# Binanual Movements and Chronic Stroke Rehabilitation: Looking Back and Looking Forward
## 3 Bimanual Kinematic and Kinetic Functions in Chronic Stroke
Moreover, bimanual forces produced by participants post-stroke tended to be more regular (i.e., greater force regularity) as indicated by higher values of approximate entropy [79; 82; 83], and these patterns indicated decreased motor adaptability during force control tasks [84]. Asymmetrical muscular functions between the paretic and non-paretic hands as well as impaired sensorimotor processing may be responsible for lower submaximal bimanual force control performances from 5% to 50% of maximum voluntary contraction (MVC) [78; 83]. Importantly, interlimb force coordination patterns were additionally impaired after stroke onset. [PERSON] and colleagues reported lower values of cross-correlation strength with increased time-lag as compared with age-matched controls during bimanual isometric wrist and fingers extension tasks [83]. These findings suggested that stroke may interfere with temporal coordination between paretic and non-paretic hands, and further non-paretic hands presumably modulated their forces to compensate for lacking forces generated by paretic hands during bimanual force control [81; 85]. These deficits in interlimb coordination in individuals with stroke were additionally observed in dynamical force control tasks (e.g., force increment and decrement phases) [86]. Moreover, altered bimanual force coordination in patients with stroke were significantly associated with motor impairments as indicated by various clinical assessments (e.g., the Fugl-Meyer assessment and Pegboard assembly score) [81; 82; 86]. Proposed neurophysiological mechanisms underlying impairments abound for bimanual movements and bimanual coordination [39; 40], including altered sensorimotor integration capabilities post-stroke such as online motor correction using simultaneous visual information [87]. Further, increased interhemispheric inhibition from the contralesional hemisphere typically suppresses cortical activation of the ipsilesional hemisphere, which may send biased efferent signals to the paretic and non-paretic arms, causing impaired interlimb coordination functions [73; 88]. Indeed, changes in somatosensory feedback influenced by stroke appear to be a crucial reason in weakening interlimb coordination because prior studies showed more deficits in force coordination without a visual feedback condition for chronic stroke patients [79; 89]. Beyond the altered bimanual motor control functions within a trial, recent studies explored changes in bimanual coordination strategies across multiple trials for post-stroke individuals. [PERSON] and colleagues proposed the importance of bimanual motor synergies reflecting different cooperative behaviors between hands across multiple trials in stroke motor rehabilitation [43]. According to the uncontrolled manifold hypothesis [90; 91; 92], motor variability consists of two components: good and bad variability. During multiple trials of a bimanual force control task, the fundamental elements can include pairs of left and right mean forces within a trial. Good variability is the variance of fundamental elements projected on the uncontrolled manifold line that does not influence the stability of task performance (e.g., overall force accuracy across multiple trials). However, greater good variability indicates that the motor system produces more possible motor solutions (i.e., motor flexibility), whereas less good variability denotes that the motor system selects a more consistent motor solution (i.e., motor optimality). Bad variability is the variance of fundamental elements projected on the line orthogonal to the uncontrolled manifold line that does influence the stability of task performance. Increased bad variability impairs the stabilization of task performance across multiple trials.