Anthropomorphic robotics. II. Analysis of manipulator dynamics and the output motor impedance

What muscle variable(s) does the nervous system control in limb movements?

To control force accurately under a wide range of behavioral conditions, the central nervous system would either require a detailed, continuously updated representation of the state of each muscle (and the load against which each is acting) or else force feedback with sufficient gain to cope with variations in the properties of the muscles and loads. The evidence for force feedback with adequate gain or for an appropriate central representation is not sufficient to conclude that force is the major controlled variable in normal limb movements.

Morton's hypothesis, that length is controlled by a follow-up servo, has a number of difficulties related to the delays, gains, variability, and specificity in feedback pathways comprising potential servo loops. However, experimental evidence is consistent with these pathways providing servo assistance for some movements produced by coactivation of α- and static γ-motoneurons. Dynamic γ-motoneurons may provide an additional input for adaptive control of different types of movements.

The idea that feedback is used to compensate for changes in muscle stiffness has received experimental support under static postural conditions. However, reflexes tend to increase rather than decrease the range of variation in muscle stiffness during some cyclic movements. Theoretical problems associated with the regulation of stiffness are also discussed. The possibilities of separate control systems for velocity or viscosity are considered, but the evidence is either negative or lacking. I conclude that different physical variables can be controlled depending on the type of limb movement required. The concept of stiffness regulation is also useful under some conditions, but should probably be extended to the regulation of the visco-elastic properties (i.e., the mechanical impedance) of a muscle or joint.

The contributions of muscles and reflexes to the regulation of joint and limb mechanics

The property of muscular stiffness is essential to the regulation of posture and interjoint coordination. The mechanical properties of a skeletal muscle arise from the integration of sensory feedback with the intrinsic mechanics of the muscle. Damage to peripheral nerves and probably the muscle itself can result in a permanent loss of sensory feedback with consequent alterations in muscular stiffness and interjoint coordination. Additional study of neural feedback, its vulnerability to damage, and the means to promote its recovery after muscular damage is an important direction for the investigation and treatment of musculoskeletal disorders.

Superposition of ballistic on steady contractions in man

In a visual reaction time task, human subjects superimposed isometric ballistic contractions on a maintained activity in the soleus or anterior tibial muscle. Since there were good reasons to believe that the supraspinal motor commands for the ballistic contractions were independent of those for the background activity, the interaction between the motor commands for the ballistic and for the steady contractions could be studied at the spinal level. If ballistic and steady contractions were in the same direction, the EMG burst and torque changes associated with the ballistic contraction were nearly constant irrespective of the maintained steady flexion force. This was true if a muscle was activated to about 5% of its maximum force as the soleus muscle during plantar flexions and if it was activated to about 40% of its maximum force as the anterior tibial muscle during dorsal flexions. If ballistic and steady contractions were in opposite directions the torque changes related to the ballistic contraction increased linearly with the background activity. This relation was caused by a reduction in antagonist activity starting about 50 ms before the agonist EMG burst and not by an increased agonist burst, the latter remaining independent of background activity. These results imply that the input-output relationship of the motoneuronal pool is nearly linear. The functional basis of this relation is the size principle which is valid during continuous and ballistic contractions. The number of motor units recruited for the ballistic contraction is adjusted according to their force such that the contraction amplitude remains constant.

Changes in limb dynamics during the practice of rapid arm movements

In our study we examined Bernstein's hypothesis that practice alters the motor coordination among the muscular and passive joint moments. In particular, we conducted dynamical analyses of a human multisegmental movement during the practice of a task involving the upper extremity. Seven male human volunteers performed maximal-speed, unrestrained vertical arm movements whose upward and downward trajectories between two target endpoints required the hand to round a barrier, resulting in complex shoulder, elbow, and wrist joint movements. These movements were recorded by high-speed ciné film, and myopotentials from selected upper-extremity muscles were recorded. The arm was modeled as interconnected rigid bodies, so that dynamical interactions among the upper arm, forearm, and hand could be calculated. With practice, subjects achieved significantly shorter movement times. As movement times decreased, all joint-moment components (except gravity) increased, and the moment-time and EMG profiles were changed significantly. Particularly during reversals in movement direction, the changes in moment-time and EMG profiles were consistent with Bernstein's hypothesis relating practice effects and intralimb coordination: with practice, motor coordination was altered so that individuals employed reactive phenomena in such a way as to use muscular moments to counterbalance passive-interactive moments created by segment movements.

An analysis of the sources of musculoskeletal system impedance

When antagonistic muscles co-contract, the impedance of musculoskeletal systems to applied loads is known to increase. In this paper a physiologically-based, higher-order, nonlinear antagonistic muscle-joint model is utilized to clarify the sources of impedance modulation during a variety of tasks, ranging from resisting transient loads to holding steady loads to making fast movements in unpredictable surroundings. It is shown that impedance modulation occurs automatically as a function of the specific operating ranges utilized during a given task by each of four different muscle-joint mechanical relations. The relative contribution of each relation depends on the type of task, with impedance during quasi-static conditions sensitive to muscle tension-length and sometimes joint parallel elastic properties and during dynamic tasks dominated by the series element and muscle force-velocity properties. Elimination of any of these causes a decrease in built-in biomechanical capabilities. These findings raise questions concerning past theories on stiffness-impedance modulation which appear to underestimate the role of inherent biomechanical properties.

Parallel Computations for Controlling an Arm

In order to control a reaching movement of the arm and body, several different computational problems must be solved. Some parallel methods that could be implemented in networks of neuron-like processors are described. Each method solves a different part of the overall task. First, a method is described for finding the torques necessary to follow a desired trajectory. The methods is more economical and more versatile than table look-up and requires very few sequential steps. Then a way of generating an internal representation of a desired trajectory is described. This method shows the trajectory one piece at a time by applying a large set of heuristic rules to a "motion blackboard" that represents the static and dynamic parameters of the state of the body at the current point in the trajectory. The computations are simplified by expressing the positions, orientations, and motions of parts of the body in terms of a single, non-accelerating, world-based frame of reference, rather than in terms of the joint-angles or an egocentric frame based on the body itself.