Are stance ankle plantar flexor muscles necessary to generate propulsive force during human gait initiation?

The study examined whether the generation of the forward propulsive force (PF) during gait initiation resulted mainly from the electromyogram activity of stance ankle plantar flexor muscles (APF) which 'push' on the ground as is generally claimed in the literature. Six unilateral above-knee amputees performed a specific gait initiation protocol, i.e. they were asked to walk as fast as possible from an upright posture. Data from a force platform were collected and processed to obtain gait parameters (centre of mass (CoM) acceleration, anteroposterior (A/P) progression velocity, step length, etc.). The results showed that the A/P CoM velocity at the time of foot-off differed depending on the state of the lower limb (sound or prosthetic limb) performing the step. However, the A/P velocity of the CoM reached at the time of foot contact was similar whatever the state of the lower limb initiating the gait. Thus, the absence of ankle and knee muscles did not affect the velocity of body progression, i.e. the generation of the PF in gait initiation. Furthermore, the comparable slopes of the A/P velocity between the stance sound limb and the stance prosthetic limb suggest that the organization of the motor synergy underlying the production of the PF remained the same and did not directly involve the APF. However, other mechanisms could explain PF generation. PF could be generated by the swing leg oscillation, by the trunk movement, or by other mechanisms such as the energy transfer and the exchange of gravity potential energy into kinetic energy.