Influence of muscle preactivation of the lower limb on impact dynamics in case of frontal collision

Accidentology or shock biomechanics are research domains mainly devoted to the development of safety conditions for the users of various transport modes in case of an accident. The objective of this study was to improve the knowledge of the biomechanical behaviour of the lower limb facing sudden dynamic loading during a frontal collision. We aimed at establishing the relationship between the level of muscular activity prior to impact, called 'preactivation', of the lower limb extensors and the mechanical characteristics of impact. Relationships were described between the level of preactivation, the impact peak force values, the minimum force after unloading and the associated loading and unloading rates. The existence of reflex mechanisms that were affected by the level of voluntary muscular preactivation for the lower limb muscles was demonstrated. In conclusion, the existence of specific mechanism acting mainly at the knee level may result from the level of preactivation. Muscle behavior has to be included in numerical models of the human driver to better evaluate the overall stiffness of the body before and at impact

Does visual experience influence arm proprioception and its lateralization? Evidence from passive matching performance in congenitally-blind and sighted adults

In humans, body segments' position and movement can be estimated from multiple senses such as vision and proprioception. It has been suggested that vision and proprioception can influence each other and that upper-limb proprioception is asymmetrical, with proprioception of the non-dominant arm being more accurate and/or precise than proprioception of the dominant arm. However, the mechanisms underlying the lateralization of proprioceptive perception are not yet understood. Here we tested the hypothesis that early visual experience influences the lateralization of arm proprioceptive perception by comparing 8 congenitally-blind and 8 matched, sighted right-handed adults. Their proprioceptive perception was assessed at the elbow and wrist joints of both arms using an ipsilateral passive matching task. Results support and extend the view that proprioceptive precision is better at the non-dominant arm for blindfolded sighted individuals. While this finding was rather systematic across sighted individuals, proprioceptive precision of congenitally-blind individuals was not lateralized as systematically, suggesting that lack of visual experience during ontogenesis influences the lateralization of arm proprioception

Scoliose idiopathique de l’adolescence : apport de la modélisation numérique dans le traitement orthopédique

La scoliose, déformation tridimensionnelle du rachis, peut être traitée par corset orthopédique. Notre étude vise à améliorer leur conception, d’une part par la réalisation / exploitation d’un modèle numérique du tronc et d’autre part par l’application de nouvelles stratégies correctives. Les résultats montrent l’efficacité de bandes modulables. Cette étude met en avant la faisabilité et l’efficacité d’une modélisation qualitative

How about running on Mars? Influence of sensorimotor coherence on running and spatial perception in simulated reduced gravity

Motor control, including locomotion, strongly depends on the gravitational field. Recent developments such as lower-body positive pressure treadmills (LBPPT) have enabled studies on Earth about the effects of reduced body weight (BW) on walking and running, up to 60% BW. The present experiment was set up to further investigate adaptations to a more naturalistic simulated hypogravity, mimicking a Martian environment with additional visual information during running sessions on LBPPT. Twenty-nine participants performed three sessions of four successive five-min runs at preferred speed, alternating Earth- or simulated Mars-like gravity (100% vs. 38% BW). They were displayed visual scenes using a virtual reality headset to assess the effects of coherent visual flow while running. Running performance was characterized by normal ground reaction force and pelvic accelerations. The perceived upright and vection (visually-induced self-motion sensation)in dynamic visual environments were also investigated at the end of the different sessions. We found that BW reduction induced biomechanical adaptations independently of the visual context. Active peak force and stance time decreased, while flight time increased. Strong inter-individual differences in braking and push-off times appeared at 38% BW, which were not systematically observed in our previous studies at 80% and 60% BW. Additionally, the importance given to dynamic visual cues in the perceived upright diminished at 38% BW, suggesting an increased reliance on the egocentric body axis as a reference for verticality when the visual context is fully coherent with the previous locomotor activity. Also, while vection was found to decrease in case of a coherent visuomotor coupling at 100% BW (i.e., post-exposure influence), it remained unaffected by the visual context at 38% BW. Overall, our findings suggested that locomotor and perceptual adaptations were not similarly impacted, depending on the -simulated- gravity condition and visual context

Dynamique des ajustements neuromusculaires à la réduction progressive des forces externes

International audienceINTRODUCTION:Previous studies have investigated the running pattern adjustments to sudden perturbations, such as unexpected changes in terrain stiffness or height (1). Interestingly, lower body positive pressure treadmills (LBPPTs) allow for progressive unweighting. During the transition phase, positive correlations have been reported between changes in braking force and changes in vasti and soleus muscle activity (2). However, these correlations were not examined separately for each participant, likely masking inter-individual variability. The current study investigated the dynamics of neuromuscular adjustments to the progressive reduction of external forces, using an individualised analysis. METHODS:Forty men (19±1yr) ran on a LBPPT in 3 consecutive conditions performed at 100, 60 and 100% body weight. The analysis focused on the unweighing transition which included 18±3 right strides. Normal ground reaction force and surface EMG activity of 11 right lower limb muscles were averaged over the braking phase and normalised to their mean value recorded at 100% body weight. Spearman’s correlation tests were used to quantify the relationship between the normalized mean braking force and mean EMG activity. Chi-squared tests were used to compare the proportions of positive and negative significant correlations. RESULTS:Braking force was positively correlated with VM (ρ=0.67±0.11), VL (0.66±0.10) and RF (0.68±0.10) activity in 65, 60 and 63% of the participants, respectively. It was negatively correlated with STSM (-0.67±0.10) and BF (-0.68±0.10) activity in 50 and 28% of them. Braking force was positively correlated with shank muscle activity in few participants (<13%), but negatively correlated with SOL (-0.59±0.13), GaM (-0.58±0.15), GaL (-0.70±0.15), TA (-0.59±0.10) and PL (-0.63±0.10) activity in 8, 13, 28, 23 and 25% of them. Positive correlations predominated for the quadriceps (VM: X²=26, VL: 21.2, RF:25, p<.001), whereas negative correlations predominated for the hamstrings (STSM: X²=20, BF:20, p<.001) and three shank muscles (GaL: X²=6.4, TA: 6.2, PL: 3.8, p<.05).CONCLUSION:The progressive reduction of external forces mainly affected thigh muscle activity. It showed a decrease in quadriceps activity, as previously reported (2), but an increase in hamstring activity in most participants. Operating in the optimal range of their force-length relationship, the hamstrings may have contributed to the reduced peak knee flexion during the stance phase (3). On the other hand, unweighting resulted in opposite adjustments in shank muscle activity between participants. Nevertheless, their activity increased in most of them, suggesting an increased ankle stabilisation. This is attributed to the shift towards a more forefoot strike pattern (3). This adjustment appears to be transient as it is no longer observed after 3 minutes of unweighted running (4).REFERENCES:(1)Daley, in Understanding Mammalian Locomotion, 2016(2)Sainton et al., PloS One, 2016(3)Neal et al., J Orthop Sports Phys Ther, 2016(4)Fazzari et al., Front Physiol, 202

Plantar Sole Unweighting Alters the Sensory Transmission to the Cortical Areas

International audienceIt is well established that somatosensory inputs to the cortex undergo an early and a later stage of processing. The later has been shown to be enhanced when the earlier transmission decreased. In this framework, mechanical factors such as the mechanical stress to which sensors are subjected when wearing a loaded vest are associated with a decrease in sensory transmission. This decrease is in turn associated with an increase in the late sensory processes originating from cortical areas. We hypothesized that unweighting the plantar sole should lead to a facilitation of the sensory transmission. To test this hypothesis, we recorded cortical somatosensory evoked potentials (SEPs) of individuals following cutaneous stimulation (by mean of an electrical stimulation of the foot sole) in different conditions of unweighting when standing still with eyes closed. To this end, the effective bodyweight (BW) was reduced from 100% BW to 40% BW. Contrary to what was expected, we found an attenuation of sensory information when the BW was unweighted to 41% which was not compensated by an increase of the late SEP component. Overall these results suggested that the attenuation of sensory transmission observed in 40 BW condition was not solely due to the absence of forces acting on the sole of the feet but rather to the current relevance of the afferent signals related to the balance constraints of the task

Partial Unweighting in Obese Persons Enhances Tactile Transmission From the Periphery to Cortical Areas: Impact on Postural Adjustments

International audienceTactile plantar information is known to play an important role in balance maintenance and to contribute to the setting of anticipatory postural adjustments (APAs) prior to stepping. Previous studies have suggested that somatosensory processes do not function optimally for obese individuals due to the increased pressure of the plantar sole resulting in balance issues. Here, we investigated whether decreasing the compression of the mechanoreceptors by unweighting the plantar sole would enhance tactile sensory processes leading to an increased stability and an accurate setting of the APAs in obese individuals. More specifically, we tested the hypothesis that the somatosensory cortex response to electric stimulation (SEP) of the plantar sole in standing obese persons will be greater with reduced body weight than with their effective weight. The level of unweighting was calculated for each participant to correspond to a healthy body mass index. We showed an increase SEP amplitude in the unweighted condition compared to the effective body weight for all participants. This increase can be explained by the reduction of weight itself but also by the modified distribution of the pressure exerted onto the foot sole. Indeed, in the unweighted condition, the vertical ground reaction forces are evenly distributed over the surface of the foot. This suggests that decreasing and equalizing the pressure applied on the plantar mechanoreceptors results in an increase in somatosensory transmission and sensory processes for obese persons when unweighted. These sensory processes are crucial prior to step initiation and for setting the anticipatory postural adjustments (i.e., thrust). These cortical changes could have contributed to the observed changes in the spatiotemporal characteristics of the thrust prior to step initiation

Neuro-mechanical adjustments to shod versus barefoot treadmill runs in the acute and delayed stretch-shortening cycle recovery phases

International audienceIn habitually shod recreational runners, we studied the combined influence of footwear and stretch-shortening cycle (SSC) fatigue on treadmill running pattern, paying special attention to neuro-mechanical adjustments in the acute and 2-day delayed recovery periods. The SSC exercise consisted of a series of 25 sub-maximal rebounds on a sledge apparatus repeated until exhaustion. The acute and delayed functional fatigue effects were quantified in a maximal drop jump test. The neuro-mechanical adjustments to fatigue were examined during two submaximal treadmill run tests of 3 min performed either barefoot or with shoes on. Surface electromyographic (EMG) activities, tibial accelerations and kinematics of the right lower limb were recorded during the first and last 15 s of each run. The main result was that neuro-mechanical differences between the shod and barefoot running patterns, classically reported in the absence of fatigue, persisted in the fatigued state. However, in the delayed recovery phase, rearfoot eversion was found to significantly increase in the shod condition. This specific footwear effect is considered as a potential risk factor of overuse injuries in longer runs. Therefore, specific care should be addressed in the delayed recovery phase of SSC fatigue and the use of motion control shoes could be of interest

Joint Specificity and Lateralization of Upper-Limb Proprioceptive Perception

International audienceProprioception is the sense of position and movement of body segments. The widespread distribution of proprioceptors in human anatomy raises questions about proprioceptive uniformity across different body parts. For the upper limbs, previous research, using mostly active and/or contralateral matching tasks, has suggested better proprioception of the non-preferred arm, and at the elbow rather than the wrist. Here we assessed proprioceptive perception through an ipsilateral passive matching task by comparing the elbow and wrist joints of the preferred and non-preferred arms. We hypothesized that upper limb proprioception would be better at the elbow of the non-preferred arm. We found signed errors to be less variable at the non-preferred elbow than at the preferred elbow and both wrists. Signed errors at the elbow were also more stable than at the wrist. Across individuals, signed errors at the preferred and non-preferred elbows were correlated. Also, variable signed errors at the preferred wrist, non-preferred wrist, and preferred elbow were correlated. These correlations suggest that an individual with relatively consistent matching errors at one joint may have relatively consistent matching errors at another joint. Our findings also support the view that proprioceptive perception varies across upper limb joints, meaning that a single joint assessment is insufficient to provide a general assessment of an individual’s proprioception

Kinetics and Muscle Activity Patterns during Unweighting and Reloading Transition Phases in Running

International audienceAmongst reduced gravity simulators, the lower body positive pressure (LBPP) treadmill is emerging as an innovative tool for both rehabilitation and fundamental research purposes as it allows running while experiencing reduced vertical ground reaction forces. The appropriate use of such a treadmill requires an improved understanding of the associated neuromechanical changes. This study concentrates on the runner's adjustments to LBPP-induced unweighting and reloading during running. Nine healthy males performed two running series of nine minutes at natural speed. Each series comprised three sequences of three minutes at: 100% bodyweight (BW), 60 or 80% BW, and 100% BW. The progressive unweighting and reloading transitions lasted 10 to 15 s. The LBPP-induced unweighting level, vertical ground reaction force and center of mass accelerations were analyzed together with surface electromyographic activity from 6 major lower limb muscles. The analyses of stride-to-stride adjustments during each transition established highly linear relationships between the LBPP-induced progressive changes of BW and most mechanical parameters. However, the impact peak force and the loading rate systematically presented an initial 10% increase with unweighting which could result from a passive mechanism of leg retraction. Another major insight lies in the distinct neural adjustments found amongst the recorded lower-limb muscles during the pre-and post-contact phases. The preactivation phase was characterized by an overall EMG stability, the braking phase by decreased quad-riceps and soleus muscle activities, and the push-off phase by decreased activities of the shank muscles. These neural changes were mirrored during reloading. These neural adjustments can be attributed in part to the lack of visual cues on the foot touchdown. These findings highlight both the rapidity and the complexity of the neuromechanical changes associated with LBPP-induced unweighting and reloading during running. This in turn emphasizes the need for further investigation of the evolution over time of these neurome-chanical changes