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.

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. Keywordsarm; rehabilitation; strength; stroke; synergy Motor recovery following stroke is characterized by a progressive reduction in sensorimotor impairment. 6,31 However, weakness, changes in stretch reflex excitability, and constraints on available joint torque patterns due to abnormal muscle synergies persist in severely affected individuals. 1,3,10,12 An important question is whether severe stroke results in a permanent reduction in available muscle coactivation and associated joint torque patterns. Modifiability of these impairments would support continued effort and likelihood of success in developing effective rehabilitation interventions. To answer this question, a protocol was devised to alter and preferably reduce abnormal joint torque coupling by training stroke survivors to generate joint torque patterns away from the abnormal patterns.We previously demonstrated 12 that, during shoulder abduction, individuals with moderate to severe chronic stroke spontaneously generate near-maximal elbow-flexion torque, with lesser torques in shoulder extension and external rotation. This abnormal torque-coupling pattern reflected coactivation of shoulder abductors (deltoid) with elbow flexors (biceps brachii, brachialis, and brachioradialis). 12 Subsequent studies have described the detrimental impact

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.

Context:The effects of medical and surgical treatments for obesity on peptide YY (PYY) levels, in patients with similar weight loss, remain unclear.Objective: The objective of the study was to assess PYY and appetite before and after Roux-en-Y gastric bypass (RYGB), sleeve gastrectomy (SG), and medical treatment (MED).Design: This was a prospective, controlled, nonrandomized study. Setting:The study was conducted at the Departments of Nutrition and Digestive Surgery at a university hospital.Participants: Participants included three groups of eight patients with similar body mass indexes (RYGB 37.8 Ϯ 0.8, SG 35.3 Ϯ 0.7, and MED 39.1 Ϯ 1.7 kg/m 2 , P ϭ NS) and eight lean controls (body mass index 21.7 Ϯ 0.7 kg/m 2 ). Main Outcome Measures:Total plasma PYY, hunger, and satiety visual analog scales in fasting and after ingestion of a standard test meal were measured.Results: At baseline there were no differences in the area under the curve (AUC) of PYY, hunger, or satiety in obese groups. Two months after the interventions, RYGB, SG, and MED groups achieved similar weight loss (17.7 Ϯ 3, 14.9 Ϯ 2.4, 16.6 Ϯ 4%, respectively, P ϭ NS). PYY AUC increased in RYGB (P Ͻ 0.001) and SG (P Ͻ 0.05) and did not change in MED. PYY levels decreased at fasting, 30 min, and 180 min after a standard test meal in MED (P Ͻ 0.05). Hunger AUC decreased in RYGB (P Ͻ 0.05). Satiety AUC increased in RYGB (P Ͻ 0.05) and SG (P Ͻ 0.05). Appetite did not change in MED. PYY AUC correlated with satiety AUC (r ϭ 0.35, P Ͻ 0.05).Conclusion: RYGB and SG increased PYY and reduced appetite. MED failed to produce changes. Different effects occur despite similar weight loss. This suggests that the weight-loss effects of these procedures are enhanced by an increase in PYY and satiety. O ver the last 30 yr, the prevalence of obesity has increased worldwide, becoming one of the world's most important public health problems and resulting in great efforts toward the development of strategies to treat this condition (1). However, medical interventions for obesity, including diet, physical activity, behavioral therapy, and weight loss drugs, are associated with poor weight reduction and are difficult to sustain in the long term (2). These modest results can be explained by several factors, one being that the human body responds to weight