Hogan - 1985 - The mechanics of multi-joint posture and movement control

Citation

Hogan N. The mechanics of multi-joint posture and movement control. Biol Cybern. 1985;52(5):315-31. PUBMED

10 Word Summary

Utilization of mechanical feedback simplifies control.

Abstract

The dependence of muscle force on muscle length gives rise to a "spring-like" behavior which has been shown to play a role in the execution of single-joint posture and movement. This paper extends this concept and considers the influence of the apparent mechanical behavior of the neural, muscular and skeletal system on the control and coordination of multiple degree of freedom posture and movement. A rigorous definition of "spring-like" behavior is presented. From it a numerically quantifiable, experimental test of spring-like behavior is formulated. It is shown that if the steady-state force-displacement behavior of a limb is not spring-like, this can only be due to the action of inter-muscular feedback, and can not be due to intrinsic muscle properties. The directional character of the spring-like behavior of a multiple degree of freedom system is described. The unique way in which synergistic coactivation of polyarticular muscles may modulate the directional properties of the spring-like behavior of a multiple degree of freedom system is explained. Dynamic aspects of postural behavior are also considered. The concept of mechanical impedance is presented as a rigorous dynamic generalisation of the postural stiffness of the limb. The inertial behavior of the system is characterised by its mobility. As with the stiffness or impedance, in the multiple degree of freedom case it has a directional property. The way in which the apparent kinematic redundancy of the musculo-skeletal system may be used to modify its dynamic behavior is explained. Whereas the inertial behavior of a single limb segment is not modifiable, it is shown that the apparent inertial behavior of a multiple degree of freedom system may be modulated by repositioning the joints. A unified description of the posture and movement of a multi-joint system is presented by defining a "virtual trajectory" of equilibrium positions for the limb which may be specified by the neuro-muscular system. The way in which this approach may lead to a simplification of some the apparent computational difficulties associated with the control of multi-joint motion is discussed.

Notes

• Idea that the nervous system takes advantage of the mechanical behavior of the neuro-musculo-skeletal system
  • Modulation of: stiffness, damping and inertia
  • Nervous system may modulate skeletal "admittance" and muscular "impedance" to achieve a movement task
• A spring is a device where the integral of force with respect to displacement, elastic energy, is defined.
• Non-spring-like behavior would occur based inter-muscular feedback
• Poly-articular muscles are required to vary all possible elements of the end-point stiffness
• Describing the muscles and moment-arms as impedances allows for combining them with just addition for total impedance and no loss in generality.
• A possible neural strategy is to control impedance creating an equilibrium point trajectory, which is analogous to ball following a movable depression.
• Neural control of impedance allows for the removal of the "inverse kinematics" problem.
• Multiple degrees of freedom allow for flexibility in modulating impedance.
• Mechanical feedback is likely a necessity to account for sudden disturbances.