Agonistic and antagonistic interaction in speed/accuracy tradeoff: A delta-lognormal perspective

This paper reports the results of a model-based analysis of movements gathered in a 4 × 4 experimental design of speed/accuracy tradeoffs with variable target distances and width. Our study was performed on a large (120 participants) and varied sample (both genders, wide age range, various health conditions). The delta-lognormal equation was used for data modeling to investigate the interaction between the output of the agonist and the antagonist neuromuscular systems. Empirical observations show that the subjects must correlate more tightly the impulse commands sent to both neuromuscular systems in order to achieve good performances as the difficulty of the task increases whereas the correlation in the timing of the neuromuscular action co-varies with the size of the geometrical properties of the task. These new phenomena are discussed under the paradigm provided by the Kinematic Theory and new research hypotheses are proposed for further investigation of the speed/accuracy tradeoffs.     

Shoulder joint position sense is not enhanced at end range in an unconstrained task

Shoulder joint position sense (JPS) is important for maintaining stability and contributing to coordinated movements. It is provided by afferent and centrally-derived signals interpreted and integrated by the central nervous system (CNS) for subsequent use. Shoulder JPS is enhanced as the joint approaches end range of motion (ROM) in studies involving internal and external rotation with the arm supported, but this finding has not been confirmed in unconstrained movements. To address this issue, the present study examined the effect of shoulder position in the horizontal plane on JPS at a constant elevation. Twenty-three healthy individuals were recruited from a university campus. Subjects attempted to actively replicate various target positions in both plane and elevation. Target positions consisted of five positions in the horizontal plane, normalized to individual horizontal abduction ROM, at 90° of arm elevation. All target positions were tested three times, and average absolute and variable errors were analyzed for each position. No differences in either absolute (p = .312) or variable (p = .185) errors were observed between positions. These results further support the contention that the muscle spindles are a dominant source of afferent feedback regarding shoulder JPS in unconstrained movements, even approaching end ROM, when the capsuloligamentous receptors are active.     

Descending control to the nonparetic limb degrades the cyclic activity of paretic leg muscles

During anti-phased locomotor tasks such as cycling or walking, hemiparetic phasing of muscle activity is characterized by inappropriate early onset of activity for some paretic muscles and prolonged activity in others. Pedaling with the paretic limb alone reduces inappropriate prolonged activity, suggesting a combined influence of contralesional voluntary commands and movement-related sensory feedback. Five different non-target leg movement state conditions were performed by 15 subjects post-stroke and 15 nonimpaired controls while they pedaled with the target leg and EMG was recorded bilaterally. Voluntary engagement of the non-lesioned motor system increased prolonged paretic vastus medialis (VM) activity and increased phase-advanced rectus femoris (RF) activity. We suggest bilateral descending commands are primarily responsible for the inappropriate activity in the paretic VM during anti-phase pedaling, and contribute to the dysfunctional motor output in the paretic RF. Findings from controls suggest that even an undamaged motor system can contribute to this phenomenon.     

Increased dynamic regulation of postural tone through Alexander Technique training

Gurfinkel and colleagues (2006) recently found that healthy adults dynamically modulate postural muscle tone in the body axis during anti-gravity postural maintenance and that this modulation is inversely correlated with axial stiffness. Our objective in the present study was to investigate whether dynamic modulation of axial postural tone can change through training. We examined whether teachers of the Alexander Technique (AT), who undergo “long-term” (3-year) training, have greater modulation of axial postural tone than matched control subjects. In addition, we performed a longitudinal study on the effect of “short-term” (10-week) AT training on the axial postural tone of individuals with low back pain (LBP), since short term AT training has previously been shown to reduce LBP. Axial postural tone was quantified by measuring the resistance of the neck, trunk and hips to small (±10°), slow (1°/s) torsional rotation during stance. Modulation of tone was determined by the torsional resistance to rotation (peak-to-peak, phase-advance, and variability of torque) and axial muscle activity (EMG). Peak-to-peak torque was lower (∼50%), while phase-advance and cycle-to-cycle variability were enhanced for AT teachers compared to matched control subjects at all levels of the axis. In addition, LBP subjects decreased trunk and hip stiffness following short-term AT training compared to a control intervention. While changes in static levels of postural tone may have contributed to the reduced stiffness observed with the AT, our results suggest that dynamic modulation of postural tone can be enhanced through long-term training in the AT, which may constitute an important direction for therapeutic intervention.     

A single bout of resistance exercise can enhance episodic memory performance

Acute aerobic exercise can be beneficial to episodic memory. This benefit may occur because exercise produces a similar physiological response as physical stressors. When administered during consolidation, acute stress, both physical and psychological, consistently enhances episodic memory, particularly memory for emotional materials. Here we investigated whether a single bout of resistance exercise performed during consolidation can produce episodic memory benefits 48 h later. We used a one-leg knee extension/flexion task for the resistance exercise. To assess the physiological response to the exercise, we measured salivary alpha amylase (a biomarker of central norepinephrine), heart rate, and blood pressure. To test emotional episodic memory, we used a remember-know recognition memory paradigm with equal numbers of positive, negative, and neutral IAPS images as stimuli. The group that performed the exercise, the active group, had higher overall recognition accuracy than the group that did not exercise, the passive group. We found a robust effect of valence across groups, with better performance on emotional items as compared to neutral items and no difference between positive and negative items. This effect changed based on the physiological response to the exercise. Within the active group, participants with a high physiological response to the exercise were impaired for neutral items as compared to participants with a low physiological response to the exercise. Our results demonstrate that a single bout of resistance exercise performed during consolidation can enhance episodic memory and that the effect of valence on memory depends on the physiological response to the exercise.     

Larger plantar flexion torque variability implies less stable balance in the young: An association affected by knee position

The present study examined the association between plantar flexion torque variability during isolated isometric contractions and during quiet bipedal standing. For plantar flexion torque measurements in quiet stance (QS), subjects stood still over a force plate. The mean plantar flexion torque level exerted by each subject in QS (divided by 2 to give the torque due to a single leg) served as the target torque level for right leg force-matching tasks in extended knee (KE) and flexed knee (KF) conditions. Muscle activation levels (EMG amplitudes) of the triceps surae and mean, standard deviation and coefficient of variation of plantar flexion torque were computed from signals acquired during periods with and without visual feedback. No significant correlations were found between EMG amplitudes and torque variability, regardless of the condition and muscle being analyzed. A significant correlation was found between torque variability in QS and KE, whereas no significant correlation was found between torque variability in QS and KF, regardless of vision availability. Therefore, torque variability measured in a controlled extended knee plantar flexion contraction is a predictor of torque variability in the anterior-posterior direction when the subjects are in quiet standing. In other words, larger plantar flexion torque variability in KE (but not in KF) implies less stable balance. The mechanisms underlying the findings above are probably associated with the similar proprioceptive feedback from the triceps surae in QS and KE and poorer proprioceptive feedback from the triceps surae in KF due to the slackening of the gastrocnemii. An additional putative mechanism includes the different torque contributions of each component of the triceps surae in the two knee angles. From a clinical and research standpoint, it would be advantageous to be able to estimate changes in balance ability by means of simple measurements of torque variability in a force matching task.     

Reorganised anticipatory postural adjustments due to experimental lower extremity muscle pain

Automated movements adjusting postural control may be hampered during musculoskeletal pain leaving a risk of incomplete control of balance. This study investigated the effect of experimental muscle pain on anticipatory postural adjustments by reaction task movements. While standing, nine healthy males performed two reaction time tasks (shoulder flexion of dominant side and bilateral heel lift) before, during and after experimental muscle pain. On two different days experimental pain was induced in the m. vastus medialis (VM) or the m. tibialis anterior (TA) of the dominant side by injections of hypertonic saline (1 ml, 5.8%). Isotonic saline (1 ml, 0.9%) was used as control injection. Electromyography (EMG) was recorded from 13 muscles. EMG onset, EMG amplitude, and kinematic parameters (shoulder and ankle joint) were extracted. During shoulder flexion and VM pain the onset of the ipsilateral biceps femoris was significantly faster than baseline and post injection sessions. During heels lift in the VM and TA pain conditions the onset of the contralateral TA was significantly faster than baseline and post injection sessions in bilateral side. VM pain significantly reduced m. quadriceps femoris activity and TA pain significantly reduced ipsilateral VM activity and TA activity during bilateral heel lift. The EMG reaction time was delayed in bilateral soleus muscles during heels lift with VM and TA pain. The faster onset of postural muscle activity during anticipatory postural adjustments may suggest a compensatory function to maintain postural control whereas the reduced postural muscle activity during APAs may indicate a pain adaptation strategy to avoid secondary damage.     

Muscle coordination and force variability during static and dynamic tracking tasks

This study examined muscular activity patterns of extensor and flexor muscles and variability of forces during static and dynamic tracking tasks using compensatory and pursuit display. Fourteen volunteers performed isometric actions in two conditions: (i) a static tracking task consisting of flexion/pronation, ulnar deviation, extension/supination and radial deviation of the wrist at 20% maximum voluntary contraction (MVC), and (ii) a dynamic tracking task aiming at following a moving target at 20% MVC in the four directions of contraction. Surface electromyography (SEMG) from extensor carpi ulnaris, extensor carpi radialis, flexor carpi ulnaris and flexor digitorum superficialis muscles and exerted forces in the transverse and sagittal plane were recorded. Normalized root mean square and mutual information (index of functional connectivity within muscles) of SEMGs and the standard deviation and sample entropy of force signals were extracted. Larger SEMG amplitudes were found for the dynamic task (p < .05), while normalized mutual information between muscle pairs was larger for the static task (p < .05). Larger size of variability (standard deviation of force) concomitant with smaller sample entropy was observed for the dynamic task compared with the static task (p < .01 for both). These findings underline a rescaling of the muscles’ respective contribution influencing force variability relying on feedback and feed-forward control strategies in relation to display modes during static and dynamic tracking tasks.     

Dynamics of the ankle joint analyzed through moment-angle loops during human walking: gender and age effects

Aim of this study was to provide a non-invasive assessment of the dynamic properties of the ankle joint during human locomotion, with specific focus on the effects of gender and age. Accordingly, flexion-extension angles and moments, obtained through gait analysis, were used to generate moment–angle loops at the ankle joint in 120 healthy subjects walking at a same normalized speed. Four reproducible types of loops were identified: Typical Loops, Narrow, Large and Yielding loops. No significant changes in the slopes of the main loop phases were observed as a function of gender and age, with the exception of a relative increase in the slope of the descending phase in elderly males compared to adult females. As for the ergometric parameters, the peak ankle moment, work produced and net work along the cycle were slightly, but significantly affected, with progressively decrease in the following order: Adult Males, Adult Females, Elderly Males and Elderly Females. The evidence that only few of the quantitative aspects of moment–angle loops were affected suggests that the control strategy which regulates the biomechanical properties of the ankle joint during walking is rather robust and qualitatively consistent across genders and age.     

Role of physical activity and perceived adequacy on peak aerobic power in children with developmental coordination disorder

The purpose of this study was to determine the mediating role of physical activity and perceived adequacy towards physical activity on peak aerobic power (VO₂peak) in children with developmental coordination disorder. This case-control study involved 61 male and female subjects age 12-13 years with motor impairments and 61 healthy controls matched for age, gender and school location. Subjects were assessed for motor proficiency and classified as probable developmental coordination disorder (p-DCD) or healthy control using the Movement Assessment Battery for Children, 2nd Edition. VO₂peak was assessed by a progressive exercise test on a cycle ergometer. Perceived adequacy towards physical activity was estimated using the Children’s Self-perception of Adequacy and Predilection for Physical Activity scale. Physical activity was monitored for seven days using accelerometry. Children with p-DCD had significantly lower VO₂peak adjusted for lean mass (48.8 ± 7.2 ml/kg LM/min; p ? 0.05) compared to controls (53.1 ± 8.2 ml/kg LM/min). Regression analysis demonstrated that perceived adequacy and physical activity were significant mediators in the relationship between p-DCD and VO₂peak (R-squared = 24.3%). In conclusion, using a stringent laboratory assessment, the current study verifies earlier non laboratory findings, adding low aerobic power, the most important component of cardiorespiratory fitness, to the list of health consequences associated with developmental coordination disorder.