Philippe Corbeil

Increased risk for falling associated with obesity: mathematical modeling of postural control

Recent epidemiological studies report that obesity is positively related to fracture incidence. In the present experiment, a model of postural control was used to examine the impact of an abnormal distribution of body fat in the abdominal area upon postural stability. Obese and lightweight humanoids were destabilized by imposing a small initial angular speed from a neutral standing position. To avoid a loss of stability yielding a stepping reaction or a fall, an ankle torque is necessary to counteract the perturbation. Three torque parameters-ankle torque onset, time to peak torque, and muscular ankle torque-were entered in a program to simulate the intrinsic variability of the human postural control system. A loss of stability was detected when the center of pressure exceeded stability margins. The most striking observation is the nonlinear increase of torque needed to stabilize the humanoid when the motor response was chararterized by delayed temporal parameters. The effect was more pronounced when an anterior position of the center of mass was included in the simulations. This suggests that, when submitted to daily postural stresses and perturbations, obese persons (particularly those with an abnormal distribution of body fat in the abdominal area) may be at higher risk of falling than lightweight individuals.

Perturbation of the postural control system induced by muscular fatigue

In this experiment, we induced muscular fatigue of ankle plantar-flexors to examine how it deteriorates the regulation of bipedal quiet upright standing. Postural stability was assessed in conditions with and without vision over 60 s period to examine not only classical postural variables (time- and frequency-domain analyses), but also structural variables (stabilogram-diffusion analysis). Muscular fatigue was induced with repeated plantar-flexion of both legs. With muscular fatigue, subjects exhibited an increased postural sway (faster center of pressure (CP) velocity, and greater CP mean and median frequency) and a decreased long-term scaling exponent compared with the control conditions. The fatigue conditions, however, did not modify the range of oscillations and the variability of the postural oscillations around the mean position of CP. The effects of muscular fatigue were similar with eyes open and eyes closed. These results suggest that fatigue did induce some changes in the control mode of postural stability, but the detection/action capabilities of the sensorimotor system remained partly efficient when the ankle plantar-flexors were fatigued. Furthermore, the decreased long-term scaling exponent observed with fatigue suggests that the control of upright stance operates in a less stochastic and more antipersistent manner when fatigue is present (i.e. past and future behaviors were more negatively correlated and thus more tightly regulated). Altogether, the present results suggest that, compared with the no-fatigue conditions, fatigue places higher demands on the postural control system by increasing the frequency of actions needed to regulate the upright stance.

Weight loss and muscular strength affect static balance control

Objective:Overweight individuals sway more than normal weight individuals. Major weight loss improves their balance control despite a related decrease in muscle strength. Presumably, muscular strength is an important factor for balance control. This study investigated the effect that a change in body mass has on relative strength and balance control.Methodology:Force (isometric knee extension) and balance control (center of pressure speed and range) were studied in three groups; normal weight (BMI <25 kg m−2), obese (30 kg m−2 <BMI< 40 kg m−2) and excess obese (BMI >40 kg m−2) Caucasian male individuals.Results:The excess obese individuals who underwent bariatric surgery as a weight loss strategy were studied before, 3 and 12 months after losing on average, 66.9 kg (±95% CI 55.8, 77.9 kg; on average, 45% of their weight). The obese individuals who underwent diet modifications were studied before dieting and when resistance to weight loss occurred after losing on average 11.7 kg (±95% CI 9.3, 14.2 kg; on average, 12% of their weight). The control group was studied twice, 50 weeks apart. In obese and excess obese individuals, losing weight reduced absolute knee muscular strength on average, by 8.2 kg (±95% CI 3.9, 12.5 kg; on average, 10% of their strength) and 23.9 kg (±95% CI 12.1, 35.8 kg; on average, 33% of their strength). However, it also increased balance control measured with speed of the center of foot pressure, on average, by 0.10 cm s–1 (±95% CI 0.05, 0.14 cm s–1; or increased of 12%) and 0.28 cm s–1 (±95% CI 0.07, 0.47 cm s–1; increased of 27%), respectively. Relative strength increased approximately by 22% for only the excess obese group 12 months post surgery.Conclusion:This suggests, in overweight individuals, weight loss is more efficient at improving balance control than increasing, or even maintaining muscle strength. In these individuals, training programs aimed at improving balance control should primarily target weight loss.

Effects of 24 wk of treadmill training on gait performance in Parkinson's disease.

PURPOSE Recent studies suggest that walking on a treadmill improves gait, mobility, and quality of life of patients with Parkinsons disease (PD). Still, there is a need for larger-scale randomized controlled studies that demonstrate the advantages of treadmill training (TT) with control groups that receive similar amounts of attention. Moreover, to date, no study has combined speed and incline as parameters of progression. The aim of the study was to evaluate the effects of 24 wk of TT, with and without the use of incline, on gait, mobility and quality of life in patients with PD. METHODS The sample comprised 34 patients with PD, at the Hoehn and Yahr stage 1.5 or 2. Participants were randomized to speed TT, mixed TT, and control groups. The intervention consisted of 72 one-hour exercise sessions for 24 wk. The main outcome measures are the Movement Disorder Society-Unified Parkinsons Disease Rating Scale, the 39-item Parkinsons Disease Questionnaire, spatiotemporal parameters of gait and 6-min walking distance. The measures were taken at baseline, mid-term and after 6 months. RESULTS Both TT groups improved in terms of speed, cadence, and stride length during self-selected walking conditions at the study end point. Both groups also showed improvements in distance traveled. Only the Mixed TT group improved their quality of life. The Control group showed no progress. CONCLUSIONS Participants in this study showed significant improvements in walking speed and walking endurance after 6 months of TT. Improvements were observed after 3 months of intensive TT and persisted at 6 months. It appears that individuals with poorer baseline performance may benefit most from TT.

Effects of intensity and locus of painful stimulation on postural stability.

&NA; Stimulation of small diameter afferents can influence motor behavior. Little is known about how a prolonged painful stimulation of these small afferents may affect essential motor behavior such as the maintenance of an erect stance. The present study documents the effects of 10‐s weak, moderate and extreme painful stimulations applied to the dorsum of the feet on the postural stability. Also, the moderate painful stimulation was applied to the metacarpal heads to determine if a painful stimulation to a limb not involved in the maintenance of the erect stance affects the postural control mechanisms. Increasing the intensity of the painful stimulation applied to the feet yielded larger postural oscillations whereas stimulation to the hands did not affect the control of posture. This suggests that the painful stimulation mainly affected the postural control mechanisms via sensorimotor processes rather than via cognitive resources related to the perception of pain.

Short term alteration of balance control after a reduction of plantar mechanoreceptor sensation through cooling.

Proprioceptive inputs from the plantar sole contribute to balance control during normal quiet standing. This study investigated the cooling of plantar sole mechanoreceptors through ice immersion and its effects on balance control and lower leg muscle activity. Ten healthy males participated in this study. Plantar sole sensitivity was tested using the two point discriminatory test and the Von Frey monofilaments test. Plantar sole cooling was achieved through foot immersion in ice water. Balance control was measured using a force platform with seven trials (30s) performed before and after ice water foot immersion. Lower limb balance control muscle activity was measured with electromyography. Ice cooling reduced the plantar sole sensitivity of the foot. A short term alteration in balance control was observed with only the first trial showing significantly greater speed and RMS of the velocity of the centre of pressure in the cooling condition when compared to control trials before cooling. Muscular activity increased following the first trial. The adaptation observed after the short term alteration of balance control, could result from sensory reweighting processes. It is suggested that the muscular activity increase is evidence of sensory reweighting and contributes to the regulation of balance control when the plantar sole sensation is partially inhibited.

Does lower limb preference influence gait initiation

Gait initiation (GI) has been the focus of many investigations in order to determine the kinematic and kinetic parameters associated with this process. In these studies, the parameters are observed during GI with the preferential lower limb. However, none of these studies have looked at the impact on GI parameters when the start is achieved with the non-preferential limb. This investigation focused on the effects of lower limb preference on the kinematic and kinetic parameters of GI. Upon display of a visual cue, subjects stepped with preferential limb or non-preferential limb at natural speed. The duration of GI phases, the medio-lateral component of the center of mass (CM) displacement, the medio-lateral distance between the center of pressure and the CM, the step width as well as the medio-lateral impulse, were observed. When subjects started with the non-preferential limb, the bodyweight transfer was facilitated by a greater impulse during the anticipatory postural adjustment (APA) phase. Conversely, a more lateral CM displacement during the execution phase and a more lateral step in preferential start were observed. Asymmetry in frontal plane body motion was observed during weight transfer following APA, as well as during assistive control of ballistic body motion during the execution phase of the first step. In both conditions, the non-preferential limb provided the greater lateral impulse on the ground. This may have clinical relevance especially in individuals with unilateral limb dyscontrol and postural asymmetry that may require rehabilitation.

The effect of time to peak ankle torque on balance stability boundary: experimental validation of a biomechanical model

Pai and Patton (1997), using a biomechanical model, determined a set of feasible center of mass (CM) velocity-position combinations (balance stability boundary) that guarantee upright stability. In their study, the magnitude of the restoring ankle torque was used to study the subject’s ability to recover balance. Recent studies have suggested that the ability to maintain a stable posture depends not only on the magnitude of the restoring torque but also on the time to generate this torque. The objectives of the present study were: (1) to build a biomechanical model that predicts the balance stability boundary which includes time to peak ankle torque, (2) to determine the capability of the model to predict successful and failed experimental balance recovery trials, and (3) to compare the predictive capability of the biomechanical model with that of a statistical model (logistic regression). A single-link-plus-foot biomechanical model was used to determine a set of balance stability boundaries, computed from the combination of maximum CM velocity and related CM position, for various times to peak ankle torque. An experiment was conducted to validate the biomechanical model. The participants self-initiated a forward destabilization and were asked to regain balance using an ankle feet-in-place strategy. Also, a forward stepwise logistic regression (predictors: CM position and velocity and time to peak ankle torque) was used to discriminate between successful and failed experimental trials. (1) The outcomes of the biomechanical model confirmed that the time to peak ankle torque drastically constrained the stability boundaries. (2) The biomechanical model predicted 79.9% of the failed experimental trials and 74.5% of the successful experimental trials. (3) The stepwise logistic regression included all independent variables and predicted 57.2% of the failed and 93.7% of the successful experimental trials. Hence, the biomechanical model showed better predictive capability than the statistical model for identifying unsuccessful balance recovery. It is noteworthy that the balance stability boundaries constrained by the speed of ankle torque development predicted the outcome of the experimental trial earlier in the time series than balance stability boundary constrained by constant ankle torque. Overall, the present biomechanical model may serve as an assessment tool to develop specific interventions towards improving a patient’s speed of ankle torque development and to possibly reduce falling frequency.

Current state of knowledge of post-traumatic stress, sleeping problems, obesity and cardiovascular disease in paramedics

Purpose The impacts of emergency work on firefighters have been well documented and summarised, but this is not the case for paramedics. This paper explores the literature regarding the impact of work stress on paramedics. Objective To identify the literature available on the effect of paramedics’ jobs on their health status. Methods Electronic database used: MEDLINE (Ovid, PubMed, National Library of Medicine) between 2000 and 2011. Key words used for the computer searches were: paramedics, emergency responders, emergency workers, shift workers, post-traumatic symptoms, obesity, stress, heart rate variability, physiological response, blood pressure, cardiovascular and cortisol. Exclusion criteria were: studies in which participants were not paramedics, participants without occupational exposure, physical fitness assessment in paramedics and epidemiological reports regarding death at work. Results The electronic databases cited 42 articles, of which we excluded 17; thus, 25 articles are included in this review. It seems clear that paramedics accumulate a set of risk factors, including acute and chronic stress, which may lead to development of cardiovascular diseases. Post-traumatic disorders, sleeping disorders and obesity are prevalent among emergency workers. Moreover, their employers use no inquiry or control methods to monitor their health status and cardiorespiratory fitness. Conclusions More studies are needed to characterise paramedics’ behaviour at work. These studies could allow the development of targeted strategies to prevent health problems reported in paramedics.

The effects of muscle strength on center of pressure-based measures of postural sway in obese and heavy athletic individuals.

INTRODUCTION Obesity affects postural sway during normal quiet standing; however, the reasons for the increased postural sway are unknown. Improving muscular strength is regarded as a potential way to improve postural control, particularly for obese and overweight subjects. The purpose of this investigation is to evaluate the role of muscular strength on postural sway in obese and overweight individuals. METHODS Fifteen healthy weight (control group), seventeen obese (obese group) subjects and nine football players (heavy athletic group) participated in this investigation. Isometric knee extension force and postural sway were measured. Muscular strength was calculated in absolute measures as well as relative to body mass (muscular strength to body mass). RESULTS The heavy athletic group demonstrated significantly stronger (absolute) lower limb strength (1593.9 N (95% CI 1425.5, 1762.3)) than both the obese (796.2N (95% CI 673.8, 824.5)) and control (694.1N (95% CI 563.7, 824.5)) groups. As well, when muscular strength was expressed as a ratio to body mass the heavy athletic group had significantly higher values (1.27 (95% CI 1.11, 1.43)) than obese (0.78 (95% CI 0.66, 0.89) and control (1.00 (95% CI (0.88, 1.12)) individuals. Despite this, they swayed similarly to the obese (mean center of pressure speed of 0.83 cms(-1) (95% CI 0.72, 0.93) vs. 0.87 cms(-1) (95% CI 0.80, 0.95)), that is, significantly more than the controls (0.60 cms(-1) (95% CI 0.52, 0.68)). CONCLUSION Isometric knee extensor strength has a minimal effect on postural sway in heavier athletic individuals during normal quiet stance.