Sex Differences in Endurance Running.

In recent years, there has been a significant expansion in female participation in endurance (road and trail) running. The often reported sex differences in maximal oxygen uptake (VO2max) are not the only differences between sexes during prolonged running. The aim of this narrative review was thus to discuss sex differences in running biomechanics, economy (both in fatigue and non-fatigue conditions), substrate utilization, muscle tissue characteristics (including ultrastructural muscle damage), neuromuscular fatigue, thermoregulation and pacing strategies. Although males and females do not differ in terms of running economy or endurance (i.e. percentage VO2max sustained), sex-specificities exist in running biomechanics (e.g. females have greater non-sagittal hip and knee joint motion compared to males) that can be partly explained by anatomical (e.g. wider pelvis, larger femur-tibia angle, shorter lower limb length relative to total height in females) differences. Compared to males, females also show greater proportional area of type I fibres, are more able to use fatty acids and preserve carbohydrates during prolonged exercise, demonstrate a more even pacing strategy and less fatigue following endurance running exercise. These differences confer an advantage to females in ultra-endurance performance, but other factors (e.g. lower O2 carrying capacity, greater body fat percentage) counterbalance these potential advantages, making females outperforming males a rare exception. The present literature review also highlights the lack of sex comparison in studies investigating running biomechanics in fatigue conditions and during the recovery process.

Predicting cumulative load during running using field-based measures.

The main objective was to investigate whether the cumulative load of the lower limbs, defined as the product of external load and step rate, could be predicted using spatiotemporal variables gathered with a commercially available wearable device in running. Therefore, thirty-nine runners performed two running tests at 10 and 12 km/h, respectively. Spatiotemporal variables (step rate, ground contact time, and vertical oscillation) were collected using a commercially available wearable device. Kinetic variables, measured with gold standard equipment (motion capture system and instrumented treadmill) and used for the calculation of a set of variables representing cumulative load, were peak vertical ground reaction force (peak vGRF), vertical instantaneous loading rate (VILR), vertical impulse, braking impulse, as well as peak extension moments and angular impulses of the ankle, knee and hip joints. Separate linear mixed-effects models were built to investigate the prediction performance of the spatiotemporal variables for each measure of cumulative load. BMI, speed, and sex were included as covariates. Predictive precision of the models ranged from .11 to .66 (R2 m ) and .22 to .98 (R2 c ), respectively. Greatest predictive performance was obtained for the cumulative peak vGRF (R2 m  = .66, R2 c  = .97), VILR (R2 m  = .43, R2 c  = .97), braking impulse (R2 m  = .52, R2 c  = .98), and peak hip extension moment (R2 m  = .54, R2 c  = .90). In conclusion, certain variables representing cumulative load of the lower limbs in running can be predicted using spatiotemporal variables gathered with a commercially available wearable device.

Maturation-related changes in the development and etiology of neuromuscular fatigue.

PURPOSE: The aim of the present study was to investigate the role of maturation on the etiology of neuromuscular fatigue induced by repeated maximal voluntary isometric contractions (MVIC). METHODS: Nine prepubertal boys (9.9 ± 1.3 years), eight male adolescents (13.6 ± 1.3 years) and eleven men (23.4 ± 3.0 years) performed a series of repeated isometric MVICs of the knee extensors until the MVIC torque reached 60% of its initial value. Magnetic stimulations were delivered to the femoral nerve every five MVICs to follow the course of voluntary activation level (VA) and the potentiated twitch torque (Qtwpot). RESULTS: Task failure was reached after 52.9 ± 12.7, 42.6 ± 12.5, and 26.6 ± 6.3 repetitions in boys, adolescents and men, respectively. VA remained unchanged in men whereas it decreased significantly and similarly in boys and adolescents (p < 0.001). In contrast, Qtwpot remained unchanged in boys and decreased significantly less in adolescents than adults (p < 0.05). CONCLUSIONS: Children and adolescents experience less peripheral and more central fatigue than adults. However, adolescents experience more peripheral fatigue than children for a comparable amount of central fatigue. This finding supports the idea that the tolerance of the central nervous system to peripheral fatigue could increase during maturation.

Influence of shoe drop on running kinematics and kinetics in female runners.

This study investigated the effects of shoe drop on lower limb kinematics and kinetics in female runners. Fifteen healthy female runners ran on a 15-m runway at their preferred speed with three different shoe-drop conditions: 0 (D0), 6 (D6) and 10 (D10) mm. Three-dimensional marker positions and ground reaction forces were recorded to analyse kinetic and kinematic parameters using zero- (0D) and one-dimensional (1D) metrics (statistical parametric mapping, SPM). Regarding 0D parameters, significantly higher loading rates and transient peaks were found in D0 compared to D6 and D10 conditions (both p < .01). For 1D analysis, significantly higher ankle dorsiflexion moments were found in D0 compared to D6 and D10 during the braking phase (p < .01). Lower knee extension moments between 52% and 55% and 61% and 65% of contact time (p < .05) were also found. No difference was found between D6 and D10 conditions (p > .05). As previously shown in men, this study demonstrates that shoe drop influences running kinematic and kinetic patterns. Using SPM analysis in conjunction with classical analysis, the study adds new understanding on the influence of shoes on joint moment during contact time.

Neuromechanical adaptations to slippery sport shoes.

Although shoe friction has been widely studied in occupational ergonomics, information was lacking about friction in sport shoes. The purpose of the study was to examine the neuromechanical adaptations to different shoe-surface interface in an aerobic-gym specific movement. Sixteen females performed 10 change of direction movements in two shoe conditions differing by their outsoles (ethyl-vinyl-acetate: EVA and rubber: RB) to ensure significant differences in mechanical coefficients of friction (EVA = 0.73 ±â€¯0.07 and RB = 1.46 ±â€¯0.15). The kinematics, kinetics and muscle activities of the right lower-limb were analysed. Statistical parametric mapping was used to investigate the kinematics and kinetics adaptation to the different shoe-surface coefficients of friction. The participants had a longer stance duration in the EVA compared to the RB condition (526 ±â€¯160 ms vs. 430 ±â€¯151 ms, p < .001). The ankle and knee joints powers and works were lower during both the braking and the push-off phases in the EVA as compared to the RB condition. Preactivation of the agonist muscles (soleus, gastrocnemius medialis and vastus medialis) decreased in the EVA compared to the RB condition (-28.5%, -26.5% and -49.0%, respectively). Performing a change of direction movement with slippery shoes reduced the ankle and knee joints loadings, but impaired the stretch-shortening cycle performance. Participants demonstrated thus a different neuromechanical strategy to control their movement which was associated with a reduced performance.

Perceiving slipperiness and grip: A meaningful relationship of the shoe-ground interface.

The present study investigated the relationship between objective measurements of the available (CoFA), the utilized (CoFU) coefficient of friction and subjective perception of grip or slipperiness. It was hypothesized that significant correlations exist between the perception of grip or slip and the CoF during sports movement and that a minimum CoF was needed to ensure an optimal grip/slipperiness perception. Eighteen healthy active females performed forward and backward cutting tasks onto a forceplate. Six shoes and two floors were used to induce different grip conditions. Subjective ratings and CoFU were assessed for each shoe-floor combination, and mechanical CoFA was also measured in a specific test bed. Significant relationships (p<0.001) were found between grip, slipperiness ratings or CoFA with the CoFU (r=0.98, r=-0.97, r=0.88, respectively). Individual sensory thresholds of the minimum required CoFU were also determined using probit models between the CoFU and the grip acceptability. The mean threshold defined in the present study was 0.70±0.11. This meant that below this threshold, the grip perception was not acceptable, whereas above this threshold, the grip was felt good enough to perform the task. In conclusion, strong relationships between subjective perceptions and objective measurements of friction were found in sports-like movements. Moreover, a minimum friction requirement was defined for indoor dry shoe-floor conditions. The present study gives new insights of the shoe-floor interaction and outlines friction requirements for the manufacturers of sports floor or footwear.

Effect of muscle length on voluntary activation of the plantar flexors in boys and men.

PURPOSE: The aim of the present study was to compare the effect of muscle length on the maximal voluntary activation level (VA) of the plantar-flexors between children and adults. METHODS: Fourteen boys (10.0 ± 1.0 years) and fifteen men (24.6 ± 4.2 years) performed 5-s maximal isometric voluntary contractions (MVC) of the plantar-flexor muscles at seven ankle angles [from 10° in dorsi-flexion (DF) to 20° in plantar-flexion (PF); 0° = reference position; the angle between the plantar surface and leg is a right angle]. Single magnetic stimulations were delivered to the posterior tibial nerve during MVCs to determine VA. RESULTS: Results showed a higher absolute torque of the plantar-flexor muscles at long (10° DF) than at short muscle length (20° PF) in men (89.4 ± 19.4 vs. 46.8 ± 17.0 N m, P < 0.001) and boys (44.9 ± 18.5 vs. 26.6 ± 12.8 N m, P < 0.001). On average, VA was significantly higher in men than in boys (92.4 ± 1.7 vs. 87.6 ± 1.6 %, P < 0.05). However, no significant main effect of the ankle angle was observed on VA. CONCLUSIONS: The VA partly accounts for the plantar-flexors MVC torque difference between children and adults but is not affected by the muscle length changes in both groups. Therefore, VA cannot account for the shape of the torque-angle relationship on the plantar-flexor muscles.

Perception of Affordance during Short-Term Exposure to Weightlessness in Parabolic Flight.

We investigated the role of the visual eye-height (VEH) in the perception of affordance during short-term exposure to weightlessness. Sixteen participants were tested during parabolic flight (0g) and on the ground (1g). Participants looked at a laptop showing a room in which a doorway-like aperture was presented. They were asked to adjust the opening of the virtual doorway until it was perceived to be just wide enough to pass through (i.e., the critical aperture). We manipulated VEH by raising the level of the floor in the visual room by 25 cm. The results showed effects of VEH and of gravity on the perceived critical aperture. When VEH was reduced (i.e., when the floor was raised), the critical aperture diminished, suggesting that widths relative to the body were perceived to be larger. The critical aperture was also lower in 0g, for a given VEH, suggesting that participants perceived apertures to be wider or themselves to be smaller in weightlessness, as compared to normal gravity. However, weightlessness also had an effect on the subjective level of the eyes projected into the visual scene. Thus, setting the critical aperture as a fixed percentage of the subjective visual eye-height remains a viable hypothesis to explain how human observers judge visual scenes in terms of potential for action or "affordances".

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

In 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.

Influence of exercise type on metabolic cost and gross efficiency: elliptical trainer versus cycling trainer.

BACKGROUND: Elliptical trainers are known as a good mean to develop physical fitness. However, the pedaling efficiency on an elliptical trainer has not been reported in the literature. The aim of the present study was to compare metabolic cost and gross efficiency for two different trainers - the elliptical trainer (ET) and the cycling trainer (CT). METHODS: Fourteen participants were tested on ET and CT during two exercise sessions. Participants pedaled at 9 different power outputs for 3 minutes each. Oxygen consumption (VO2) and heart rate (HR) were recorded. Gross efficiency (GE) was calculated during the last 30s of each 3min period. Maximal aerobic power (MAP) was estimated for each participant for each condition. RESULTS: MAP was found to be significantly greater in CT (237±88W) compared to ET (151±51 W). Significant positive correlations were found between power output and VO2 in both CT (r=0.93) and ET conditions (r=0.97). Regarding the inter-individual variability in MAP, GE was significantly correlated to the relative power output (%MAP) (r=0.75 in CT and r=0.69 in ET). CONCLUSIONS: The aim of the present study was to investigate metabolic demand of different exercise type using %MAP in each condition. The results confirmed that metabolic cost of ET was greater than CT at similar %MAP. Gross efficiency was lowered in ET condition compared to CT. This could be explained through the additional use of arms and the standing position during ET.

Insights into the Mechanisms of Neuromuscular Fatigue in Boys and Men.

PURPOSE: The aim of the present study was to investigate the role of central and peripheral factors in neuromuscular fatigue induced by repeated maximal contractions in children and adults. METHODS: Eleven boys (9.9 ± 1.2 yr) and 12 men (23.9 ± 3.5 yr) completed a fatigue protocol consisting in a repetition of 5-s maximal isometric voluntary contractions (MVC) of the knee extensors separated by 5-s passive recovery periods until the generated torque reached 60% of its initial value. Single magnetic stimulations were delivered to the femoral nerve every five MVC to follow the course of voluntary activation level and the amplitude of the potentiated twitch torque (Qtw(pot)) and vastus lateralis and rectus femoris concomitant M-waves (Mmax). RESULTS: Torque reached 60% of initial value after 49.5 ± 16.8 and 34.0 ± 19.6 repetitions in boys and men, respectively (P < 0.05). Furthermore, men showed significantly higher knee extensor MVC decline than boys between 50% and 90% of total repetitions (P < 0.05). Voluntary activation remained unchanged in men, whereas it decreased significantly in boys (P < 0.05). In contrast, whereas Qtw(pot) remained unchanged in boys, Qtw(pot) decreased progressively up to 60% of total repetitions in men (P < 0.001). Finally, Mmax remained unchanged for vastus lateralis and rectus femoris muscles in both groups. CONCLUSIONS: Children experienced no apparent peripheral fatigue and higher central fatigue than adults. The greater fatigue resistance in children could be related to a strategy of the CNS aimed at limiting the recruitment of motor units to prevent any extensive peripheral fatigue.

Effect of muscle length on voluntary activation level in children and adults.

PURPOSE: The aim of the present study was to compare the effect of muscle length on the level of voluntary activation (VA) at short and long muscle lengths between children and adults. METHODS: Thirteen prepubertal boys (10.2 ± 1.1 yr) and 10 men (23.9 ± 2.9 yr) performed 5-s maximal isometric voluntary contractions of the knee extensor muscles at three muscular angles (20°, 90°, 100°; 0°, full extension) interspersed with at least 60-s passive recovery periods. Single magnetic stimulations were delivered to the femoral nerve during maximal isometric voluntary contractions to determine the level of VA using the twitch interpolation technique. The specific torque was calculated as the absolute torque divided by thigh muscle mass, as assessed using dual-energy x-ray absorptiometry. Finally, the theoretical specific torque that could be produced with a complete (i.e., 100%) activation level (specific torque at 100% VA) was estimated from the values of specific torque and VA. RESULTS: Results showed a higher specific torque in adults at 90° and 100° but not at 20°. Accordingly, VA was significantly higher in adults at 90° (94% ± 4% vs 88% ± 8%, P < 0.05) and 100° (93% ± 6% vs 86% ± 8%, P < 0.05), whereas no significant difference was observed at 20°. Interestingly, the specific torque at 100% VA was not different between groups whatever the joint angle. CONCLUSIONS: The lower ability of children to fully activate their motor units at long muscle length could account for their lower specific torque because no difference in theoretical specific torque was observed between groups at 90° and 100°.

Postural mechanisms to control body displacements in the performance of lateral gaze shifts.

Medialateral postural control mechanisms (bodyweight distribution and center of pressure location) have been studied in static conditions. Our objective was to determine how these mechanisms are adjusted to perform voluntary movements, in our case 80° lateral gaze shifts at 0.125 Hz and 0.25 Hz. In healthy, young adults, we expected body marker (neck, lower back) and center of pressure displacements to be significantly greater in gaze shift conditions than in the stationary gaze condition. To explain these changes in center of pressure displacement, the amplitude contribution of both mechanisms was expected to increase significantly. All these results were found accordingly. Unexpectedly, the active contribution of the bodyweight distribution mechanism was negatively related to body marker displacements in the gaze shift conditions (ns in stationary condition). Moreover, changes in the contribution of the mechanisms were statistically weaker in effect size than changes in body displacement. However, the participants were not unstable because they performed the visual tasks as requested. We propose that the strength of medialateral postural control mechanisms may not only be strengthened to control challenging ML stance conditions but also slightly weakened to allow the performance of adequate body motions in ongoing tasks.

The electrically assisted bicycle: an alternative way to promote physical activity.

OBJECTIVE: The aim of this study was to analyze the physiologic demand of pedaling on an electrically assisted bicycle to test its potential as an alternative method to perform exercise. DESIGN: Twenty participants (ten trained vs. ten untrained in endurance) novice in the electrically assisted bicycle performed the same cycling protocol composed of nine 5-min submaximal cycling periods without electrical support (NO), with a light support (S1), or with a high support (S4). The cycling conditions were tested at two different imposed speeds (16 and 21 km·hr(-1)) and one freely chosen speed (mean [SD], 18.1 [0.7] km·h(-1)). RESULTS: Mean power output, intensity, and energy expenditure were significantly lowered by the use of an electrical support, in similar proportions between the groups. For all subjects, the intensity of the electrically assisted bicycle in all measured conditions (S1 or S4) was sufficiently high to achieve the standards for moderate-intensity health-enhancing physical activity for adults (>3 metabolic equivalents). However, a vigorous intensity (>6 metabolic equivalents) was reached with the NO and S1 supports in the untrained group whatever the cycling speed and only at 21 km·hr(-1) with the light support (S1) in the endurance-trained group. CONCLUSIONS: By reducing the perceived sensation of effort while maintaining a sufficiently high energy expenditure, the electrically assisted bicycle has a great potential to promote physical activity in industrialized societies.

Acute and 2 days delayed effects of exhaustive stretch-shortening cycle exercise on barefoot walking and running patterns.

This study investigated the acute and 2 days delayed influences of exhaustive stretch-shortening cycle exercise (SSC) on barefoot walking and running gait patterns. The SSC exercise was performed on a sledge apparatus, on which the subjects (N = 10) repeated until exhaustion intermittent series of 25 bilateral submaximal rebounds. Maximal drop-jumps and submaximal barefoot treadmill walking and running were performed before (PRE) and after (POST) the exhaustive exercise and repeated 48 h (D2) later. Electromyographic activity and 3D kinematics of the right lower limb and foot were recorded for 15 s at gait initiation (BEG) and at the end (END: at 3 min of walk and 5 min of run). The exhaustive SSC exercise resulted in 6% reductions in maximal drop jump performance at POST and D2, and affected mostly both gait patterns at D2. The walking pattern presented compensatory neural adjustments within the triceps surae muscle group. This expected pain-induced protective strategy of the soleus muscle was sufficient to preserve the kinematics pattern. The running condition revealed a major knee strategy, which might support the concept of pain protective strategy of knee extensor muscles at the expense of impact cushioning. Regardless the testing session, most parameters showed fatigue-induced changes at gait initiation (BEG), which were opposite to subsequent BEG to END adjustments. This is likely to support anticipatory strategies rather than progressive adjustments during the exercise.

Time course of neuro-mechanical changes underlying stretch-shortening cycle during intermittent exhaustive rebound exercise.

This study analysed the time course of neuro-mechanical changes underlying stretch-shortening cycle during intermittent exhaustive rebound exercise. On a sledge apparatus, ten subjects repeated until exhaustion a series of 30 unilateral submaximal rebounds, with intermediate 3-min rest periods. Rebound height, ground reaction force, 3D tibial acceleration and electromyographic activity of major lower limb muscles were recorded. A maximal drop jump test performed before and after the exhaustive exercise revealed a 10% drop in maximal stretch-shortening cycle (SSC) performance. Specific investigation of the neuro-mechanical changes along the exhaustive exercise included classical comparison of the first (BEG) and last (END) rebound series. From the initial accommodation phase, an optimized (OPTIM) series was individually determined as the first of at least two subsequent series with significantly shorter contact time than in the BEG series. The OPTIM series was reached after 3 ± 1 series, with associated increased lower limb stiffness during the braking phase and decreased muscle activities during the push-off. The major result was that the early (BEG-OPTIM) changes explained most of the BEG-END ones whereas the actual (OPTIM-END) fatigue effects remained quite limited. This confirmed our expectation that erroneous quantification of the SSC fatigue effects might be drawn when using the early beginning of rebound exercise on the sledge as a reference. Actual fatigue effects included medio-lateral instability as suggested by increased peroneus longus preactivation and medio-lateral tibial acceleration. The present methodology is thus considered as improving the distinction between SSC optimization and its deterioration with fatigue.

The influence of footwear on foot motion during walking and running.

There are evidences to suggest that wearing footwear constrains the natural barefoot motion during locomotion. Unlike prior studies that deduced foot motions from shoe sole displacement parameters, the aim of this study was to examine the effect of footwear motion on forefoot to rearfoot relative motion during walking and running. The use of a multi-segment foot model allowed accurate both shoe sole and foot motions (barefoot and shod) to be quantified. Two pairs of identical sandals with different midsole hardness were used. Ten healthy male subjects walked and ran in each of the shod condition. The results showed that for barefoot locomotion there was more eversion of the forefoot and it occurred faster than for shod locomotion. In this later condition, the range of eversion was reduced by 20% and the rate of eversion in late stance by 60% in comparison to the barefoot condition. The sole constrained both the torsional (eversion/inversion) and adduction range of motion of the foot. Interestingly, during the push-off phase of barefoot locomotion the rate and direction of forefoot torsion varied between individuals. However, most subjects displayed a forefoot inversion direction of motion while shod. Therefore, this experiment showed that the shoes not only restricted the natural motion of the barefoot but also appeared to impose a specific foot motion pattern on individuals during the push-off phase. These findings have implications for the matching of footwear design characteristics to individual natural foot function.