Common synaptic input, synergies and size principle: Control of spinal motor neurons for movement generation

Understanding how movement is controlled by the CNS remains a major challenge, with ongoing debate about basic features underlying this control. In current established views, the concepts of motor neuron recruitment order, common synaptic input to motor neurons and muscle synergies are usually addressed separately and therefore seen as independent features of motor control. In this review, we analyse the body of literature in a broader perspective and we identify a unified approach to explain apparently divergent observations at different scales of motor control. Specifically, we propose a new conceptual framework of the neural control of movement, which merges the concept of common input to motor neurons and modular control, together with the constraints imposed by recruitment order. This framework is based on the following assumptions: (1) motor neurons are grouped into functional groups (clusters) based on the common inputs they receive; (2) clusters may significantly differ from the classical definition of motor neuron pools, such that they may span across muscles and/or involve only a portion of a muscle; (3) clusters represent functional modules used by the CNS to reduce the dimensionality of the control; and (4) selective volitional control of single motor neurons within a cluster receiving common inputs cannot be achieved. Here, we discuss this framework and its underlying theoretical and experimental evidence

Motor Adaptations to Pain during a Bilateral Plantarflexion Task: Does the Cost of Using the Non-Painful Limb Matter?

During a force-matched bilateral task, when pain is induced in one limb, a shift of load to the non-painful leg is classically observed. This study aimed to test the hypothesis that this adaptation to pain depends on the mechanical efficiency of the non-painful leg. We studied a bilateral plantarflexion task that allowed flexibility in the relative force produced with each leg, but constrained the sum of forces from both legs to match a target. We manipulated the mechanical efficiency of the non-painful leg by imposing scaling factors: 1, 0.75, or 0.25 to decrease mechanical efficiency (Decreased efficiency experiment: 18 participants); and 1, 1.33 or 4 to increase mechanical efficiency (Increased efficiency experiment: 17 participants). Participants performed multiple sets of three submaximal bilateral isometric plantarflexions with each scaling factor during two conditions (Baseline and Pain). Pain was induced by injection of hypertonic saline into the soleus. Force was equally distributed between legs during the Baseline contractions (laterality index was close to 1; Decreased efficiency experiment: 1.16±0.33; Increased efficiency experiment: 1.11±0.32), with no significant effect of Scaling factor. The laterality index was affected by Pain such that the painful leg contributed less than the non-painful leg to the total force (Decreased efficiency experiment: 0.90±0.41, P<0.001; Increased efficiency experiment: 0.75±0.32, P<0.001), regardless of the efficiency (scaling factor) of the non-painful leg. When compared to the force produced during Baseline of the corresponding scaling condition, a decrease in force produced by the painful leg was observed for all conditions, except for scaling 0.25. This decrease in force was correlated with a decrease in drive to the soleus muscle. These data highlight that regardless of the overall mechanical cost, the nervous system appears to prefer to alter force sharing between limbs such that force produced by the painful leg is reduced relative to the non-painful leg

APPLIED SESSION: ELASTOGRAPHY FOR MUSCLE BIOMECHANICS

The purpose of this applied session is to demonstrate the potential of shear wave elastography for the study of muscle biomechanics using both real-time demo and recent results, with a special focus on sport applications (stretching, fatigue, pain, damage)

Influence of experimental pain on the perception of action capabilities and performance of a maximal single-leg hop

Changes in an individual's state - for example, anxiety/chronic pain - can modify the perception of action capabilities and physical task requirements. In parallel, considerable literature supports altered motor performance during both acute and chronic pain. This study aimed to determine the effect of experimental pain on perception of action capabilities and performance of a dynamic motor task. Performance estimates and actual performance of maximal single-leg hops were recorded for both legs in 13 healthy participants before, during, and after an episode of acute pain induced by a single bolus injection of hypertonic saline into vastus lateralis of 1 leg, with the side counterbalanced among participants. Both estimation of performance and actual performance were smaller (

ELECTROMECHANICAL DELAY AND ITS MECHANISMS ARE NOT IMPAIRED FOLLOWING ECCENTRIC EXERCISE

The aim of the present study was to assess the effect of exercise-induced muscle damage on both electrochemical and mechanical components involved in the electromechanical delay in the gastrocnemius medialis muscle. 15 healthy participants completed 10 sets of 30 maximal eccentric contractions of the plantar flexor muscles at a constant angular velocity of 45°.s-1. Delayed onset muscular soreness, maximal isometric torque, and electromechanical delay were measured before, 1h, and 48h following eccentric exercise. The present study revealed that the time required for both electrochemical and mechanical process involved in electromechanical delay are not impaired by exercise induced muscle damage. This study suggests that the long lasting reduction in force after eccentric exercise cannot be associated to an alteration of the force transmission efficiency

The effects of acute experimental hip muscle pain on dynamic single-limb balance performance in healthy middle-aged adults

Middle-aged adults with painful hip conditions show balance impairments that are consistent with an increased risk of falls. Pathological changes at the hip, accompanied by pain, may accelerate pre-existing age-related balance deficits present in midlife. To consider the influence of pain alone, we investigated the effects of acute experimental hip muscle pain on dynamic single-limb balance in middle-aged adults. Thirty-four healthy adults aged 40–60 years formed two groups (Group-1: n\ua0=\ua016; Group-2: n\ua0=\ua018). Participants performed four tasks: Reactive Sideways Stepping (ReactSide); Star Excursion Balance Test (SEBT); Step Test; Single-Limb Squat; before and after an injection of hypertonic saline into the right gluteus medius muscle (Group-1) or ∼5\ua0min rest (Group-2). Balance measures included the range and standard deviation of centre of pressure (CoP) movement in mediolateral and anterior-posterior directions, and CoP total path velocity (ReactSide, Squat); reach distance (SEBT); and number of completed steps (Step Test). Data were assessed using three-way analysis of variance. Motor outcomes were altered during the second repetition of tasks irrespective of exposure to experimental hip muscle pain or rest, with reduced SEBT anterior reach (−1.2\ua0±\ua04.1\ua0cm, P\ua0=\ua00.027); greater step number during Step Test (1.5\ua0±\ua01.7 steps, P\ua

The influence of isoinertial-pneumatic ratio on force-velocity-power relationships

Introduction: Isoinertial contractions are effective to generate maximal force during the initiation of the movement whereas they do not provide an appropriate training stimulus to generate force once accelerative phase has been developed (1). Pneumatic resistance is one alternative that has been developed to overcome the aforementioned limitations associated with isoinertial contractions. This technique allow higher initial velocity and reduce the decrease of force towards the end of the concentric phase (1). There is some training interest by combining isoinertial and pneumatic loading. The aim of this study was to determine how different isoinertial-pneumatic ratio influence the force-velocity-power relationships during bench-press. Methods: Fifteen participants performed bench press at 30%, 45%, 60%, 75%, and 90% of their 1RM, with five isoinertial(I)-pneumatic(P) resistance ratio : 100%I/0%P, 75%I/25%P, 50%I/50%P, 25%I/75%P, and 0%I/100%P. Velocity, force and power were assessed using a linear transducer and mechanical parameters measured by the pneumatic ergometer. Force-, velocity- and power-time patterns were averaged over the push-off phase to build the corresponding force-velocity and power-velocity relationships for each resistance ratio. Results: The increase in pneumatic part in resistance ratio elicited higher movement velocity and lower force level from 0% to 80% of the concentric phase. The increase in isoinertial part in resistance balance resulted in higher velocity towards the end of the movement. As a consequence, the use of isoinertial resistance oriented the force-velocity relationship towards force, whereas pneumatic resistance elicited a more velocity-oriented profile. Conclusion: Pneumatic-oriented resistance could be used to develop initial velocity and force towards the end of the push-off. Isoinertial-oriented resistance should be used to develop maximal force and maximal power. Resistance modality could be modulated according to training objectives. Références : 1. Frost et al. A comparison of the kinematics, kinetics and muscle activity between pneumatic and free weight resistance. Eur J Appl Physiol. 2008;104:937-56

Application of the speed-duration relationship to normalize the intensity of high-intensity interval training

The tolerable duration of continuous high-intensity exercise is determined by the hyperbolic Speed-tolerable duration (S-tLIM) relationship. However, application of the S-tLIM relationship to normalize the intensity of High-Intensity Interval Training (HIIT) has yet to be considered, with this the aim of present study. Subjects completed a ramp-incremental test, and series of 4 constant-speed tests to determine the S-tLIM relationship. A sub-group of subjects (n = 8) then repeated 4 min bouts of exercise at the speeds predicted to induce intolerance at 4 min (WR4), 6 min (WR6) and 8 min (WR8), interspersed with bouts of 4 min recovery, to the point of exercise intolerance (fixed WR HIIT) on different days, with the aim of establishing the work rate that could be sustained for 960 s (i.e. 4×4 min). A sub-group of subjects (n = 6) also completed 4 bouts of exercise interspersed with 4 min recovery, with each bout continued to the point of exercise intolerance (maximal HIIT) to determine the appropriate protocol for maximizing the amount of high-intensity work that can be completed during 4×4 min HIIT. For fixed WR HIIT tLIM of HIIT sessions was 399±81 s for WR4, 892±181 s for WR6 and 1517±346 s for WR8, with total exercise durations all significantly different from each other (P&#60;0.050). For maximal HIIT, there was no difference in tLIM of each of the 4 bouts (Bout 1: 229±27 s; Bout 2: 262±37 s; Bout 3: 235±49 s; Bout 4: 235±53 s; P&#62;0.050). However, there was significantly less high-intensity work completed during bouts 2 (153.5±40. 9 m), 3 (136.9±38.9 m), and 4 (136.7±39.3 m), compared with bout 1 (264.9±58.7 m; P&#62;0.050). These data establish that WR6 provides the appropriate work rate to normalize the intensity of HIIT between subjects. Maximal HIIT provides a protocol which allows the relative contribution of the work rate profile to physiological adaptations to be considered during alternative intensity-matched HIIT protocols