outcomes, particularly cardiorespiratory fitness and upper-body muscular endurance, with stronger associations at higher PA intensities. Further research is needed to identify additional modifiable determinants of physical fitness and support long-term functional independence.