Table 1. Predictive methods, input variables and critical equations.

Method	Variable name	Methodology	Input variables	Critical equations
ACSM-walk	ACSM	Statistical	Velocity (v), body mass (m), regression coefficients	=  (0.1v+0.0583)m
Force production	EXT-FP	Measured, calculated, statistical	Body mass, velocity, ground contact time	=  ƒ (vm/contact time)
CoM-GRF	EXT-GRF	Measured, calculated	Ground reaction forces, body mass, acceleration (a)	F = mav =  ∫a dtEnergy = mgh+0.5mv2External work  =  ∑ dEnergy (time)
CoM-sacrum-model	EXT-MAT	Simulated	Velocity, body mass, segment lengths, angles	x  =  ƒ (segment length, angles)v = dx/dtEnergy = mgh+0.5mv2External work  =  ∑ dEnergy (time)
CoM- sacrum-measured	EXT-SAC	Measured, calculated	Motion of sacrum (x), body mass	v =  dx/dtEnergy = mgh+0.5mv2External work =  ∑ dMEnergy (time)
Internal work	INT-MAT	Simulated	Velocity, angular velocity (ω), segment lengths, angles, segment mass and moments of inertia (I)	Energy  =  ∑ 0.5 (mv2 + Iω2)(of segments)Internal work  =  ∑ dEnergy (time)
Joint moments	COMB-JM	Measured, calculated	Ground reaction forces (F), joint shape (r/R), motion of ankle, knee and hip joints	Muscle force  =  r/R FPower  =  Muscle force * vCombined work  =  ∑ Power/time (of joints)
Model (int + ext work)	COMB-MAT	Simulated, calculated	Internal (INT-MAT) and external (EXT-MAT) work	Combined work  = internal +external work

Note that the ACSM method is a metabolic energy approach while others are mechanical energy approaches. The mechanical energy approaches are grouped into those the approximate external work (the energy required to move the body), internal work (the energy required to move the legs relative to the body) and combined work (external and internal work).