Evolution of Motor Control: From Reflexes and Motor Programs to the Equilibrium-Point Hypothesis

Abstract

This brief review analyzes the evolution of motor control theories along two lines that emphasize active (motor programs) and reactive (reflexes) features of voluntary movements. It suggests that the only contemporary hypothesis that integrates both approaches in a fruitful way is the equilibrium-point hypothesis. Physical, physiological, and behavioral foundations of the EP-hypothesis are considered as well as relations between the EP-hypothesis and the recent developments of the notion of motor synergies. The paper ends with a brief review of the criticisms of the EP-hypothesis and challenges that the hypothesis faces at this time.

Figure 1

An illustration of single-muscle control within the EP-hypothesis. A: A central command (λ) defines a force-length characteristic. Given an external load (L), only one equilibrium point is possible (EP₁). Any deviations (filled points) from EP₁ will result in motion back to EP₁. B: To perform an active movement, a change in λ is required (λ₁ to λ₂). As a result, a new equilibrium point (EP₂) is established, and a motion to EP₂ happens. C: Movements can occur passively, as a result of a change in the load (L₁ to L₂).

Figure 2

Control of a joint may be described with two variables, λ_f and λ_e on the torque-angle plane. The joint torque-angle haracteristic (thick lines) will represent the algebraic sum of the corresponding muscle characteristics. A: Shifts of both λ_f and λ_e in the same direction result in a shift of the joint characteristic parallel to the angle axis and may be associated with a reciprocal command (r). Shifts of λ_f and λ_e in opposite directions lead to a change in the slope of the joint characteristic and may be associated with a coactivation command (c).

Figure 3

An illustration of a hypothetical hierarchy for a multi-joint movement. A command for a desired motion of a particular point on the body (for example, the endpoint of a multi-joint limb) may be associated with a command {R,C}. It forms an input into a synergy using {r,c} commands to individual joints as elemental variables. Each {r,c} pair is the input into a multi-muscle synergy with λs as elemental variables.