Table 1.

Mean (SD, n =10) of the impact of each lower limb joint moment on the COM stability s (Impact Factor A), and the state variables (Impact Factors B): center of mass (COM) position, x_COM; its velocity, ẋ_COM; base of support (BOS) position x_BOS, and its velocity, ẋ_BOS. The impact factor is measured by the change at the end of the single-stance phase in these variables (Δx_COM and Δx_BOS normalized by foot length, l_BOS, Δẋ_COM and Δẋ_BOS normalized by $\sqrt{g \times b h}$ , where g is the gravitational acceleration and bh the body height; and Δs dimensionless) caused by one unit increment in joint moments (i.e., when Δτ = 1bm×g×bh Nm, bm represents the body mass) or by the slope of the simulated results for these relationship depicted in Fig 4, which had a near-perfect linear fit across the simulation range (R² > 0.99). Positive slope means a unit of increase in these dominant moments will bring an increase in the state variables or in the COM stability, and vice versa for a negative slope. Hip extensor, knee flexor, and ankle plantar-flexor moments were found to be the dominant moments that occurred in stance limb for > 90% of single-stance phase in all subjects, meanwhile the swing limb was dominated by hip flexor and knee extensor moments and ankle dorsiflexor moments (Fig 2).

Joints

Sides

Impact Factor A

Impact Factors B

x_COM/l_BOS

{\dot{x}}_{COM} / \sqrt{g \times b h}

x_BOS/l_BOS

{\dot{x}}_{BOS} / \sqrt{g \times b h}

Hip

Stance (Extensor)^a

18.57 (6.08)^b

−1.08 (1.54)

−0.14 (0.25)

−4.98 (4.72)^b

−22.33 (5.55)^b

Swing (Flexor)

3.29 (0.81)^b

−0.26 (0.41)^b

−0.02 (0.07)^b

−0.98 (0.92)^b

−3.69 (1.46)^b

Knee

Stance (Flexor)^a

49.12 (9.85)^b

−3.13 (5.64)^b

−0.53 (0.73)^b

−18.57 (7.75)^b

−57.72 (10.46)^b

Swing (Extensor)

−0.99 (0.66)

0.10 (0.10)

0.01 (0.02)

0.36 (0.21)^b

1.18 (0.41)^b

Ankle

Stance (Plantar-flexor)^a

−9.36 (1.97)^b

0.62 (2.67)

0.10 (0.06)

2.10 (1.03)^b

13.09(2.27)^b

Swing (Dorsiflexor)

−0.33 (0.16)

0.005 (0.05)

0.004 (0.004)

0.10 (0.09)^b

0.41 (0.24)^b

p < 0.001 vs. swing limb;

p < 0.001 vs. the other two joints on the same side.