Jean Paul Weber

Pilot study demonstrating that sole mechanosensitivity can be affected by insole use

Insoles are known to alter plantar loads and thus plantar sensory input. We therefore hypothesised that plantar somatosensory sensation could be modified over time by use of hard metatarsal pads. A sample of 12 healthy female participants was randomly allocated to either soft metatarsal pads (n=6, latex foam, Shore A11) or hard metatarsal pads groups (n = 6, thermoplastic, ShoreA65). All wore the same shoe type and pedometers measured daily activities. Using a bespoke actuated device, multiple mechanical stimuli were applied to the forefoot and rearfoot before and after 8 and 30 days of wearing the pads. A control test comprised estimation of multiple auditory sensations at day 0, 8 and 30. Changes in detection of the mechanical and sound stimuli were estimated using the Stevens power function, Ψ = k × Φ(n) (estimate = Ψ; stimulus = Φ). The k coefficient measured the sensitivity, i.e. the lowest detectable load/sound, and the n coefficient the gain in perception over time. After 30 days, hard metatarsal pads group had increased plantar sensitivity in the forefoot but not the rearfoot. The soft metatarsal pads group showed no changes in plantar sensitivity and the detection of auditory sensation remained stable over the 30 days.Metatarsal pads with relatively high hardness increased the perception of the lowest mechanical stimulus in the forefoot compared to soft metatarsal pads. This provides initial evidence of the potential for changes in plantar somatosensory sensation due to choice of orthotic designs in patients with foot-related problems.

Decreased foot inversion force and increased plantar surface after maximal incremental running exercise

Formulating the hypothesis that a maximal running exercise could induce fatigue of some foot muscles, we searched for electromyographic (EMG) signs of fatigue in the tibialis anterior (TA), peroneus longus (PL), and gastrocnemius medialis (GM) muscles. We also searched for post-exercise alterations of the stationary upright standing in normal-arched feet subjects. Healthy subjects performed a maximal running exercise. Surface EMGs of the TA, PL, and GM muscles were analysed during maximal dynamic efforts. Before and after the running bout, we measured the evoked compound muscle potential (M-wave) in TA, the maximal force into inversion (MIF), and the repartition of the plantar and barycentre surfaces with a computerised stationary platform. During maximal running exercise, the median frequency of the EMG spectra declined in TA while it remained stable in the PL and GM muscles. After the exercise, MIF decreased, and both the rearfoot plantar surface and the barycentre surface increased. We concluded that a maximal running bout elicits EMG signs of fatigue, though only in the TA muscle. It also elicits post-exercise changes in the foot position during stationary upright standing which indicates a foot eversion. These data solely concern a maximal running test and they can not be extrapolated to walking or running at a low speed.

Emergency braking is affected by the use of cruise control

ABSTRACT Objective: We compared the differences in the braking response to vehicle collision between an active human emergency braking (control condition) and cruise control (CC) or adaptive cruise control (ACC). Methods: In 11 male subjects, age 22 to 67 years, we measured the active emergency braking response during manual driving using the accelerator pedal (control condition) or in condition mimicking CC or ACC. In both conditions, we measured the brake reaction time (BRT), delay to produce the peak braking force (PBD), total emergency braking response (BRT + PBD), and peak braking force (PBF). Electromyograms of leg and thigh muscles were recorded during braking. The tonic vibratory response (TVR), Hoffman reflex (HR), and M-waves were recorded in leg muscles to explore the change in sensorimotor control. Results: No difference in PBF, TVR amplitude, HR latency, and Hmax/Mmax ratio were found between the control and CC/ACC conditions. On the other hand, BRT and PBD were significantly lengthened in the CC/ACC condition (240 ± 13 ms and 704 ± 70 ms, respectively) compared to control (183 ± 7 ms and 568 ± 36 ms, respectively). BRT increased with the age of participants and the driving experience shortened PBD and increased PBF. Conclusions: In male subjects, driving in a CC/ACC condition significantly delays the active emergency braking response to vehicle collision. This could result from higher amplitude of leg motion in the CC/ACC condition and/or by the age-related changes in motor control. Car and truck drivers must take account of the significant increase in the braking distance in a CC/ACC condition.

Consequences of simulated car driving at constant high speed on the sensorimotor control of leg muscles and the braking response

Due to the increase in time spent seated in cars, there is a risk of fatigue of the leg muscles which adjust the force exerted on the accelerator pedal. Any change in their sensorimotor control could lengthen the response to emergency braking. Fourteen healthy male subjects (mean age: 42 ± 4 years) were explored. Before and after a 1‐h driving trial at 120 km h−1, we measured the braking response, the maximal leg extension and foot inversion forces, the tonic vibratory response (TVR) in gastrocnemius medialis (GM) and tibialis anterior (TA) muscles to explore the myotatic reflex, and the Hoffmann reflex (H‐reflex). During driving, surface electromyograms (EMGs) of GM and TA were recorded and the ratio between high (H) and low (L) EMG energies allowed to evaluate the recruitment of high‐ and low‐frequency motor unit discharges. During driving, the H/L ratio decreased in TA, whereas modest and often no significant H/L changes occurred in GM muscle. After driving, the maximal foot inversion force decreased (−19%), while the leg extension force did not vary. Reduced TVR amplitude (−29%) was measured in TA, but no H‐reflex changes were noted. The braking reaction time was not modified after the driving trial. Driving at constant elevated speed reduced the myotatic reflex and the recruitment of motor units in TA muscle. The corresponding changes were rarely present in the GM muscle that plays a key role in the braking response, and this could explain the absence of a reduced braking reaction time.

Skin Hardness and Epidermal Thickness Affect the Vibration Sensitivity of the Foot Sole

Objective: The cutaneous mechanoreceptors of the foot sole detect the changes in the application of mechanical loads on the plantar surface during gait and standing, and contribute to controlling the standing balance and postural reflexes in healthy subjects. A local thickening of the foot sole skin occurs in response to repetitive load application. We hypothesized that an elevated skin hardness of the foot sole could reduce its mechano sensitivity. Methods: In healthy subjects, we quantified the sensation produced by different amplitudes of vibratory stimulations at two frequencies (25 and 150 Hz). The vibration threshold was determined on the 1st or 2nd, and 5th metatarsal heads, and the heel at each vibration frequency. The Stevens power function (Ψ=k.Φn) allowed to obtain regression equations between the estimate (Ψ) of the vibratory stimuli and their physical magnitude (Φ). Any increase in the absolute k value (all were negative) indicated a reduced sensitivity to the lowest loads. The n coefficient measured the global perception. The highest skin hardness (Shore) was measured on the 5th metatarsal head and the heel. In some subjects, superficial skin abrasion of the 5th metatarsal head was performed and the vibration sensitivity was tested again. Results: The vibration threshold was significantly higher at the level of the 5th metatarsal head and the heel. The k value was significantly higher at the 25 and 150 Hz frequencies for the 5th metatarsal head, and only at 25 Hz for the heel. At both vibration frequencies, negative correlations were obtained between the k values and skin hardness. After skin abrasion, the n coefficient was significantly higher at both vibration frequencies. Conclusion: Skin hardness affects the foot sole mechano sensitivity and could alter the control of posture during standing and walking. This indicates that foot care by podiatrist are relevant to improve posture control.

Psychophysical estimate of plantar vibration sensitivity brings additional information to the detection threshold in young and elderly subjects

Highlights • Psychophysical estimate of plantar vibration perception (n coefficient) was obtained.• In all subjects higher n values were measured at the 150 Hz vibration frequency.• n values bring complementary information of vibration detection threshold.

Consequences of Car Driving on Foot and Ankle Mobility and Reflexes

Car driving could induce fatigue and an altered sensorimotor control of foot muscles. Also, the use of a cruise controller (CC) or an adaptive cruise controller (ACC) could delay the brake reaction time when an emergency braking response is needed. The literature brings very few information on fatigue of the leg muscles during prolonged car driving and no data was found on any lengthened brake reaction time in CC/ACC condition. We recently showed that 1 hour driving at constant speed (120 or 60 km/hour) induced fatigue of the tibialis anterior (TA) muscle. TA fatigue was associated with a reduced myotatic reflex, a situation which reduced the sensorimotor control of muscles maintaining the foot on the accelerator pedal. Driving in CC/ACC condition increased the amplitude of leg displacement during emergency braking and markedly lengthened the brake reaction time, increasing the braking distance. The brake reaction time increased with age in the CC/ACC condition. Thus, car driving modifies the sensorimotor control of foot muscles and the use of new tools to control the speed of a motor vehicle significantly lengthens the brake reaction time. This could result from an increased amplitude of leg motion and/or an age-related decrease in reflex control.