Ana Maria Acosta

Multiple-input, multiple-output system identification for characterization of limb stiffness dynamics.

Abstract. This study presents time-domain and frequency-domain, multiple-input, multiple-output (MIMO) linear system identification techniques that can be used to estimate the dynamic endpoint stiffness of a multijoint limb. The stiffness of a joint or limb arises from a number of physiological mechanisms and is thought to play a fundamental role in the control of posture and movement. Estimates of endpoint stiffness can therefore be used to characterize its modulation during physiological tasks and may provide insight into how the nervous system normally controls motor behavior. Previous MIMO stiffness estimates have focused upon the static stiffness components only or assumed simple parametric models with elastic, viscous, and inertial components. The method presented here captures the full stiffness dynamics during a relatively short experimental trial while assuming only that the system is linear for small perturbations. Simulation studies were performed to investigate the performance of this approach under typical experimental conditions. It was found that a linear MIMO description of endpoint stiffness dynamics was sufficient to describe the displacement responses to small stochastic force perturbations. Distortion of these linear estimates by nonlinear centripetal and Coriolis forces was virtually undetectable for these perturbations. The system identification techniques were also found to be robust in the presence of significant output measurement noise and input coupling. These results indicate that the approach described here will allow the estimation of endpoint stiffness dynamics in an experimentally efficient manner with minimal assumptions about the specific form of these properties.

Modifiability of abnormal isometric elbow and shoulder joint torque coupling after stroke.

Unlike individuals with mild stroke, individuals with severe stroke are constrained to stereotypical movement patterns attributed to abnormal coupling of shoulder abductors with elbow flexors, and shoulder adductors with elbow extensors. Whether abnormal muscle coactivation and associated joint torque patterns can be changed in this population is important to determine given that it bears on the development of effective rehabilitation interventions. Eight subjects participated in a protocol that was designed to reduce abnormal elbow/shoulder joint torque coupling by training them to generate combinations of isometric elbow and shoulder joint torques away from the constraining patterns. After training, subjects demonstrated a significant reduction in abnormal torque coupling and a subsequent significant increase in ability to generate torque patterns away from the abnormal pattern. We suggest the rapid time‐course of these changes reflects a residual capacity of the central nervous system to adapt to a novel behavioral training environment. Muscle Nerve, 2005

Position-dependent torque coupling and associated muscle activation in the hemiparetic upper extremity

Previous studies have demonstrated abnormal joint torque coupling and associated muscle coactivations of the upper extremity in individuals with unilateral stroke. We investigated the effect of upper limb configuration on the expression of the well-documented patterns of shoulder abduction/elbow flexion and shoulder adduction/elbow extension. Maximal isometric shoulder and elbow torques were measured in stroke subjects in four different arm configurations. Additionally, an isometric combined torque task was completed where subjects were required to maintain various levels of shoulder abduction/adduction torque while attempting to maximize elbow flexion or extension torque. The dominant abduction/elbow flexion pattern was insensitive to changes in limb configuration while the elbow extension component of the adduction/extension pattern changed to elbow flexion at smaller shoulder abduction angles. This effect was not present in control subjects without stroke. The reversal of the torque-coupling pattern could not be explained by mechanical factors such as muscle length changes or muscle strength imbalances across the elbow joint. Potential neural mechanisms underlying the sensitivity of the adduction/elbow extension pattern to different somatosensory input resultant from changes in limb configuration are discussed along with the implications for future research.

A robotic manipulator for the characterization of two-dimensional dynamic stiffness using stochastic displacement perturbations

Experimental techniques for estimating the two-dimensional dynamic stiffness of the human arm over a wide range of conditions have been developed. A robotic manipulator has been developed to create loads against which subjects perform various tasks and also to impose perturbations onto the endpoint of the arm to allow estimation of its mechanical properties. The manipulator can produce static endpoint forces exceeding 220 N in any direction in its plane of motion, and this plane can be vertically translated and tilted over wide ranges to study arm dynamic stiffness in many functionally relevant planes. It can impose stochastic position and force perturbations whose bandwidth exceeds that of the arm. These random perturbations avoid undesirable volitional reactions and allow the efficient estimation of stiffness dynamics using experimental trials of short duration. The ability of this manipulator to characterize inertial-viscoelastic systems was tested using several two-dimensional physical systems whose properties were independently characterized. The endpoint dynamic stiffness properties of a human arm were estimated as an example of the use of the manipulator in studying upper limb mechanical properties. The system properties characterized by these methods will be useful in probing normal neural arm control strategies and in developing rehabilitation interventions to improve arm movements in disabled individuals.

Pilot study to test effectiveness of video game on reaching performance in stroke

Robotic systems currently used in upper-limb rehabilitation following stroke rely on some form of visual feedback as part of the intervention program. We evaluated the effect of a video game environment (air hockey) on reaching in stroke with various levels of arm support. We used the Arm Coordination Training 3D system to provide variable arm support and to control the hockey stick. We instructed seven subjects to reach to one of three targets covering the workspace of the impaired arm during the reaching task and to reach as far as possible while playing the video game. The results from this study showed that across subjects, support levels, and targets, the reaching distances achieved with the reaching task were greater than those covered with the video game. This held even after further restricting the mapped workspace of the arm to the area most affected by the flexion synergy (effectively forcing subjects to fight the synergy to reach the hockey puck). The results from this study highlight the importance of designing video games that include specific reaching targets in the workspace compromised by the expression of the flexion synergy. Such video games would also adapt the target location online as a subjects success rate increases.

Measurement of isometric elbow and shoulder moments: position-dependent strength of posterior deltoid-to-triceps muscle tendon transfer in tetraplegia

This report describes an apparatus which has been developed to measure several isometric elbow and shoulder forces and moments simultaneously and also allows this characterization to be performed across a range of shoulder and elbow joint angles in a horizontal plane. This apparatus was used to characterize the elbow extension strength in individuals with tetraplegia resulting from cervical level spinal cord injury. In all of these individuals, voluntary elbow extension was provided exclusively by the posterior deltoid muscle, which had previously been surgically transferred to the tendon of the paralyzed triceps muscle. Elbow extension is essential for many daily activities, such as reaching above shoulder level and pushing objects away from the body; the widely used posterior deltoid-to-triceps muscle tendon transfer surgery restores some degree of voluntary control to this important function. The apparatus contained a six-axis force-moment transducer to which the arm of each subject was attached. The six outputs of the transducer were transformed to correspond to physiological elbow and shoulder moments and forces. A customized table allowed the shoulder and elbow angles of the subject to be varied over a wide range in a horizontal plane so that the effects of posterior deltoid muscle length could be characterized over the likely functional range of the subject within this plane. It was found that elbow extension strength varied widely across subjects with C5 or C6 tetraplegia, from quite weak to strong enough to propel a manual wheelchair. Furthermore, the elbow extension strength of most subjects showed a strong dependence on both elbow and shoulder angles. Elbow extension was typically weak when the upper arm was elevated to shoulder level at the side, which unfortunately corresponds to the position often adopted by these individuals due to shoulder weakness.

A neuroprosthesis for high tetraplegia

Abstract Background: This case report describes a neuroprosthesis that restored shoulder and elbow function in a 23-year-old man with chronic C3 complete tetraplegia. Before implementation of the neuroprosthesis, electrodiagnostic testing revealed denervation from CS to T1, with the greatest degree of denervation in the C8 and T1 myotomes. Thirteen percutaneous intramuscular electrodes were implanted into muscles acting on the shoulder and elbow of one upper limb. Before functional testing, the subject underwent a conditioning regimen to maximize the strength and endurance of the implanted muscles. Results: After completion of the 8-week exercise regimen, stimulated active range of motion against gravity included 60° of shoulder abduction, 45° of shoulder flexion, 10° of shoulder external rotation with the shoulder passively abducted to 90°, and 110° of elbow flexion. Stimulated elbow extension lacked 20° of full extension with gravity eliminated. After system setup, the subject was able to pick up mashed potatoes on a plate with a utensil and bring them to his mouth using the neuroprosthesis and a balanced forearm orthosis. A switch mounted on the headrest of the subjects wheelchair and a position sensor mounted on the co tralateral shoulder allowed the subject to control movement of his upper limb.

Three-dimensional shoulder kinematics in individuals with C5-C6 spinal cord injury

Abstract The shoulder kinematics of five able-bodied subjects and those of five arms in three subjects with spinal cord injuries at C5 or C6 levels were measured as the subjects elevated their arms in three different planes: coronal, scapular and sagittal. The range of humeral elevation was significantly reduced in all spinal cord injury (SCI) subjects relative to able-bodied subjects. Over this restricted range of humeral motion, the scapula of SCI subjects tended to be medially rotated, relative to ablebodied subjects, and the protraction and spinal tilt angles of the scapula of the SCI subjects indicated scapular winging. These results are consistent with paralysis or at least with significant weakness of the serratus anterior muscle. If further study confirms this hypothesis, functional neuromuscular stimulation of the serratus anterior muscle via a nerve cuff electrode may be an effective intervention for improving shoulder function in C5-C6 SCI.

A Multiple Degree of Freedom Lower Extremity Isometric Device to Simultaneously Quantify Hip, Knee, and Ankle Torques

Characterization of the joint torque coupling strategies used in the lower extremity to generate maximal and submaximal levels of torque at either the hip, knee, or ankle is lacking. Currently, there are no available isometric devices that quantify all concurrent joint torques in the hip, knee, and ankle of a single leg during maximum voluntary torque generation. Thus, joint-torque coupling strategies in the hip, knee, and concurrent torques at ankle and/or coupling patterns at the hip and knee driven by the ankle have yet to be quantified. This manuscript describes the design, implementation, and validation of a multiple degree of freedom, lower extremity isometric device (the MultiLEIT) that accurately quantifies simultaneous torques at the hip, knee, and ankle. The system was mechanically validated and then implemented with two healthy control individuals and two post-stroke individuals to test usability and patient acceptance. Data indicated different joint torque coupling strategies used by both healthy individuals. In contrast, data showed the same torque coupling patterns in both post-stroke individuals, comparable to those described in the clinic. Successful implementation of the MultiLEIT can contribute to the understanding of the underlying mechanisms responsible for abnormal movement patterns and aid in the design of therapeutic interventions.

The effects of asymmetric tonic neck reflex during reaching movement following stroke: Preliminary results

Previous studies and clinical observations reveal that stroke survivors show the resurgence of the asymmetric tonic neck reflex (ATNR) both in static and dynamic conditions during maximal efforts. This observation may imply more reliance on the brainstem pathways following stroke. However, the effect of ATNR during a dynamic condition that represents more natural movement, such as reaching, has not been studied before. During reaching movements, the application of a robot controlled haptic environment is important to quantify the effect of ATNR following stroke. Therefore, this paper reports the use of a novel setup using the ACT3D robotic device to investigate and quantify this reflexive behavior. Our preliminary results demonstrate that the effect of ATNR is significant in the stroke population when abducting the shoulder at 25% of maximum ability. These results show that the ATNR affects reaching distance especially when shoulder loading in abduction is required. In conclusion, these preliminary results provide evidence that the effect of ATNR in stroke subjects during reaching task can be quantified by using a novel 3-D robotic setup.