Background/Purpose: While foot pain has been linked to poor outcomes, little is known about how the foot might affect physical functioning or, specifically, leg muscle mass. As no studies have examined the association between leg muscle mass and foot pain, structure or function, the purpose was to evaluate the relation of leg muscle mass to these characteristics, in a population-based study of older men and women. We hypothesized that foot pain and poor foot function/structure (e.g. supination or pronation) would be linked to low leg muscle mass.

Methods: Framingham Foot Study participants with complete data on leg muscle mass as well as foot pain, structure and function (2002-08) were included in this study. Whole body DXA (Lunar DPX-L) was used to measure leg muscle mass (kg). Foot pain (y/n) was present if pain, aching or stiffness was reported on most days of the month. Data from a Tekscan Matscan pressure mat were used to calculate foot structure, as the modified arch index (MAI) during bipedal standing, and foot function, as the center of pressure excursion index (CPEI) while walking using the two-step method. Feet in the top or bottom 20% of CPEI values were classified as supinators or pronators, respectively, and feet in the middle 60% were the referent group. The bottom and top 20% of MAI values were considered high and low arched feet, respectively, with the referent the middle 60%. The foot with CPEI or MAI value farthest from the respective median value was chosen as the foot of interest for each participant. Crude and adjusted (age, body mass index (BMI, kg/m²), sex) multinomial (for foot structure and function outcomes) logistic regression was used to determine the association of a 1 standard deviation (SD) increase in leg muscle mass, both crude and normalized to height, with foot pain, structure and function. Sex-specific models were also examined.

Results: Of the 1798 participants (age: 67 ± 10 years; BMI: 28 ± 4.9 kg/m²; 57% women), the average leg muscle mass was 17.51 ± 2.23 kg in men and 11.60 ± 1.51 kg in women. 21% reported foot pain. A 1 SD increase in leg muscle mass was associated with 18% lower odds of foot pain, 25% lower odds of pronation and 19% higher odds of supination, compared to the referent. A 1 SD increase in muscle mass was associated with 14% higher odds of low arch and 15% lower odds of high arch. Adjustment for age and BMI did not change the results, but associations were attenuated after adding sex to the model. Results were similar for muscle mass normalized to height and sex-specific models.

Conclusion: Our results suggest that although leg muscle mass was associated with foot pain, foot structure and foot function in our population of middle-aged and older adults, these relations are confounded by sex. Leg muscle mass is likely not a determinant of foot pain, structure or function. These results highlight the need for future work to examine the role of foot pain, structure and function in understanding other aspects of impairment and physical function.

Table. Odds ratios and 95% confidence intervals for the associated between one standard deviation increase in leg muscle mass and foot pain, foot structure (MAI), and foot function (CPEI)
		Crude Model		Adjusted Model (age, BMI, sex)
	n (%)	OR (95% CI)	p-value	OR (95% CI)	p-value
Foot pain	385 (21)	0.83 (0.74, 0.94)	0.0023	0.93 (0.74, 1.18)	0.5727
Foot Structure (MAI)
low arch	474 (26)	1.13 (1.01, 1.27)	0.0296	0.90 (0.71, 1.13)	0.3671
high arch	548 (30)	0.85 (0.76, 0.95)	0.0048	0.82 (0.65, 1.03)	0.0949
Foot Function (CPEI)
pronator	559 (31)	0.74 (0.66, 0.83)	<.0001	0.96 (0.76, 1.21)	0.7233
supinator	535 (30)	1.17 (1.05, 1.31)	0.0055	0.84 (0.67, 1.05)	0.1197

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/leg-muscle-mass-is-not-affected-by-foot-pain-structure-or-function-the-framingham-foot-study/