Cécile Smeesters

Effects of orthopaedic immobilization of the right lower limb on driving performance: an experimental study during simulated driving by healthy volunteers.

BACKGROUND The effects of immobilization of the right lower limb on driving performance are unknown. Therefore, clinicians and legislators cannot put forth recommendations for road safety for patients requiring such immobilization. The objective of the present study was to evaluate the effect of two orthopaedic immobilization devices on the braking performances of healthy volunteers under simulated driving conditions. METHODS The braking performances of forty-eight healthy volunteers were evaluated under three conditions: wearing a running shoe, wearing a walking cast, and wearing an Aircast Walker on the right lower limb. A computerized driving simulator was used to measure the maximum force applied on the brake pedal during braking as well as the braking reaction time and the total braking time during emergency braking with and without a distractor. RESULTS The mean braking forces applied with the shoe, the walking cast, and the Aircast Walker were 293.8, 275.4, and 287.2 lb (133.3, 124.9, and 130.3 kg), respectively. The value with the walking cast was significantly lower than that with the shoe or Aircast Walker (p < 0.0001); there was no difference between the shoe and the Aircast Walker. The adjusted mean braking reaction times during emergency braking without a distractor were 0.580 second (shoe), 0.609 second (cast), and 0.619 second (Aircast Walker). The value with the running shoe was significantly lower than that with either type of immobilization (p < or = 0.0001). With a distractor, the mean braking reaction time was shorter with the running shoe than it was with either form of immobilization (p < or = 0.0001); the mean time was also shorter with the walking cast than it was with the Aircast Walker (p = 0.003). During both emergency braking tasks (with and without a distractor), the mean total braking time was shorter with the shoe than it was with either type of immobilization (p < 0.0001). With a distractor, the adjusted mean total braking time was shorter with the walking cast than it was with the Aircast Walker (p = 0.035). CONCLUSIONS Immobilization of the right lower limb affects the braking force as well as the braking reaction time and total braking time during emergency braking by healthy volunteers. While these changes are significant, their impact on the ability to drive safely during emergency braking situations is questionable. Future research into the impact of such immobilization on the emergency braking performances of patients is warranted to confirm these observations.

Comparison between younger and older drivers of the effect of obstacle direction on the minimum obstacle distance to brake and avoid a motor vehicle accident

Our objectives were to determine the effects of age and direction of appearance of an obstacle on the minimum obstacle distance to brake and avoid a motor vehicle accident. Ten younger and 10 older drivers were tested in a custom-made driving simulator using an animation of a straight suburban road. Driving at 50 km/h, participants suddenly encountered pedestrians appearing at increasingly closer distances in front, from the left or from the right. They then had to brake as fast as possible and come to a complete stop before running over the pedestrian. Results showed that older drivers had 1.8m greater minimum obstacle distance than younger drivers. This decreased ability with age appeared to be due to a decline in response initiation. Pedestrians appearing from the periphery also resulted in 2.1m (left) and 2.8m (right) greater minimum obstacle distance than those appearing directly in front. This decreased ability with obstacle direction appeared to be mostly due to declines in response initiation and response geometry. Finally, the difference with age was greater when pedestrians appeared from the right compared to the left or front. Therefore, it is important to include both temporal and geometrical performance measures in studies on motor vehicle accidents.

Maximum allowable force on a safety harness cable to discriminate a successful from a failed balance recovery

A safety harness system is essential to ensure participant safety in experiments at the threshold of balance recovery where avoiding a fall is not always possible. The purpose of this study was to propose a method to determine the maximum allowable force on a safety harness cable to discriminate a successful from a failed balance recovery. Data from 12 younger adults, who participated in experiments to determine the maximum forward lean angles that participants could be suddenly released from and still recover balance using three different limits on the number of steps, were used. For each participant, the coefficients of an asymptotic exponential regression, between the maximum vertical force on the safety harness cable and the initial lean angle at each trial, were evaluated by a least squares method. A proposed threshold for the maximum allowable vertical force of five force constants ensured that the initial lean angle reached 99% of its steady state value with respect to its initial value. It should thus discriminate well a successful (below the threshold) from a failed (above the threshold) balance recovery. Furthermore, although the amplitude of the horizontal forces should not be neglected in safety harness system designs, the contributions of the medial-lateral and anterior-posterior forces can be neglected in experiments at the threshold of balance recovery. Finally, although our five force constants method could be used, the actual value obtained for the maximum allowable vertical force may vary with other safety harness systems and postural perturbations.

Effects of age and lean direction on the threshold of single-step balance recovery in younger, middle-aged and older adults

Several studies have quantified and compared balance recovery between healthy younger and older adults, using a variety of large postural perturbations and loss of balance directions. However, to the best of our knowledge, no studies at the threshold of balance recovery, where avoiding a fall is not always possible, have included middle-aged adults. We thus determined the maximum lean angle from which 20 younger, 16 middle-aged and 16 older healthy adults could be suddenly released and still recover balance using a single step for forward, sideways and backward leans. Results showed that the maximum lean angles of younger adults were 23% greater than middle-aged adults and 48% greater than older adults. The maximum lean angles for forward leans were 23% greater than sideways leans and 22% greater than backward leans. These declines with age and lean direction were associated with declines in response initiation, execution and geometry. Finally exponential regressions showed that the critical ages at which the ability to recover balance and avoid a fall significantly decreases were 51.0, 60.6 and 69.9 yrs for forward, sideways and backward leans, respectively. Therefore, we have demonstrated that age affects the ability to recover balance nearly a decade earlier than the rate of falls. Future studies should thus not only include older adults over 65 yrs, but also middle-aged adults under 65 yrs, or recruit all ages from 18 to 85 yrs. Finally, the critical ages identified in this study may justify an earlier screening of aging adults to prevent future falls, especially the first fall.

Kinematics of the threshold of balance recovery are not affected by instructions limiting the number of steps in younger adults.

To our knowledge, the effect of instructions limiting the number of steps on the threshold of balance recovery has not been quantified, which could make comparisons between studies difficult. We determined the maximum forward lean angles from which 28 younger adults could be suddenly released and still recover balance using: (i) only a single step, (ii) no more than two steps and (iii) no limit on the number of steps. Results showed that instructions limiting the number of steps significantly affected the maximum lean angle but only by a maximum of 1 degree. At the maximum lean angles, they also significantly affected reaction time, first weight transfer time, first stride velocity, first stride length and second stride velocity but only by 8% on average. However, they did not affect muscular latencies, first stride time, first stride height, first stride width or other second step variables. Moreover, at lean angles larger than their maximum, participants were able to respect the instructions but unable to recover balance without safety harness assistance. Given that the effect was too small to be pertinent, we can state that, between the three limits on the number of steps, not only were the maximum lean angles similar but the first steps were nearly identical and the additional steps did not help to increase the maximum lean angle. Therefore, we have demonstrated that instructions limiting or not limiting the number of steps appear to be equally valid to study the threshold of balance recovery in younger adults.

A sit-ski design aimed at controlling centre of mass and inertia

Abstract This article introduces a sit-ski developed for the Canadian Alpine Ski Team in view of the Vancouver 2010 Paralympic games. The design is predominantly based on controlling the mass distribution of the sit-ski, a critical factor in skiing performance and control. Both the antero-posterior location of the centre of mass and the sit-ski moment of inertia were addressed in our design. Our design provides means to adjust the antero-posterior centre of mass location of a sit-ski to compensate for masses that would tend to move the antero-posterior centre of mass location away from the midline of the binding area along the ski axis. The adjustment range provided is as large as 140 mm, thereby providing sufficient adaptability for most situations. The suspension mechanism selected is a four-bar linkage optimised to limit antero-posterior seat movement, due to suspension compression, to 7 mm maximum. This is about 5% of the maximum antero-posterior centre of mass control capacity (151 mm) of a human participant. Foot rest inclination was included in the design to modify the sit-ski inertia by as much as 11%. Together, these mass adjustment features were shown to drastically help athletes’ skiing performance.

Determining Fall Direction and Impact Location for Various Disturbances and Gait Speeds Using the Articulated Total Body Model

Because fall experiments with volunteers can be both challenging and risky, especially with older volunteers, we wished to develop computer simulations of falls to provide a theoretical framework for understanding and extending experimental results. To perform a preliminary validation of the articulated total body (ATB) model for passive falls, we compared the model predictions of fall direction, impact location, and impact velocity as a function of disturbance type (faint, slip, step down, trip) and gait speed (fast, normal, slow) to experimental results with young adult volunteers. The three-dimensional ATB model had 17 segments and 16 joints. Its physical characteristics, environment definitions, contact functions, and initial conditions were representative of our experiment. For each combination of disturbance and gait speed, the ATB model was left to fall passively under gravity once disturbed, i.e., no joint torques were applied, until impact with the floor occurred. Finally, we also determined the sensitivity of the model predictions to changes in the models parameters. Our model predictions of fall angles and impact angles were qualitatively in agreement with those observed experimentally for ten and seven of the 12 original simulations, respectively. Quantitatively, the model predictions of fall angles, impact angles, and impact velocities were within one experimental standard deviation for seven, three, and nine of the 12 original simulations, respectively, and within two experimental standard deviations for ten, nine, and 11 of the 12 original simulations, respectively. Finally, the fall angle and impact angle region did not change for 92% and 95% of the 74 input variation simulations, respectively, and the impact velocities were within the experimental standard deviations for 78% of the 74 input variation simulations. Based on our simulations and a sensitivity analysis, we conclude that our preliminary validation of the ATB model for passive falls was successful. In fact, these ATB model simulations represent a significant step forward in fall simulations. We believe that with additional work, the ATB model could be used to accurately simulate a variety of human falls and may be useful in further understanding the etiology and mechanisms of fall injuries such as hip fractures.

Contribution of limb momentum to power transfer in athletic wheelchair pushing

Pushing capacity is a key parameter in athletic racing wheelchair performance. This study estimated the potential contribution of upper limb momentum to pushing. The question is relevant since it may affect the training strategy adopted by an athlete. A muscle-free Lagrangian dynamic model of the upper limb segments was developed and theoretical predictions of power transfer to the wheelchair were computed during the push phase. Results show that limb momentum capacity for pushing can be in the order of 40J per push cycle at 10m/s, but it varies with the specific pushing range chosen by the athlete. Although use of momentum could certainly help an athlete improve performance, quantifying the actual contribution of limb momentum to pushing is not trivial. A preliminary experimental investigation on an ergometer, along with a simplified model of the upper limb, suggests that momentum is not the sole contributor to power transfer to a wheelchair. Muscles substantially contribute to pushing, even at high speeds. Moreover, an optimal pushing range is challenging to find since it most likely differs if an athlete chooses a limb momentum pushing strategy versus a muscular exertion pushing strategy, or both at the same time. The study emphasizes the importance of controlling pushing range, although one should optimize it while also taking the dynamics of the recovery period into account.