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Robert J. Gregor, Judith L. Smith, Dagan W. Smith, Alanna Oliver, and Boris I. Prilutsky

Different forms of locomotion have been studied in the cat in an effort to understand the neural mechanisms involved in movement control. Recent studies have focused on the roles of one- and two-joint muscles, the integration of central commands with sensory input, and the notion that the control system may be organized around the mechanical actions of muscles and the number of joints they span. To investigate the load-sharing between the two-joint medial gastrocnemius and one-joint soleus muscles, a single cat was trained to walk in an instrumented Plexiglas enclosed walkway at slopes ranging ±75%. Surgically implanted tendon force transducers monitored force output from each muscle. Equations in Newtonian mechanics were used to calculate joint kinetics. Results suggest that as slope angle decreased, the one-joint soleus became the primary contributor to the plantar-flexor moment calculated during stance. Unexpectedly, as slope angle increased, force in the one-joint soleus decreased while force in the two-joint medial gastrocnemius increased in the presence of the increased plantar-flexor moment calculated during stance. One explanation is that activation and force in the two-joint medial gastrocnemius should increase in the presence of a knee flexor and plantar-flexor moment. This was the case during upslope walking, as two-joint muscles increase their activation when they act as an agonist at both joints they cross. Additionally, a force-dependent inhibition of the soleus by the medial gastrocnemius has been described as part of a neural control system organized around the mechanical actions of muscles and the number of joints they span. Hence, a decrease in one-joint soleus force might be expected under certain conditions in upslope walking.

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Monica A. Perez

Most of our daily actions involve movements of the hand. The neuronal pathway contributing to the control of hand movements are complex and not yet completely understood. Recent studies highlight how task-dependent changes in cortical and subcortical pathways driven by contralateral and ipsilateral influences may open avenues to further understand the complexity of hand actions in healthy and disease. In the following section studies using transcranial magnetic and electrical stimulation in healthy subjects and in individuals with chronic incomplete spinal cord injury will be highlighted to further understand neuronal pathways involved in the control of voluntary activity by hand muscles.

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Tania Suarez, Luca Laudani, Arrigo Giombini, Vincenzo Maria Saraceni, Pier Paolo Mariani, Fabio Pigozzi, and Andrea Macaluso

Context:

Tearing of the anterior cruciate ligament (ACL) may disrupt the ability to recognize the knee position in space during limb-repositioning tasks, which is referred to as joint-position sense (JPS). Impairments in JPS have been shown to be lower during active than passive repositioning tasks, thus suggesting that coactivation patterns of the muscles surrounding the knee might compensate for the disrupted JPS and ensure accurate limb repositioning in ACL-deficient individuals.

Objective:

To investigate muscle coactivation patterns during JPS repositioning tasks in ACL-deficient and healthy individuals.

Design:

Prospective observational study.

Setting:

Functional assessment laboratory.

Participants:

8 men age 25 ± 8 y with isolated ACL rupture and 10 men age 30 ± 4 y with no history of knee injury.

Intervention:

JPS was evaluated by means of an electrogoniometer in a sitting position during either passive or active joint-positioning and -repositioning tasks with a 40° target knee angle.

Main Outcome Measures:

Root mean square (RMS) of the surface electromyogram from the vastus lateralis and biceps femoris muscles was measured during active joint positioning and repositioning.

Results:

Healthy participants showed a significant decrease in vastus lateralis RMS (−19%) and an increase in biceps femoris RMS (+26%) during joint repositioning compared with positioning. In contrast, ACL-deficient patients showed no modulation in muscle coactivation between joint positioning and repositioning, although they exhibited significantly lower RMS of the vastus lateralis (injured limb, −28%; uninjured limb, −21%) and higher RMS of the biceps femoris (injured limb, +19%; uninjured limb, +30%) than the healthy participants during joint positioning.

Conclusions:

The lack of modulation in muscle coactivation patterns between joint positioning and repositioning in ACL-deficient patients might be attributed to disrupted neural control after the injury-related loss of proprioceptive information. These results should be taken into account in the design of rehabilitation protocols with emphasis on muscle coactivation and JPS.

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Leanna Ferrand and Slobodan Jaric

The purpose of this study was to explore the effects of handedness on coordination of grip (G) and load (L) forces in static bimanual manipulation tasks. Participants (N = 10) exerted various L profiles against an externally fixed hand-held device based on presumably open-loop and closed-loop neural control mechanisms, (i.e., mediated and not mediated, respectively, by sensory feedback). Average G/L ratio and the coupling of G and L (i.e., stability of the G/L ratio and correlation between G and L) were separately assessed in each hand. The results revealed a lower average G/L ratio in the non-dominant hand suggesting a more economical grip, while the indices of G and L coupling were similar in two hands. The dominant and non-dominant hand failed to reveal relative advantages in the tasks predominantly based on open- and closed-loop control mechanisms, respectively. We conclude that, due to the static nature of the tested tasks, the particular advantage of the non-dominant hand in G and L coordination could be in line with the recently proposed specialization of the non-dominant limb for control of position. However, the overall results are not in line with classic views of the prevailing open- closed-loop neural mechanisms in the control of the dominant and nondominant limb, respectively.

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Mehmet Uygur, Goran Prebeg, and Slobodan Jaric

We compared two standard methods routinely used to assess the grip force (GF; perpendicular force that hand exerts upon the hand-held object) in the studies of coordination of GF and load force (LF; tangential force) in manipulation tasks. A variety of static tasks were tested, and GF-LF coupling (i.e., the maximum cross-correlation between the forces) was assessed. GF was calculated either as the minimum value of the two opposing GF components acting upon the hand-held object (GFmin) or as their average value (GFavg). Although both methods revealed high GF-LF correlation coefficients, most of the data revealed the higher values for GFavg than for GFmin. Therefore, we conclude that the CNS is more likely to take into account GFavg than GFmin when controlling static manipulative actions as well as that GFavg should be the variable of choice in kinetic analyses of static manipulation tasks.

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Hendrik Reimann, Tyler Fettrow, and John J. Jeka

sophisticated neural control system happen, which causes humans to fall. In recent years, falls and the resulting injuries have become a major public health concern. Maintenance of balance is a neural control problem. The standing or walking human body is mechanically unstable. It will usually fall over after a

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Mark L. Latash

Over the past years, the notion of primitives has become prominent in studies on the neural control of movements (for reviews, see Flash & Hochner, 2005 ; Giszter, 2015 ; Giszter, Patil, & Hart, 2007 ; Hogan & Sternad, 2012 ; Ivanenko, Poppele, & Lacquaniti, 2006 ; Overduin, d’Avella, Roh

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Victoria Galea, Robyn Traynor, and Michael Pierrynowski

consistency. Neural Control of Timing Our result of significant temporal inconsistency in the youngest children is in accordance with previous finger-tapping studies ( Drewing et al., 2006 ; Greene & Williams, 1993 ; Williams et al., 1992 ). Furthermore, this improvement was attributed to an age