Background/Purpose: Systemic inflammation in RA not only affects joints, but also body composition. People with RA tend to have lower lean body mass and higher body fat compared to healthy persons. Fat is also present inside the muscle, but little is known about how these fat depots are affected in RA, and their role in physical function. Skeletal muscle fat can potentially interfere with muscle fiber function and metabolic activity, and thus can be hypothesized to affect physical function. The aim of this study was to explore the association of skeletal muscle fat with outcomes of physical function and physical activity in persons with RA.

Methods: This was a cross-sectional, secondary analysis of baseline data from a study in adults diagnosed with RA as per the ACR criteria. Skeletal muscle fat was quantified as the average muscle attenuation (MA) of the mid-thigh region, and was obtained through bilateral computed tomography (CT) imaging. MA values range from 0 to 100 Hounsfield units; higher values represent greater muscle density and thus, lower fat content. Physical function measures consisted of quadriceps maximum isometric strength, single leg balance time (up to 30 seconds), and the number of seconds taken to perform functional tasks such as chair rise (five times), ascend one flight of stairs, and walk four meters. Physical activity was captured with the SenseWear Armband, and time spent in moderate level activities (≥ 3 metabolic equivalent-MET) over 7 days was calculated. Bivariate correlations were used to test the associations of mid-thigh MA with physical function and physical activity. Separate hierarchical regression models were used to assess the contribution of mid-thigh MA to each physical function and physical activity measure after controlling for body mass index (BMI), quadriceps strength and quadriceps area.

Results: Study sample consisted of 60 subjects with RA, of which 82% were female, with average age and BMI of 59 ± 10 years and 31.2 ± 7.2 kg/m², respectively. Average RA duration was 15 ± 10 years and disease activity levels were moderate (mean DAS-28 score 4 ± 1.3). MA was inversely correlated with time to ascend one flight of stairs, and walk four meters; and directly correlated with time spent in physical activity (≥ 3 MET), and single leg stance. After adjusting for BMI, quadriceps strength and area, MA continued to contribute significantly to the variability in stair climb time, single leg balance time and physical activity time ≥ 3 MET (Table). 

Conclusion: Higher skeletal muscle fat predicts lower physical function and physical activity. The contribution is above and beyond that of body size, and muscle strength and area. The mechanism by which skeletal muscle fat affects physical function is not clear, and perhaps muscle properties beyond its size and torque production need to be considered and investigated in future studies.

Table. Correlations between Muscle Attenuation, Physical Function and Physical Activity Outcomes
	Correlations  with MA (Spearman’s rho)	Contribution of MA after adjusting for BMI, quadriceps strength and quadriceps area
Outcome Variables		β-coefficient	R2 change	p-value
Quadriceps Isometric strength, Newton-meters	.454*	—	—	—
Chair rise time, sec	-.244	.083	.005	.594
Stair climb time, sec	-.576*	.262	.044	.031*
4-meter walk time, sec	-.445*	.191	.023	.200
Single Leg Balance Time-Right, sec	.409*	.493	.154	.002*
Single Leg Balance time-Left, sec	.455*	.459	.130	.002*
Time spent in Physical Activity ≥ 3 METs, min	.578*	.533	.180	.000*

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/skeletal-muscle-fat-and-its-association-with-physical-function-and-physical-activity-in-adults-with-rheumatoid-arthritis/