Impact of Tactile Sensation on Dexterity: A Cross-Sectional Study of Patients With Impaired Hand Function After Stroke

The authors sought to explore the impact of tactile sensation on manual dexterity and the validity of the strength-dexterity test in subjects with chronic impairments after stroke in a cross-sectional study of 24 patients with impaired hand function after stroke. Dexterity was assessed by the strength-dexterity test, Box and Blocks, and Nine-Hole Peg Test, and the ABILHAND questionnaire. Sensation was measured by pinprick, cotton-wool, graphesthesia, and 2-point discrimination tests. Sensation in the paretic hand had strong association with paretic hand performance in the strength-dexterity test and Nine-Hole Peg Test and explained 13% of the variance. Sensation in the nonparetic hand was associated with the results of the ABILHAND questionnaire. Among sensory tests, 2-point discrimination had the strongest association with dexterity tests. No significant correlations between sensation, pinch force, and dexterity tests were found for the nonparetic hand. The strength-dexterity test exhibited strong correlations with the other dexterity measures and with pinch force. There is an association between tactile sensation and dexterous performance in the paretic hand; activity level performance is associated with sensation in the nonparetic hand. The study supports the validity of the strength-dexterity test when applied in subjects in the chronic stage after stroke.     

Force and Directional Force Modulation Effects on Accuracy and Variability in Low-Level Pinch Force Tracking

The authors investigated how varying the required low-level forces and the direction of force change affect accuracy and variability of force production in a cyclic isometric pinch force tracking task. Eighteen healthy right-handed adult volunteers performed the tracking task over 3 different force ranges. Root mean square error and coefficient of variation were higher at lower force levels and during minimum reversals compared with maximum reversals. Overall, the thumb showed greater root mean square error and coefficient of variation scores than did the index finger during maximum reversals, but not during minimum reversals. The observed impaired performance during minimum reversals might originate from history-dependent mechanisms of force production and highly coupled 2-digit performance.     

No Relationship Between Joint Position Sense and Force Sense at the Shoulder

In practice, a single test is used to quantify an individual's proprioception. Previous studies have not found a correlation between joint position sense (JPS) and force sense (FS), which are submodalities of proprioception. The purpose of the present study is to determine if root mean square (RMS) error in JPS and FS are related at the shoulder, controlling for external load and elevation angle. Active shoulder angle and force reproduction protocols were performed. No correlation was found between JPS and FS (r = –.019, p = .941) nor were any individual angle and load combinations significant. The main effect for angle in JPS was significant (p < .001). Follow-up contrast demonstrated a significant (p < .001) decrease in RMS error with increased elevation. A significant load by angle interaction was found for FS (p = .014). Follow-up simple effects tests by angle demonstrated RMS error decreased with load at 50° and 70° but not at 90°. By load, RMS error only decreased for 120% between 50° and 90°. JPS and FS demonstrate different behavior with load and angle. This differing behavior is more likely responsible for the lack of correlation than angle and load differences in JPS and FS protocols.     

In the grip of fear: Dissociations in attentional processing of animal fearful individuals

Using a modified attention paradigm we investigated specific attentional mechanisms in processing animal feared stimuli. In this paradigm arrays of four pictures were displayed and after its disappearance from view a probe (a letter, X or P) then followed unpredictably in the location of one of the four pictures. The results showed that discriminations of probes tended to be impeded by spider stimuli, compared to snake stimuli. This effect was potentiated by high anxiety but only for those individuals fearful of spiders, since no such effect was observed for snake fearful individuals. Moreover, the discrimination of the probes was not facilitated when presented after the feared stimuli. The implications of these findings are discussed as a function of the cognitive bias involved in specific fear.     

Muscle force compensation among synergistic muscles after fatigue of a single muscle

PurposeThe aim of this study was to examine control strategies among synergistic muscles after fatigue of a single muscle. It was hypothesized that the compensating mechanism is specific for each fatigued muscle.MethodsThe soleus (SOL), gastrocnemius lateralis (GL) and medialis (GM) were fatigued in separate sessions on different days. In each experiment, subjects (n = 11) performed maximal voluntary contractions prior to and after fatiguing a single muscle (SOL, GL or GM) while the voluntary muscle activity and torque were measured. Additionally, the maximal single twitch torque of the plantarflexors and the maximal spinal reflex activity (H-reflex) of the SOL, GL and GM were determined. Fatigue was evoked using neuromuscular stimulation.ResultsFollowing fatigue the single twitch torque decreased by −20.1%, −19.5%, and −23.0% when the SOL, GL, or GM, have been fatigued. The maximal voluntary torque did not decrease in any session but the synergistic voluntary muscle activity increased significantly. Moreover, we found no alterations in spinal reflex activity.ConclusionsIt is concluded that synergistic muscles compensate each other. Furthermore, it seems that self-compensating mechanism of the fatigued muscles occurred additionally. The force compensation does not depend on the function of the fatigued muscle.     

Force Control Is Greater in the Upper Compared With the Lower Extremity

The authors investigated whether force control is similar between the upper and lower limbs and between contractions that involve 1 or 2 joints. Six volunteers (27.5 ± 11.2 years of age) attempted to produce consistent discrete rapid force responses of 30, 60, and 90 N by using 6 different body postures, 3 with the upper and 3 with the lower limb. One of the postures for each limb involved 2 joints. The standard deviation of peak force and impulse (aggregate of the force-time curve) was significantly greater (?25%) for the lower limb than for the upper limb (p < .01). Contractions that involved 1 or 2 joints within a limb had similar variability. Therefore, the upper limb might have better control of force than the lower limb because of its extensive use in fine motor tasks in daily activities.     

The Effect of Mental Practice on Performance in a Sequential Reaction Time Task

The effect of mental practice on performance in a dot-location RT task was investigated. Participants (N = 40) were required either to mentally practice, physically practice, or do no practice on an RT task in which the signals appeared in a repeating sequence. Correct mental practice, as opposed to incorrect mental practice and no practice, was predicted to have a positive (enhancing) effect on performance of the RT task. Despite previous evidence that mental rehearsal does enhance performance in many perceptual-motor tasks, neither correct nor incorrect mental rehearsal affected subsequent sequence learning; that is, no mental practice effect was observed. That surprising result is discussed in terms of motivational, psychoneuromuscular, separate memory systems, and transfer-appropriate processing explanations of mental practice.     

Coordination of Grip Configurations as a Function of Force Output

In this investigation, the authors examined the coordination and control of force production by the digits of the hand as a function of criterion force level and grip configuration. Each adult participant (N = 6: 3 men and 3 women) was required to place the thumb and a finger (or fingers) upon load cells that were fixed to a grasping apparatus that was clamped to a table. In the task, participants had to match a criterion continuous constant total force level displayed on a computer screen. There were 10 trials at each grip configuration and criterion force level combination on each of 3 consecutive days. The results showed that (a) different grip configurations minimized error at each force level; (b) there was a specific digit pairing within a given grip configuration that produced the highest correlation of force output; (c) the correlation between the force output of digits generally increased at higher force levels; (d) error was reduced at each force level and grip configuration over the practice period; and (e) the organization of the force output of each digit varied as a function of digit, force level, grip configuration, and practice. The findings are consistent with the hypothesis that coordination of the digits in prehension is reflective of an adaptive, task-specific solution that is modified with practice.     

After-Contraction Phenomenon: Influences on Performance and Learning

Three experiments investigated the influence of an after-contraction phenomenon on the performance and learning of a dynamic force-production task. The after-contraction effect refers to an involuntary potentiation (induced by a sustained pre-contraction) that is thought to summate with voluntary motor commands to bias subsequent responding. The precontraction involved a brief (20 s) static contraction. The subsequent influence of the precontraction on a dynamic force-production task was assessed. Experiments 1 and 2 were aimed at demonstrating the direct impact of the precontraction intensity on the magnitude and decay of the after-contraction effect. The results indicated that as the intensity of the precontraction increased, the magnitude of the induced bias increased. In Experiment 3, the indirect influence on subsequent retention of varying the precontraction intensity during acquisition was investigated. The results indicated that the performance of subjects experiencing varying precontraction intensities during acquisition was inferior to that of subjects experiencing a constant precontraction intensity, but the performance of the varied precontraction intensities group was superior on the test of retention. It is noted that the paradoxical reversal from acquisition to retention is similar to that found in contextual interference experiments and may arise from similar mechanisms.     

Learning to Minimize Energy Costs and Maximize Mechanical Work in a Bimanual Coordination Task

The authors addressed the hypothesis that economy in motor coordination is a learning phenomenon realized by both reduced energy cost for a given workload and more external work at the same prepractice metabolic and attentional energy expenditure. "Self-optimization" of movement parameters has been proposed to reflect learned motor adaptations that minimize energy costs. Twelve men aged 22.3 ± 3.9 years practiced a 90° relative phase, upper limb, independent ergometer cycling task at 60 rpm, followed by a transfer test of unpracticed (45 and 75 rpm) and selfpaced cadences. Performance in all conditions was initially unstable, inaccurate, and relatively high in both metabolic and attentional energy costs. With practice, coordinative stability increased, more work was performed for the same metabolic and attentional costs, and the same work was done at a reduced energy cost. Selfpaced cycling was initially below the metabolically optimal, but following practice at 60 rpm was closer to optimal cadence. Given the many behavioral options of the motor system in meeting a variety of everyday movement task goals, optimal metabolic and attentional energy criteria may provide a solution to the problem of selecting the most adaptive coordination and control parameters.