The novel finding of this study is that premenopausal SLE patients with low disease activity show lower dynamic muscle strength (upper- and lower-limb) when compared with their healthy peers. Furthermore, we provided the evidence that lower dynamic muscle strength was associated with fatigue, low functional performance, and poor quality of life (namely, role-emotional functioning) in SLE patients.
Our results are in agreement with those by Tench et al.  and Stockton et al. , who demonstrated that SLE patients have lower isometric muscle strength when compared with healthy controls. However, the aforementioned studies evaluated muscle strength using isometric tests. In this regard, one may argue that dynamic strength tests may be more informative than static tests in terms of physical function evaluation because daily living functioning primarily encompasses dynamic rather than isometric contractions . In fact, the significant association between dynamic strength (i.e., 1-RM) and physical function assessments (i.e., chair timed-stands) observed in the current study further supports this notion.
Fatigue scores (as assessed by the FSS questionnaire) lower than 4.0 suggest that fatigue is not severe enough to limit participation in daily living physical activities. Conversely, FSS scores higher than 4.0 suggest that fatigue is perceived to adversely affect the ability to engage in physical and social activities . In the current study, however, the SLE patients scored 3.5 on average (with 15 of 25 patients having FSS scores lower than 4). However, the patients experienced decreased physical function, low dynamic muscle strength capacity, and poor quality of life, suggesting that either “residual” fatigue or other factors (e.g., long-term medication or systemic inflammation) may have contributed to the poor health-related findings demonstrated in this study. Further studies must elucidate the role of fatigue on health-related parameters in SLE patients.
We observed that even with low fatigue and low disease activity scores, 20% (5 of 25) of the SLE patients showed handgrip strength between 17 and 20 kg. Notably, these values are considered a marker of sarcopenia . The handgrip strength test has been considered a clinical marker of mobility [23, 31] and lower limb muscle strength . Moreover, a 5-kg increase in handgrip strength has been associated with a significantly reduced mortality risk . This fact, along with the fact that the handgrip strength test has been proven reliable in SLE patients , make this simple and inexpensive tool an emerging marker of clinical relevance. Further prospective studies should test its ability as a prognostic marker in SLE.
Our study must be interpreted in light of its strengths and limitations. Although a few studies have also demonstrated lower physical function in SLE patients [7, 8], these studies did not control for important confounding factors that affect physical performance, such as obesity , fibromyalgia [9, 10], perimenopause , smoking , use of beta-blocker  and statins , activities of daily living, physical activity level , and socioeconomic-status . In the present study, we did correct for these confounding factors; therefore, their influence on the muscle strength may be considered minimal.
However, this study is not without limitations. First, the cross-sectional nature of this study precluded us to establish cause-effect relationships between muscle strength and health-related parameters in SLE patients (e.g., role-emotional functioning from SF-36). Second, our homogeneous sample comprised premenopausal SLE patients with low disease activity and who were free of comorbidities and associated diseases. Therefore, one cannot extrapolate the present results to older or younger patients with more severe disease. Finally, our sample size was relatively low. Further studies should test the accuracy of our multivariate model in a larger patient cohort.