Impact reduction during running: efficiency of simple acute interventions in recreational runners

International audienceRunning-related stress fractures have been associated with the overall impact intensity, which has recently been described through the loading rate (LR). Our purpose was to evaluate the effects of four acute interventions with specific focus on LR: wearing racing shoes (RACE), increasing step frequency by 10 % (FREQ), adopting a midfoot strike pattern (MIDFOOT) and combining these three interventions (COMBI). Nine rearfoot-strike subjects performed five 5-min trials during which running kinetics, kinematics and spring-mass behavior were measured for ten consecutive steps on an instrumented treadmill. Electromyographic activity of gastrocnemius lateralis, tibialis anterior, biceps femoris and vastus lateralis muscles was quantified over different phases of the stride cycle. LR was significantly and similarly reduced in MIDFOOT (37.4 ± 7.20 BW s(-1), -56.9 ± 50.0 %) and COMBI (36.8 ± 7.15 BW s(-1), -55.6 ± 29.2 %) conditions compared to NORM (56.3 ± 11.5 BW s(-1), both P<0.001). RACE (51.1 ± 9.81 BW s(-1)) and FREQ (52.7 ± 11.0 BW s(-1)) conditions had no significant effects on LR. Running with a midfoot strike pattern resulted in a significant increase in gastrocnemius lateralis pre-activation (208 ± 97.4 %, P<0.05) and in a significant decrease in tibialis anterior EMG activity (56.2 ± 15.5 %, P<0.05) averaged over the entire stride cycle. The acute attenuation of foot-ground impact seems to be mostly related to the use of a midfoot strike pattern and to a higher pre-activation of the gastrocnemius lateralis. Further studies are needed to test these results in prolonged running exercises and in the long term

A simple method for measuring power, force, velocity properties, and mechanical effectiveness in sprint running

International audienceThis study aimed to validate a simple field method for determining force– and power–velocity relationships and mechanical effectiveness of force application during sprint running. The proposed method, based on an inverse dynamic approach applied to the body center of mass, estimates the step-averaged ground reaction forces in runner's sagittal plane of motion during overground sprint acceleration from only anthropometric and spatio-temporal data. Force– and power–velocity relationships, the associated variables, and mechanical effectiveness were determined (a) on nine sprinters using both the proposed method and force plate measurements and (b) on six other sprinters using the proposed method during several consecutive trials to assess the inter-trial reliability. The low bias (<5%) and narrow limits of agreement between both methods for maximal horizontal force (638 ± 84 N), velocity (10.5 ± 0.74 m/s), and power output (1680 ± 280 W); for the slope of the force–velocity relationships ; and for the mechanical effectiveness of force application showed high concurrent validity of the proposed method. The low standard errors of measurements between trials (<5%) highlighted the high reliability of the method. These findings support the validity of the proposed simple method, convenient for field use, to determine power, force, velocity properties, and mechanical effectiveness in sprint running

Mechanical determinants of 100-m sprint running performance

International audienceSprint mechanics and field 100-m performances were tested in 13 subjects including 9 non-specialists, 3 French national-level sprinters and a world-class sprinter, to further study the mechanical factors associated with sprint performance. 6-s sprints performed on an instrumented treadmill allowed continuous recording of step kinematics, ground reaction forces (GRF), and belt velocity and computation of mechanical power output and linear force–velocity relationships. An index of the force application technique was computed as the slope of the linear relationship between the decrease in the ratio of horizontal-to-resultant GRF and the increase in velocity. Mechanical power output was positively correlated to mean 100-m speed (P0.683; P0.21). Last, anthropometric data of body mass index and lowerlimb- to-height ratio showed no significant correlation with 100-m performance. We concluded that the main mechanical determinants of 100-m performance were (1) a ‘‘velocity-oriented’’ force–velocity profile, likely explained by (2) a higher ability to apply the resultant GRF vector with a forward orientation over the acceleration, and (3) a higher step frequency resulting from a shorter contact time

Effect of the Fatigue Induced by a 110-km Ultramarathon on Tibial Impact Acceleration and Lower Leg Kinematics

Ultramarathon runners are exposed to a high number of impact shocks and to severe neuromuscular fatigue. Runners may manage mechanical stress and muscle fatigue by changing their running kinematics. Our purposes were to study (i) the effects of a 110-km mountain ultramarathon (MUM) on tibial shock acceleration and lower limb kinematics, and (ii) whether kinematic changes are modulated according to the severity of neuromuscular fatigue. Twenty-three runners participated in the study. Pre- and post-MUM, neuromuscular tests were performed to assess knee extensor (KE) and plantar flexor (PF) central and peripheral fatigue, and a treadmill running bouts was completed during which step frequency, peak acceleration, median frequency and impact frequency content were measured from tibial acceleration, as well as foot-to-treadmill, tibia-to-treadmill, and ankle flexion angles at initial contact, and ankle range of motion using video analysis. Large neuromuscular fatigue, including peripheral changes and deficits in voluntary activation, was observed in KE and PF. MVC decrements of ~35% for KE and of ~28% for PF were noted. Among biomechanical variables, step frequency increased by ~2.7% and the ankle range of motion decreased by ~4.1% post-MUM. Runners adopting a non rearfoot strike pre-MUM adopted a less plantarflexed foot strike pattern post-MUM while those adopting a rearfoot strike pre-MUM tended to adopt a less dorsiflexed foot strike pattern post-MUM. Positive correlations were observed between percent changes in peripheral PF fatigue and the ankle range of motion. Peripheral PF fatigue was also significantly correlated to both percent changes in step frequency and the ankle angle at contact. This study suggests that in a fatigued state, ultratrail runners use compensatory/protective adjustments leading to a flatter foot landing and this is done in a fatigue dose-dependent manner. This strategy may aim at minimizing the overall load applied to the musculoskeletal system, including impact shock and muscle stretch

CHANGES IN ROWING ERGOMETER TECHNIQUE AND POWER OUTPUT AFTER A SPECIFIC CORE TRAINING PROGRAM

The purpose of this study was to evaluate the changes in core stability, core strength, ergometer rowing technique and power output after a a specific core training program. 13 young rowers followed an 8-week training program consisting of 20min additional training realized 4 to 5 times a week composed of low load core stability, high load dynamic strength and ergometer specific exercises. After the program, rowers’ global core stability was improved (p=0.003), but not trunk maximal isometric strength. Ergometer rowing tests showed significant improvement of trunk technique (higher mean to peak power and work ratio, lower trunk negative power) and trunk power (+35W) at 20spm but only for trunk technique at a competitive stroke rate. This training program could be considered by the rowers and trainers to develop sport-specific core capacity to produce and transfer power

Sex differences in the rate of torque development and torque–velocity relationship are due to maximal strength only

Purpose: This study aimed to analyse the sex differences in the rate of torque development (RTD) and torque-velocity parameters with and without normalisation for maximal voluntary torque (MVT). Methods: Right-leg knee extensors were tested in 64 healthy and active participants (31 F and 33 M). MVT and RTD were obtained under isometric conditions. Individual torque-velocity relationships were obtained using a curvilinear model on averaged torque and velocity over 80°-to-140° knee angle. Dynamic data were acquired through an incremental protocol on a leg extension machine, going from the lightest to the unmovable load despite maximal effort. Results: Independent samples t test revealed (p &lt; 0.001) that males possess greater RTD measured at 50&nbsp;ms (d = -1.2), 100ms (d = -2.1) and 150ms (d = -2.3), peak RTD (d = -1.3) and MVT (d = 2.1). When normalised by MVT, the sex differences in RTD disappeared. Curvilinear hyperbolic TV relationship well-fitted (R2 = 0.99). In FV parameters, maximal theoretical torque (d = -1.7), maximal power (Pmax) (d = -2.0), and torque at Pmax (d = -1.7) were greater in males (p &lt; .05), while maximal theoretical velocity (V0) and velocity at Pmax did not differ. Conclusions: The sex differences in explosiveness (i.e., rapid isometric and dynamic force production) were mainly due to greater maximal strength in males than in females. These findings suggest that, in non-sedentary people, males do not present higher contraction velocity capacities, i.e. higher maximal velocity until which muscles can produce force, than females in knee extension

In-situ MMP-cadence relationship for 2-, 5- and 20-min duration: a proof of concept in U19 cyclists

Background. Power profiling has been very well studied with Mean Maximal Power (MMP) but the cadence at which power has been produced has never been taken into account. However, the power- cadence relationship defines that the power production is limited according to the rate. A maximal power (Pmax) can only be produced in optimal torque (Topt) and cadence (Copt) conditions. This study aimed to propose and evaluate a method to determine the MMP – cadence relationship for different typical exercise duration based from in-situ data. Methods. Fourteen under 19 national level cyclists participated in this study. A complete U19 season was analyzed and MMP was calculated for each cadence between 50 to 120 rpm for 2-, 5- and 20- minutes duration. the MMP-cadence relationship was fit with a second order polynomial function. Goodness of the fit (r2) and odd-even days absolute and relative reliability have been measured respectively for (Pmax), (Topt) and (Copt). Results. The goodness of the fit was very high for every duration (median r2 were 0.90, 0.89 and 0.72 for 2-, 5- and 20- minutes respectively). The relative reliability (ICC) and magnitude of the random error (SEM) was good to excellent for all parameters and durations (0.73 &lt; ICC &lt; 0.92; 2.5 &lt; SEM &lt; 8.2%). Discussion. The evaluation of a MMP – cadence relationship is feasible and reliable for 2, 5 and 20- min durations from in situ data. This profiling approach would allow to better detect the strengths and weaknesses of cyclists and to design more-effective training interventions

EFFECT OF FOOT STRIKE PATTERN ON AXIAL AND TRANSVERSE SHOCK SEVERITY DURING DOWNHILL TRAIL RUNNING

The purpose of the present study was to investigate the influence of foot strike pattern (FSP) on shock severity and attenuation during a downhill trail run. Twenty-three runners performed a 6.5-km downhill run (-1 264 m) equipped with four tri-axial accelerometers placed at tibia, sacrum, heel and metatarsals. FSP was identified using time difference between heel and metatarsals peak accelerations. Peak accelerations, median frequencies, and shock attenuation were calculated from tibial and sacral axial, anteroposterior and resultant accelerations over six sections during the run. Linear regressions analysis revealed that FSP affected differently the components of shock acceleration, i.e. although anterior FSPs enlarged shock severity along the tibial axial axis, they lowered shock severity along the tibial and sacral antero-posterior axis

Reconstruction with Voronoi Centered Radial Basis Functions

The dinosaur model is courtesy of Cyberware, other models being courtesy of the AIM@SHAPE shape repositoryWe consider the problem of reconstructing a surface from scattered points sampled on a physical shape. The sampled shape is approximated as the zero level set of a function. This function is defined as a linear combination of compactly supported radial basis functions. We depart from previous work by using as centers of basis functions a set of points located on an estimate of the medial axis, instead of the input data points. Those centers are selected among the vertices of the Voronoi diagram of the sample data points. Being a Voronoi vertex, each center is associated with a maximal empty ball. We use the radius of this ball to adapt the support of each radial basis function. Our method can fit a user-defined budget of centers: The selected subset of Voronoi vertices is filtered using the notion of lambda medial axis, then clustered to fit the allocated budget

The Rate of Torque Development as a Determinant of the Torque-Velocity Relationship

We investigate the contribution of isometric rate of torque development (RTD) and maximal voluntary torque (MVT) to the dynamic force production capacities of knee extensors obtained from the torque-velocity (TV) relationship, that is, the theoretical maximal velocity (V0), torque (T0), and maximal power (Pmax). Single-leg knee extensors were tested in 64 young adults (31 females). RTD and root mean square (RMS) of electromyographic signals from the knee extensors were recorded during isometric and incremental load dynamic (nonisokinetic) contractions. In the dynamic test, torque and velocity were continuously measured and averaged over 80°-140° knee angles to determine individual TV relationships. TV relationships were well fitted by hyperbolic regression (r2 from 0.983 to 0.993). Stepwise linear regressions showed that the main determinant of V0 was normalized RTD50 (R2 = 0.145, p = 0.004); the main determinant of T0 was MVT (R2 = 0.760, p &lt; 0.001); and the main determinant of Pmax was RTD150 (R2 = 0.612, p &lt; 0.001). V0 (when obtained from averaged values over knee extension) is partially explained by rapid torque capacity (i.e., "explosive strength"). Therefore, the capacity to produce torque at high velocity partly depends on the capacity to rise quickly the torque in the early phase of the contraction, suggesting that some underlying determinants of RFD would also affect V0