Jean-Benoı̂t Morin

Mechanical determinants of 100-m sprint running performance

Sprint 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 (Pxa0<xa00.01), as was the theoretical maximal velocity production capability (Pxa0<xa00.011), whereas the theoretical maximal force production capability was not. The ability to apply the resultant force backward during acceleration was positively correlated to 100-m performance (rsxa0>xa00.683; Pxa0<xa00.018), but the magnitude of resultant force was not (Pxa0=xa00.16). Step frequency, contact and swing time were significantly correlated to acceleration and 100-m performance (positively for the former, negatively for the two latter, all Pxa0<xa00.05), whereas aerial time and step length were not (all Pxa0>xa00.21). Last, anthropometric data of body mass index and lower-limb-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.

Changes in running mechanics and spring-mass behavior induced by a mountain ultra-marathon race

Changes in running mechanics and spring-mass behavior due to fatigue induced by a mountain ultra-marathon race (MUM, 166km, total positive and negative elevation of 9500m) were studied in 18 ultra-marathon runners. Mechanical measurements were undertaken pre- and 3h post-MUM at 12km h(-1) on a 7m long pressure walkway: contact (t(c)), aerial (t(a)) times, step frequency (f), and running velocity (v) were sampled and averaged over 5-8 steps. From these variables, spring-mass parameters of peak vertical ground reaction force (F(max)), vertical downward displacement of the center of mass (Δz), leg length change (ΔL), vertical (k(vert)) and leg (k(leg)) stiffness were computed. After the MUM, there was a significant increase in f (5.9±5.5%; P<0.001) associated with reduced t(a) (-18.5±17.4%; P<0.001) with no change in t(c), and a significant decrease in both Δz and F(max) (-11.6±10.5 and -6.3±7.3%, respectively; P<0.001). k(vert) increased by 5.6±11.7% (P=0.053), and k(leg) remained unchanged. These results show that 3h post-MUM, subjects ran with a reduced vertical oscillation of their spring-mass system. This is consistent with (i) previous studies concerning muscular structure/function impairment in running and (ii) the hypothesis that these changes in the running pattern could be associated with lower overall impact (especially during the braking phase) supported by the locomotor system at each step, potentially leading to reduced pain during running.

Do mechanical gait parameters explain the higher metabolic cost of walking in obese adolescents

Net metabolic cost of walking normalized by body mass (C(W.BM(-1)); in J.kg(-1).m(-1)) is greater in obese than in normal-weight individuals, and biomechanical differences could be responsible for this greater net metabolic cost. We hypothesized that, in obese individuals, greater mediolateral body center of mass (COM) displacement and lower recovery of mechanical energy could induce an increase in the external mechanical work required to lift and accelerate the COM and thus in net C(W.BM(-1)). Body composition and standing metabolic rate were measured in 23 obese and 10 normal-weight adolescents. Metabolic and mechanical energy costs were assessed while walking along an outdoor track at four speeds (0.75-1.50 m/s). Three-dimensional COM accelerations were measured by means of a tri-axial accelerometer and gyroscope and integrated twice to obtain COM velocities, displacements, and fluctuations in potential and kinetic energies. Last, external mechanical work (J.kg(-1).m(-1)), mediolateral COM displacement, and the mechanical energy recovery of the inverted pendulum were calculated. Net C(W.BM(-1)) was 25% higher in obese than in normal-weight subjects on average across speeds, and net C(W.BM(-67)) (J.kg(-0.67).m(-1)) was significantly related to percent body fat (r(2) = 0.46). However, recovery of mechanical energy and the external work performed (J.kg(-1).m(-1)) were similar in the two groups. The mediolateral displacement was greater in obese subjects and significantly related to percent body fat (r(2) = 0.64). The mediolateral COM displacement, likely due to greater step width, was significantly related to net C(W.BM(-67)) (r(2) = 0.49). In conclusion, we speculate that the greater net C(W.BM(-67)) in obese subjects may be partially explained by the greater step-to-step transition costs associated with wide gait during walking.

Direct measurement of power during one single sprint on treadmill

We tested the validity of an instrumented treadmill dynamometer for measuring maximal propulsive power during sprint running, and sought to verify whether this could be done over one single sprint, as shown during sprint cycling. The treadmill dynamometer modified towards sprint use (constant motor torque) allows vertical and horizontal forces to be measured at the same location as velocity, i.e. at the foot, which is novel compared to existing methods in which power is computed as the product of belt velocity and horizontal force measured by transducers placed in the tethering system. Twelve males performed 6s sprints against default, high and low loads set from the motor torque necessary to overcome the friction due to subjects weight on the belt (default load), and 20% higher and lower motor torque values. Horizontal ground reaction force, belt velocity, propulsive power and linear force-velocity relationships were compared between the default load condition and when taking all conditions together. Force and velocity traces and values were reproducible and consistent with the literature, and no significant difference was found between maximal power and force-velocity relationships obtained in the default load condition only vs. adding data from all conditions. The presented method allows one to measure maximal propulsive power and calculate linear force-velocity relationships from one single sprint data. The main novelties are that both force and velocity are measured at the same location, and that instantaneous values are averaged over one contact period, and not over a constant arbitrary time-window.

Running pattern changes depending on the level of subjects’ awareness of the measurements performed: A “sampling effect” in human locomotion experiments?

The aim of this study was to know whether subjects involved in a locomotion experiment modified their running pattern when made aware that data were being collected, and of the specific parameters studied. We used standardised actual and deceptive messages to induce increasing levels of awareness, and analysed subjects running pattern on a treadmill dynamometer, using lower limb stiffness as the key mechanical parameter. The five levels of awareness were: (1) subjects thought no sampling was performed, (2) they knew a sampling was about to take place, without knowing the parameter studied, (3) they knew this sampling was in progress, (4) they knew that sampling of lower limb stiffness was about to take place, and (5) they knew this sampling was in progress. Subjects running pattern significantly changed with the increasing level of information given, with a higher stiffness and step frequency, a reduced contact time and a lower change in leg length during contact. Post hoc tests showed that subjects changed their running pattern when knowing (i) that a sampling was performed and (ii) the mechanical parameter studied. These preliminary results suggest that experimental designs in locomotion protocols should take this potential sampling effect into account, whenever possible.

Effects of Extreme-Duration Heavy Load Carriage on Neuromuscular Function and Locomotion: A Military-Based Study

Trekking and military missions generally consist of carrying heavy loads for extreme durations. These factors have been separately shown to be sources of neuromuscular (NM) fatigue and locomotor alterations. However, the question of their combined effects remains unresolved, and addressing this issue required a representative context. Purpose The aim was to investigate the effects of extreme-duration heavy load carriage on NM function and walking characteristics. Methods Ten experienced infantrymen performed a 21-h simulated military mission (SMM) in a middle-mountain environment with equipment weighing ∼27 kg during battles and ∼43 kg during marches. NM function was evaluated for knee extensors (KE) and plantar flexors (PF) pre- and immediately post-SMM using isometric maximal voluntary contraction (MVC) measurement, neural electrical stimulation and surface EMG. The twitch-interpolation method was used to assess central fatigue. Peripheral changes were examined by stimulating the muscle in the relaxed state. The energy cost, mechanical work and spatio-temporal pattern of walking were also evaluated pre−/post-SMM on an instrumented treadmill in three equipment conditions: Sportswear, Battle and March. Results After the SMM, MVC declined by −10.2±3.6% for KE (P<0.01) and −10.7±16.1% for PF (Pu200a=u200a0.06). The origin of fatigue was essentially peripheral for both muscle groups. A trend toward low-frequency fatigue was detected for KE (5.5%, Pu200a=u200a0.08). These moderate NM alterations were concomitant with a large increase in perceived fatigue from pre- (rating of 8.3±2.2) to post-SMM (15.9±2.1, P<0.01). The SMM-related fatigue did not alter walking energetics or mechanics, and the different equipment carried on the treadmill did not interact with this fatigue either. Conclusion this study reports the first data on physiological and biomechanical consequences of extreme-duration heavy load carriage. Unexpectedly, NM function alterations due to the 21-h SMM were moderate and did not alter walking characteristics. Clinical Trial Registration Name: Effect of prolonged military exercises with high load carriage on neuromuscular fatigue and physiological/biomechanical responses. Number: NCT01127191.

Fatigue after short (100-m), medium (200-m) and long (400-m) treadmill sprints

The aim of this study was to compare the aetiology of neuromuscular fatigue following maximal sprints of different distances. It was hypothesized that increasing the distance would modify the type of peripheral and induce central fatigue. 11 subjects performed 100-, 200- and 400-m sprints on a motorized instrumented treadmill. Neuromuscular function, evaluated before (Pre), 30xa0s after (Post), and 5 and 30xa0min after the sprints (Post5 and Post30), consisted in determining maximal voluntary knee extensors torque (MVC), maximal voluntary activation of the knee extensors (%AL), maximal compound muscle action potential amplitude and duration on vastus lateralis, single twitch (Tw), and low- (Db10) and high-frequency torque. Compared with peak values, running speed decreased by 8%, (Pxa0<xa00.01), 20% (Pxa0<xa00.001) and 39% (Pxa0<xa00.001) at the end of the 100-, 200- and 400-m sprints, respectively. MVC was not altered following 100 and 200xa0m, but decreased by 14% (Pxa0<xa00.001) after the 400xa0m, was still depreciated Post5 (−11%, Pxa0<xa00.01) and went back to initial values Post30. A decrease in %AL (−6.0%, Pxa0<xa00.01) was observed Post5 for the 400xa0m. Tw, Db10 and low-to-high doublets ratio decreased Post-sprints and were not recovered Post30 after all sprints. Single maximal sprints of 100–400xa0m did not alter sarcolemmal excitability but induced progressive and substantial low-frequency fatigue and a slight reduction in neural drive with increasing sprint duration. Despite altered single or paired stimulations, MVC strength loss was detected only after the 400xa0m.

Last Word on Viewpoint: Sacrificing economy to improve running performance—a reality in the ultramarathon?

We fully agree that the number of runners at the elite level is lower in ultramarathons than shorter distances. There is also no doubt that ultramarathon running is not as universal as running of “normal” distances, i.e., up to the marathon. In particular, African runners are not represented.

SPORT-RELATED INJURIES DURING YOUTH AND NATIONAL COMBINED EVENTS CHAMPIONSHIPS

Background In International Association of Athletics Federation and European Athletics competitions, higher injury incidences were reported for athletes participating in combined events. Objective To record and analyze the incidence and characteristics of injuries incurred during Youth and National combined events Championships. Design Prospective recording of newly occurred injuries. Setting French Athletics Combined Events Championships in Athletics 2010 in Saint-Etienne, France. Participants Local Organising Committee (LOC) physicians and physiotherapists working in the Medical Centres at the stadium. Main outcome measures Incidence and characteristics of newly occurred injuries. Results A total of 51 injuries and 9 time-loss injuries were recorded among 107 registered athletes, corresponding in an incidence of 477 injuries and 84 time-loss injuries per 1000 registered athletes. 72.5% of injuries affected lower limbs. The most common diagnosis was thigh strain (17.6%), followed by trunk muscle cramps (11.8%), knee tendinopathy (9.8%), and ankle sprain (9.8%). Overuse injuries were the most common injury cause (60.8%), either with gradual (33.3%) and sudden onset (27.5%), followed by non-contact trauma (31.4%). Among the 14 dropouts recorded, about two third were caused by an injury (57.1%). Conclusions During combined events championships at the youth and national, over one third of the registered athletes incurred an injury. This represented a higher injury incidence than during international elite athletics competitions. Moreover, these injuries concerned a younger population and affecting immature musculoskeletal structures. In combined events, preventive interventions should mainly focus on overuse and thigh injuries.