Table 1 Definition of symbols and acronyms

A₉₀

N ms^-1

Scaling factor reflecting magnitude of force at 90°

a

deg^-2

Defines parabolic shape of ankle force - knee angle relationship

α_A

ms^-2

Force scaling factor in fatigue model for force-motion model parameter A

α_Km

ms^-1N^-1

Force scaling factor in fatigue model for force-motion model parameter K_m

α_τ1

N^-1

Force scaling factor in fatigue model for force-motion model parameter τ₁

b

deg^-1

Defines parabolic shape of ankle force-knee angle relationship

β_A

ms^-1deg^-1

Angular velocity x force scaling factor in fatigue model for force-motion model parameter A

β_Km

deg^-1N^-1

Angular velocity x force scaling factor in fatigue model for force-motion model parameter K_m

β_τ1

ms deg^-1N^-1

Angular velocity x force scaling factor in fatigue model for force-motion model parameter τ₁

CFT

-

Constant frequency train

C_N

-

Normalized concentration of Ca²⁺-troponin complex

F

N

Instantaneous force near the ankle due to stimulation

F_load

N

Load applied at ankle during general non-isometric leg extensions

F_M

N

Represents the resistance to knee extension due to the weight of the leg and all other passive resistance about the knee joint

TTI

N s

Torque Time Integral

I

kg m²

Net mass moment of inertia of the leg plus the applied load

K_m

-

Similar to Michaelis-Menten constant. Affinity of actin strong binding site for myosin

L

m

Effective moment arm from knee joint center of rotation to resultant force vector near ankle

λ

deg

90° minus the knee flexion angle of the resting non-isometric leg

n

-

Number of stimuli in train before time t

R₀

-

Characterizes the magnitude of enhancement in C_N from the following stimuli

R_i

-

Accounts for differences in activation for each pulse relative to first pulse of train

SCI

-

Spinal Cord Injury

t_i

ms

Time of the i^th stimulation

τ₁

ms

Time constant of force decline in the absence of strongly bound cross-bridges

τ₂

ms

Time constant of force decline due to actin-myosin friction in cross-bridges

τ_c

ms

Time constant controlling the rise and decay of C_N

τ_fat

ms

Time constant for force-motion model parameters A, K_m1, and τ₁ during fatigue

θ

deg

Knee flexion angle, where full extension was 0˚

V₁

N deg^-2

Scaling factor in the term G

V₂

deg^-1

Constant

VFT

-

Variable frequency train