Table 2.

Simple, polynomial and piecewise linear regression model summarizing the cross-sectional relationship between age and calf muscle parameters in both sexes

	Cross-sectional muscle area (mm2)						Calf muscle density (mg/cm3)
	Men			Women			Men			Women
	b ± S.E.	p	R2	b±S.E.	p	R2	b±S.E.	p	R2	b±S.E.	p	R2
Linear			0.10			0.02			0.25			0.28
Intercept	5265.9 ± 154.3	<0.0001		3752.8 ± 119.0	<0.0001		80.58 ± 0.58	<0.0001		81.02 ± 0.58	<0.0001
Age (years)	−17.1 ± 2.27	<0.0001		−6.12 ± 1.72	0.0004		−0.11 ± 0.01	<0.0001		−0.13 ± 0.01	<0.0001
Second-order polynomial			0.13			0.03			0.26			0.30
Intercept	3735.4 ± 389.1	<0.0001		2983.4 ± 292.8	<0.0001		76.30 ± 1.46	<0.0001		75.13 ± 1.41	<0.0001
Age (years)	42.62 ± 14.11	0.0028		23.03 ± 10.30	0.03		0.06 ± 0.05	0.27		0.094 ± 0.05	0.06
Age2 (years)	−0.50 ± 0.10	<0.0001		−0.25 ± 0.09	0.004		−0.001 ± 0.00	0.016		−0.002 ± 0.00	<0.0001
Piecewise			0.14			0.04			0.28			0.31
Intercept	4823.0 ± 181.0	<0.0001		3204.9 ± 194.8	<0.0001		78.94 ± 0.74	<0.0001		77.90 ± 0.89	<0.0001
Age 1a	−8.13 ± 3.02	0.007		6.80 ± 4.00	0.09		−0.07 ± 0.01	<0.0001		−0.06 ± 0.02	0.0006
Age 2a	−45.68 ± 6.70	<0.0001		−16.50 ± 3.40	<0.0001		−0.18 ± 0.02	<0.0001		−0.20 ± 0.02	<0.0001

Sex and variable specific age-threshold between the first and the second age period were identified by recursively evaluating the increment in fit (R²) piecewise models were of the age-threshold was progressively incremented from 20 to 80 years of age (Fig. 3).

^aBeta coefficient for the first (Age 1) and second (Age 2) period considered in the regression model.