. 2022 Dec 5;13:1044363. doi: 10.3389/fphys.2022.1044363

TABLE 5.

Stiffness variables for runners at endurance running speeds. Significant differences (P ≤ 0.05) identified by linear mixed effects modeling are indicated in bold.

Running speed (km/h)	F _leg,max (BW)	∆L _comp (cm)	∆L _decomp (cm)	∆L _comp (%)	∆L _decomp (%)	k _{leg, comp} (kN/m)	k _{leg, decomp} (kN/m)	k _{leg, comp} (BW/%)	k _{leg, decomp} (BW/%)	$θ_{l e g, F S}$ (deg)	$θ_{l e g, T O}$ (deg)
9	2.36 ± 0.19 ^a ^, ^b	6.5 ± 1.1	10.0 ± 1.0 ^a ^, ^b	3.7 ± 0.6	5.7 ± 0.6 ^a ^, ^b	24.6 ± 4.5	15.8 ± 2.4 ^a ^, ^b	0.66 ± 0.11	0.42 ± 0.06 ^a ^, ^b	-8.1 ± 2.6 ^a ^, ^b	13.4 ± 1.6 ^a ^, ^b
11	2.50 ± 0.19	6.5 ± 1.1	10.4 ± 1.1 ^b	3.7 ± 0.6	5.9 ± 0.6 ^b	26.3 ± 5.0	16.2 ± 2.4 ^b	0.70 ± 0.13	0.43 ± 0.06 ^b	-9.2 ± 2.5 ^b	15.0 ± 1.5 ^b
13	2.62 ± 0.19	6.4 ± 1.1	10.7 ± 1.1	3.6 ± 0.6	6.1 ± 0.6	27.8 ± 5.3	16.3 ± 2.3	0.74 ± 0.13	0.44 ± 0.06	-10.2 ± 2.4	16.5 ± 1.5
Running speed effect (P)	0.005	0.38	<0.001	0.39	<0.001	0.30	<0.001	0.13	<0.001	<0.001	<0.001
DF covariate effect (P)	$↓$ <0.001	$↓$ 0.02	$↑$ 0.03	$↓$ 0.01	$↑$ 0.01	$↓$ <0.001	$↓$ <0.001	$↓$ <0.001	$↓$ <0.001	0.59	$↑$ <0.001
SF covariate effect (P)	0.36	$↓$ <0.001	$↓$ <0.001	$↓$ <0.001	$↓$ <0.001	$↑$ <0.001	$↑$ <0.001	$↑$ <0.001	$↑$ <0.001	0.83	$↓$ <0.001

Note: values are presented as mean ± standard deviation. DF: duty factor, SF: step frequency, F _leg,max: maximum of the force vector projected along the leg, ∆L _comp and ∆L _decomp: maximum leg compression and decompression during stance, k _{leg, comp} and k _{leg, decomp}: compressive and decompressive leg stiffnesses, $θ_{l e g, F S}$ and $θ_{l e g, T O}$ : leg angle at foot-strike and toe-off. F _leg,max was normalized by body weight (BW). ∆L _comp and ∆L _decomp were expressed in absolute and relative (as a percentage of participant’s height) units and similarly for k _{leg, comp} and k _{leg, decomp}. SF covariate was normalized by $\sqrt{g / L_{0}}$ , where g is the gravitational constant and L ₀ the leg length. Up ( $↑$ ) and down ( $↓$ ) arrows indicate positive and negative effects of the covariate, respectively.

^{^a}

Significantly different from the value at 11 km/h.

^{^b}

Significantly different from the value at 13 km/h.