Unexpected impairment of INa underpins reentrant arrhythmias in a knock-in swine model of Timothy syndrome

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Motor Control of Landing from a Jump in Simulated Hypergravity

On Earth, when landing from a counter-movement jump, muscles contract before touchdown to anticipate imminent collision with the ground and place the limbs in a proper position. This study assesses how the control of landing is modified when gravity is increased above 1 g. Hypergravity was simulated in two different ways: (1) by generating centrifugal forces during turns of an aircraft (A300) and (2) by pulling the subject downwards in the laboratory with a Subject Loading System (SLS). Eight subjects were asked to perform counter-movement jumps at 1 g on Earth and at 3 hypergravity levels (1.2, 1.4 and 1.6 g) both in A300 and with SLS. External forces applied to the body, movements of the lower limb segments and muscular activity of 6 lower limb muscles were recorded. Our results show that both in A300 and with SLS, as in 1 g: (1) the anticipation phase is present; (2) during the loading phase (from touchdown until the peak of vertical ground reaction force), lower limb muscles act like a stiff spring, whereas during the second part (from the peak of vertical ground reaction force until the return to the standing position), they act like a compliant spring associated with a damper. (3) With increasing gravity, the preparatory adjustments and the loading phase are modified whereas the second part does not change drastically. (4) The modifications are similar in A300 and with SLS, however the effect of hypergravity is accentuated in A300, probably due to altered sensory inputs. This observation suggests that otolithic information plays an important role in the control of the landing from a jump

Motor control of landing from a countermovement jump in simulated microgravity.

Landing from a jump implies proper positioning of the lower limb segments and the generation of an adequate muscular force to cope with the imminent collision with the ground. This study assesses how a hypogravitational environment affects the control of landing after a countermovement jump (CMJ). Eight participants performed submaximal CMJs on Earth (1-g condition) and in a weightlessness environment with simulated gravity conditions generated by a pull-down force (1-, 0.6-, 0.4-, and 0.2-g0 conditions). External forces applied to the body, movements of the lower limb segments, and muscular activity of six lower limb muscles were recorded. 1) All subjects were able to jump and stabilize their landing in all experimental conditions, except one subject in 0.2-g0 condition. 2) The mechanical behavior of lower limb muscles switches during landing from a stiff spring to a compliant spring associated with a damper. This is true whatever the environment, on Earth as well as in environments where sensory inputs are altered. 3) The motor control of landing in simulated 1 g0 reveals an increased "safety margin" strategy, illustrated by increased stiffness and damping coefficient compared with landing on Earth. 4) The motor command is adjusted to the task constraints: muscular activity of lower limb extensors and flexors, stiffness and damping coefficient decrease according to the decreased gravity level. Our results show that even if in daily living gravity can be perceived as a constant factor, subjects can cope with altered sensory signals, taking advantage of the remaining information (visual and/or decreased proprioceptive inputs)

Human motor control of landing from a drop in simulated microgravity

Landing on the ground on one's feet implies that the energy gained during the fall be dissipated. The aim of this study is to assess human motor control of landing in different conditions of fall initiation, simulated gravity, and sensory neural input. Six participants performed drop landings using a trapdoor system and landings from self-initiated counter-movement jumps in microgravity conditions simulated in a weightlessness environment by different pull-down forces of 1-, 0.6-, 0.4-, and 0.2 g External forces applied to the body, orientation of the lower limb segments, and muscular activity of 6 lower limb muscles were recorded synchronously. Our results show that 1) subjects are able to land and stabilize in all experimental conditions; 2) prelanding muscular activity is always present, emphasizing the capacity of the central nervous system to approximate the instant of touchdown; 3) the kinetics and muscular activity are adjusted to the amount of energy gained during the fall; 4) the control of landing seems less finely controlled in drop landings as suggested by higher impact forces and loading rates, plus lower mechanical work done during landing for a given amount of energy to be dissipated. In conclusion, humans seem able to adapt the control of landing according to the amount of energy to be dissipated in an environment where sensory information is altered, even under conditions of non-self-initiated falls

THE CONTROL OF LANDING FROM A COUNTER-MOVEMENT JUMP IN HYPER-GRAVITY

It is well known that during a self-initiated fall or a drop-landing on Earth, the lower limb muscles are activated before ground contact (Melvill Jones & Watt 1971, Santello, 2005). The sensory inputs from the proprioceptive, otolithic and visual systems provide critical information to estimate the instant of touchdown, and the magnitude of the impact. The timing and amplitude of the muscular pre-activation is though to be modulated accordingly to these estimated parameters (Santello, 2005). However, there is still controversy about the modulation of this pre-activation: some authors (Santello & McDonagh, 1998) claim that it is independent of the height of the drop whereas others (Liebermann & Hoffman, 2004) have observed an increase of the pre-activation time with increased drop height. Here we explore the motor control of the counter movement jump (CMJ) in real hyper-gravity conditions up to 1,6g during ESA-parabolic flights. The question is if/how the central nervous system (CNS) predicts the time of touchdown, the characteristics of the ground reaction forces and controls the landing when gravity is increased. One can make the hypothesis that the impact with the ground will be increased, but it is unclear how the CNS will anticipate the moment of ground contact when the pull-down acceleration is increased and how the energy of the body at touchdown will be absorbed to avoid rebound on the ground. We expect that increased gravity will increase muscular amplitude while muscular pre-activation timing will remain. In order to verify these hypotheses, we recorded CMJs in the A300 zero-g during increased gravity fields obtained by turns of the airplane (1.2-1,4-1,6g). The ground reaction forces (GRF), the kinematics of the lower-limb segments and the electrical activity of the lower-limb muscles were measured in 9 subjects during 2 parabolic flight campaigns (55th-56th); a total of 7-13 jumps per subject were recorded in each hyper-gravity condition. With increased gravity, the height of the jump is decreased, but the vertical GRF peak at landing and the whole body stiffness are increased. The range of motion of each joint during landing decreased with hyper-gravity, leading to a decreased vertical downward displacement of the centre of mass. All muscles were pre-activated and the amplitude of the muscular activity increased with increased gravity. In conclusion the CNS seems able to take into account the increased downward acceleration by increasing the stiffness of the body together with an increased pre-programmed activity of the lower limb muscles. Future analysis will determine the specific effect of real hyper and hypo-g conditions on the landing pattern as well as on the factors explaining the EMG modifications observed on our preliminary results

The effect of simulated microgravity on the motor control of landing from ajump

The pre-programmed muscular activity is thought to play a key role in preparing the human body to the forthcoming impact forces in landing movements (Santello, 2005). The aim of this study was to assess the adaptability of the motor control of landing from a jump in simulated microgravity conditions. Experiments were carried out in the A300 0g during ESA parabolic flight campaigns. Participants were equipped with a loading system (Gosseye et al., 2010) creating 4 simulated gravity conditions (1g, 0.6g, 0.4g and 0.2g) during the 0g phases. Eight subjects were instructed to perform several consecutive counter-movement jumps (CMJs) and to land without rebounding. Kinetics, kinematics, and muscular activity of the lower limbs were recorded. The first 3 trials per gravity condition were rejected to avoid a learning effect. Subjects were able to perform CMJs in 0g and to land without re-bounding in the 4 gravity conditions. In the 1g condition, aerial time was 267 ± 35 ms (n=184); at landing, the peak vertical ground reaction force was 3.0 ± 0.9 times body weight with a whole body stiffness of 429 ± 386 s-2. Muscular activity was present before touchdown for most of the recordings in the 1g condition. With decreased gravity conditions, jump height and aerial time increased. Peak vertical ground reaction force decreased proportionally to the gravity condition, and whole body stiffness and amplitude of pre-landing muscular activity were reduced. These results suggest that the human body adapts whole body compliance and muscular activity with respect to the gravity condition. The presence of a pre-landing muscular activation suggests that the instant of touchdown can be predicted in different simulated gravity conditions. The fact that pre-landing muscular activity tends to disappear in 0.2g condition could be part of the specific landing control. Indeed, whole body stiffness has to be sufficiently low to avoid re-bounding on the ground in 0.2g condition

Experimental set-up (left) and typical trace in 1 <i>g</i> (right).

<p><b>Left:</b> scheme of the experimental set-up in the laboratory to simulate hypergravity. The additional pull-down force simulating hypergravity was generated by two pneumatic pistons placed horizontally on each side of the subject under the ground level. The force generated by each piston was transmitted to the harness by a rope, which passed through two pulleys. A first pulley, moving horizontally, doubled the movement of the harness as compared to the piston. A second pulley changed the direction of the force from horizontal to vertical. A force transducer placed at the level of this last pulley measured the tension in the rope and a force-plate measured the ground reaction forces under the feet (for more details see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0141574#sec002" target="_blank">Methods</a>). <b>Right:</b> typical trace of one subject (60 kg, 1.69 m, 46 yo) in 1 <i>g</i> in the laboratory: the vertical component of the ground reaction force (<i>F</i>_z normalized in <i>BW</i>), the vertical acceleration (<i>a</i>_z), velocity (<i>V</i>_z) and displacement (<i>S</i>_z) of the <i>COM</i> are expressed as a function of time, from 500 ms before take-off (<i>TO</i>) until 500 ms after touchdown (<i>TD</i>). The jump is divided into sub-periods: the instant of <i>TO</i> and the instant of <i>TD</i> delimit the aerial phase (<i>t</i>_aer). The <i>land</i>₁ is the period between <i>TD</i> and the moment at which the vertical force reaches its peak (<i>F</i>_z-peak); <i>land</i>₂ is the period between the time of <i>F</i>_z-peak and the moment at which the <i>COM</i> reaches its lowest vertical position (<i>S</i>_z-min). The dotted line on the <i>a</i>_z-time curve during <i>land</i>₁ represents the function computed from a spring-mass model. The black interrupted line after <i>land</i>₁ represents the function computed from a damped harmonic oscillator model (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0141574#sec002" target="_blank">Methods</a>).</p