Reliability of individual acceleration-speed profile in-situ in elite youth soccer players.

The aims of this study were to describe differences in the acceleration-speed (A-S) profile in-situ and to assess the week-to-week reliability of the A-S profile in-situ over a given training cycle of elite youth soccer players, in relation to the number of sessions included and analyse the effect of the inclusion or not of a specific sprint session. In this retrospective study, 18 male elite U19 football players (179.4 ± 7.1 cm; 69.0 ± 9.5 kg) participated. GPS data collected from three consecutive typical training weeks were used to calculate different combinations of A-S profile in-situ variables (theoretical maximal acceleration [A0], theoretical maximal speed [S0] and the slope of the acceleration-speed [ASslope]). The number (and content) of sessions affected mainly S0 while A0 remained similar with or without a sprint session. The reliability of the A-S profile in-situ is more related to the spread of points rather than a specific number of sessions (and thus points) and was improved when a high percentage of maximum speed (i.e. ≥ 95%) was reached. The present study showed low week-to-week variability for A0, S0 and ASslope. However, practitioners need to make sure that the values cover a sufficient range of raw data [20-95% of maximum speed] to build a clear and consistent linear regression, and in turn extrapolate meaningful A-S profile values.

Effects of the foot strike pattern on muscle activity and neuromuscular fatigue in downhill trail running.

Minimizing musculo-skeletal damage and fatigue is considered paramount for performance in trail running. Our purposes were to investigate the effects of the foot strike pattern and its variability on (a) muscle activity during a downhill trail run and (b) immediate and delayed neuromuscular fatigue. Twenty-three runners performed a 6.5-km run (1264 m of negative elevation change). Electromyographic activity of lower-limb muscles was recorded continuously. Heel and metatarsal accelerations were recorded to identify the running technique. Peripheral and central fatigue was assessed in knee extensors (KE) and plantar flexors (PF) at Pre-, Post-, and 2 days post downhill run (Post2d). Anterior patterns were associated with (a) higher gastrocnemius lateralis activity and lower tibialis anterior and vastus lateralis activity during the run and (b) larger decreases in KE high-frequency stimulus-evoked torque Post and larger decrements in KE MVC Post2d. High patterns variability during the run was associated with (a) smaller decreases in KE Db100 Post and MVC Post2d and (b) smaller decreases in PF MVC Post and Post2d. Anterior patterns increase the severity of KE peripheral fatigue. However, high foot strike pattern variability during the run reduced acute and delayed neuromuscular fatigue in KE and PF.

How 100-m event analyses improve our understanding of world-class men's and women's sprint performance.

This study aimed to compare the force (F)-velocity (v)-power (P)-time (t) relationships of female and male world-class sprinters. A total of 100 distance-time curves (50 women and 50 men) were computed from international 100-m finals, to determine the acceleration and deceleration phases of each race: (a) mechanical variables describing the velocity, force, and power output; and (b) F-P-v relationships and associated maximal power output, theoretical force and velocity produced by each athlete (Pmax , F0 , and V0 ). The results showed that the maximal sprint velocity (Vmax ) and mean power output (W/kg) developed over the entire 100 m strongly influenced 100-m performance (r > -0.80; P ≤ 0.001). With the exception of mean force (N/kg) developed during the acceleration phase or during the entire 100 m, all of the mechanicals variables observed over the race were greater in men. Shorter acceleration and longer deceleration in women may explain both their lower Vmax and their greater decrease in velocity, and in turn their lower performance level, which can be explained by their higher V0 and its correlation with performance. This highlights the importance of the capability to keep applying horizontal force to the ground at high velocities.

Changes in running mechanics over 100-m, 200-m and 400-m treadmill sprints.

PURPOSE: Compare alterations in running mechanics during maximal treadmill sprints of different distances. METHODS: Eleven physically active males performed short (100-m), medium (200-m) and long (400-m) running sprints on an instrumented treadmill. Continuous measurement of running kinetics/kinematics and spring-mass characteristics were recorded and values subsequently averaged over every 50-m distance intervals for comparison. RESULTS: Compared with the initial 50m, running velocity decreased (P<0.001) by 8±2%, 20±4% and 39±7% at the end of the 100, 200 and 400-m, respectively. All sprint distances (except for step length in the 100-m) induced significantly longer (P<0.05) contact times (+7±4%, +22±8% and +36±13%) and lower step lengths (-1±4%, -5±5% and -41±2%) and frequencies (-6±3%, -13±7% and -22±8%) at the end of the 100-m, 200-m and 400-m, respectively. Larger reductions in ground reaction forces occurred in horizontal versus vertical direction, with greater changes with increasing sprinting distance (P<0.05). Similarly, the magnitude of decrement in vertical stiffness increased with sprint distance (P<0.05), while leg stiffness decreases were smaller and limited to 200-m and 400-m runs. Overall, we observed earlier and larger alterations for the 400-m compared with other distances. CONCLUSIONS: The magnitude of changes in running velocity and mechanics over short (100-m), medium (200-m) and long (400-m) treadmill sprints increases with sprint distance. The alterations in stride mechanics occur relatively earlier during the 400-m compared with the 100-m and 200-m runs.

Acute and delayed peripheral and central neuromuscular alterations induced by a short and intense downhill trail run.

Downhill sections are highly strenuous likely contributing to the development of neuromuscular fatigue in trail running. Our purpose was to investigate the consequences of an intense downhill trail run (DTR) on peripheral and central neuromuscular fatigue at knee extensors (KE) and plantar flexors (PF). Twenty-three runners performed a 6.5-km DTR (1264-m altitude drop) as fast as possible. The electromyographic activity of vastus lateralis (VL) and gastrocnemius lateralis (GL) was continuously recorded. Neuromuscular functions were assessed Pre-, Post-, and 2-day Post-DTR (Post2d). Maximal voluntary torques decreased Post (∼ -19% for KE, ∼ -25% for PF) and Post2d (∼ -9% for KE, ∼ -10% for PF). Both central and peripheral dysfunctions were observed. Decreased KE and PF voluntary activation (VA), evoked forces, VL M-wave amplitude, and KE low-frequency fatigue were observed at Post. Changes in VL M-wave amplitude were negatively correlated to VL activity during DTR. Changes in PF twitch force and VA were negatively correlated to GL activity during DTR. The acute KE VA deficit was about a third of that reported after ultramarathons, although peripheral alterations were similar. The prolonged force loss seems to be mainly associated to VA deficit likely induced by the delayed inflammatory response to DTR-induced ultrastructural muscle damage.

Field monitoring of sprinting power-force-velocity profile before, during and after hamstring injury: two case reports.

Very little is currently known about the effects of acute hamstring injury on over-ground sprinting mechanics. The aim of this research was to describe changes in power-force-velocity properties of sprinting in two injury case studies related to hamstring strain management: Case 1: during a repeated sprint task (10 sprints of 40 m) when an injury occurred (5th sprint) in a professional rugby player; and Case 2: prior to (8 days) and after (33 days) an acute hamstring injury in a professional soccer player. A sports radar system was used to measure instantaneous velocity-time data, from which individual mechanical profiles were derived using a recently validated method based on a macroscopic biomechanical model. Variables of interest included: maximum theoretical velocity (V0) and horizontal force (F(H0)), slope of the force-velocity (F-v) relationship, maximal power, and split times over 5 and 20 m. For Case 1, during the injury sprint (sprint 5), there was a clear change in the F-v profile with a 14% greater value of F(H0) (7.6-8.7 N/kg) and a 6% decrease in V0 (10.1 to 9.5 m/s). For Case 2, at return to sport, the F-v profile clearly changed with a 20.5% lower value of F(H0) (8.3 vs. 6.6 N/kg) and no change in V0. The results suggest that the capability to produce horizontal force at low speed (F(H0)) (i.e. first metres of the acceleration phase) is altered both before and after return to sport from a hamstring injury in these two elite athletes with little or no change of maximal velocity capabilities (V0), as evidenced in on-field conditions. Practitioners should consider regularly monitoring horizontal force production during sprint running both from a performance and injury prevention perspective.

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

This 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 spatiotemporal 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.

Monitoring Locomotor Load in Soccer: Is Metabolic Power, Powerful?

The aim of the present study was to examine the validity and reliability of metabolic power (P) estimated from locomotor demands during soccer-specific drills. 14 highly-trained soccer players performed a soccer-specific circuit with the ball (3×1-min bouts, interspersed with 30-s passive recovery) on 2 different occasions. Locomotor activity was monitored with 4-Hz GPSs, while oxygen update (VO2) was collected with a portable gas analyzer. P was calculated using either net VO2 responses and traditional calorimetry principles (PVO2, W.kg(-1)) or locomotor demands (PGPS, W.kg(-1)). Distance covered into different speed, acceleration and P zones was recorded. While PGPS was 29±10% lower than PVO2 (d<- 3) during the exercise bouts, it was 85±7% lower (d<- 8) during recovery phases. The typical error between PGPS vs. PVO2 was moderate: 19.8%, 90% confidence limits: (18.4;21.6). The correlation between both estimates of P was small: 0.24 (0.14;0.33). Very large day-to-day variations were observed for acceleration, deceleration and > 20 W.kg(-1) distances (all CVs > 50%), while average Po2 and PGPS showed CVs < 10%. ICC ranged from very low- (acceleration and > 20 W.kg(-1) distances) to-very high (PVO2). PGPS largely underestimates the energy demands of soccer-specific drills, especially during the recovery phases. The poor reliability of PGPS >20 W.kg(-1) questions its value for monitoring purposes in soccer.

Sprint mechanics in world-class athletes: a new insight into the limits of human locomotion.

The objective of this study was to characterize the mechanics of maximal running sprint acceleration in high-level athletes. Four elite (100-m best time 9.95-10.29 s) and five sub-elite (10.40-10.60 s) sprinters performed seven sprints in overground conditions. A single virtual 40-m sprint was reconstructed and kinetics parameters were calculated for each step using a force platform system and video analyses. Anteroposterior force (FY), power (PY), and the ratio of the horizontal force component to the resultant (total) force (RF, which reflects the orientation of the resultant ground reaction force for each support phase) were computed as a function of velocity (V). FY-V, RF-V, and PY-V relationships were well described by significant linear (mean R(2) of 0.892 ± 0.049 and 0.950 ± 0.023) and quadratic (mean R(2) = 0.732 ± 0.114) models, respectively. The current study allows a better understanding of the mechanics of the sprint acceleration notably by modeling the relationships between the forward velocity and the main mechanical key variables of the sprint. As these findings partly concern world-class sprinters tested in overground conditions, they give new insights into some aspects of the biomechanical limits of human locomotion.

Effects of hamstring-emphasized neuromuscular training on strength and sprinting mechanics in football players.

The objective of this study was to examine the effects of a neuromuscular training program combining eccentric hamstring muscle strength, plyometrics, and free/resisted sprinting exercises on knee extensor/flexor muscle strength, sprinting performance, and horizontal mechanical properties of sprint running in football (soccer) players. Sixty footballers were randomly assigned to an experimental group (EG) or a control group (CG). Twenty-seven players completed the EG and 24 players the CG. Both groups performed regular football training while the EG performed also a neuromuscular training during a 7-week period. The EG showed a small increases in concentric quadriceps strength (ES = 0.38/0.58), a moderate to large increase in concentric (ES = 0.70/0.74) and eccentric (ES = 0.66/0.87) hamstring strength, and a small improvement in 5-m sprint performance (ES = 0.32). By contrast, the CG presented lower magnitude changes in quadriceps (ES = 0.04/0.29) and hamstring (ES = 0.27/0.34) concentric muscle strength and no changes in hamstring eccentric muscle strength (ES = -0.02/0.11). Thus, in contrast to the CG (ES = -0.27/0.14), the EG showed an almost certain increase in the hamstring/quadriceps strength functional ratio (ES = 0.32/0.75). Moreover, the CG showed small magnitude impairments in sprinting performance (ES = -0.35/-0.11). Horizontal mechanical properties of sprint running remained typically unchanged in both groups. These results indicate that a neuromuscular training program can induce positive hamstring strength and maintain sprinting performance, which might help in preventing hamstring strains in football players.

Traditional and ankle-specific vertical jumps as strength-power indicators for maximal sprint acceleration.

AIM: This study aimed to determine the demand of strength-power capabilities represented by traditional and ankle-specific vertical jump modalities ­ squat jump (SJ), counter-movement jump (CMJ), rebound-continuous jump (RJ), rebound-continuous ankle jump (AJ) ­ relative to sprint acceleration ability during the entire acceleration phase of maximal sprint. METHODS: Nineteen male sprinters performed a 60-m maximal sprint and various vertical jumps. Correlation coefficients among the vertical jump performances and between those and the 60-m sprint time and sprint acceleration at each step were calculated. RESULTS: There were significant relationships between the 60-m sprint time and SJ height, CMJ height, AJ height, and AJ index. AJ height and index had no correlation with any other jump variables. Acceleration was significantly correlated with SJ height from the 6th to the 10th steps (r=0.48-0.51) and with CMJ height from the 5th to the 11th steps (r=0.46-0.54). Acceleration was also correlated with the AJ index from the 14th to the 19th steps (r=0.48-0.54). Acceleration had no correlation with the RJ index at any step. CONCLUSION: The results suggest that the AJ allows assessment of different reactive strengths compared with traditional jump modalities. To accelerate effectively, the explosive strengths of the SJ and CMJ are important during the early stage of acceleration (from 6.6±0.4 to 17.5±0.8 m), and the reactive strength represented by the AJ is necessary during the later stage of acceleration (from 23.4±1.0 to 33.7±1.4 m). Sprinters and coaches should be aware of the different demands of strength-power capability for effective acceleration.

Association of acceleration with spatiotemporal variables in maximal sprinting.

This study clarified the association between acceleration and the rates of changes in spatiotemporal variables on a step-to-step basis during the entire acceleration phase of maximal sprinting. 21 male sprinters performed a 60-m sprint, during which step-to-step acceleration and rates of changes in step length (RSL) and step frequency (RSF) were calculated. The coefficients of correlation between acceleration and other variables were tested at each step. There were positive correlations between acceleration and the RSF up to the second step. Acceleration was positively correlated with the RSL from the 5(th) to the 19(th) step. At the third and from the 16(th) to the 22(nd) step and from the 20(th) to the 21(st) step, there was no significant correlation, but weak relationships were found between acceleration and the RSF and RSL. The results suggest that the acceleration phase can be divided into 3 sections, and for sprinting to be effective, it is important to accelerate by increasing the step frequency to the third step, increasing the step length from the 5(th) to the 15(th) step, and increasing the step length or frequency (no systematic relative importance of step length or frequency) from the 16(th) step in the entire acceleration phase.

Progression of mechanical properties during on-field sprint running after returning to sports from a hamstring muscle injury in soccer players.

The objectives of this study were to examine the consequences of an acute hamstring injury on performance and mechanical properties of sprint-running at the time of returning to sports and after the subsequent ~2 months of regular soccer training after return. 28 semi-professional male soccer players, 14 with a recent history of unilateral hamstring injury and 14 without prior injury, participated in the study. All players performed two 50-m maximal sprints when cleared to return to play (Test 1), and 11 injured players performed the same sprint test about 2 months after returning to play (Test 2). Sprint performance (i. e., speed) was measured via a radar gun and used to derive linear horizontal force-velocity relationships from which the following variables obtained: theoretical maximal velocity (V(0)), horizontal force (F(H0)) and horizontal power (Pmax). Upon returning to sports the injured players were moderately slower compared to the uninjured players. F H0 and Pmax were also substantially lower in the injured players. At Test 2, the injured players showed a very likely increase in F(H0) and Pmax concomitant with improvements in early acceleration performance. Practitioners should consider assessing and training horizontal force production during sprint running after acute hamstring injuries in soccer players before they return to sports.

Locomotor performance in highly-trained young soccer players: does body size always matter?

To examine the effects of body size on locomotor performance, 807 15-year-old French and 64 Qatari soccer players participated in the present study. They performed a 40-m sprint and an incremental running test to assess maximal sprinting (MSS) and aerobic speeds, respectively. French players were advanced in maturity, taller, heavier, faster and fitter than their Qatari counterparts (e.g., Cohen's d=+1.3 and + 0.5 for body mass and MSS). However, when adjusted for body mass (BM), Qatari players had possibly greater MSS than French players (d=+0.2). A relative age effect was observed within both countries, with the players born in the first quarter of the year being taller, heavier and faster that those born during the fourth quarter (e.g., d=+0.2 for MSS in French players). When directly adjusted for BM, these MSS differences remained (d=+0.2). Finally, in both countries, players selected in National teams were taller, heavier, faster and fitter than their non-selected counterparts (e.g., d=+0.6 for MSS in French players), even after adjustments for body size (d=+0.5). Differences in locomotor performances between players with different phenotypes are likely mediated by differences in body size. However, when considering more homogeneous player groups, body dimensions are unlikely to substantially explain the superior locomotor performances of older and/or international players.

Force-velocity profile: imbalance determination and effect on lower limb ballistic performance.

This study sought to lend experimental support to the theoretical influence of force-velocity (F-v) mechanical profile on jumping performance independently from the effect of maximal power output (P max ). 48 high-level athletes (soccer players, sprinters, rugby players) performed maximal squat jumps with additional loads from 0 to 100% of body mass. During each jump, mean force, velocity and power output were obtained using a simple computation method based on flight time, and then used to determine individual linear F-v relationships and P max values. Actual and optimal F-v profiles were computed for each subject to quantify mechanical F-v imbalance. A multiple regression analysis showed, with a high-adjustment quality (r²=0.931, P<0.001, SEE=0.015 m), significant contributions of P max , F-v imbalance and lower limb extension range (h PO ) to explain interindividual differences in jumping performance (P<0.001) with positive regression coefficients for P max and h PO and a negative one for F-v imbalance. This experimentally supports that ballistic performance depends, in addition to P max , on the F-v profile of lower limbs. This adds support to the actual existence of an individual optimal F-v profile that maximizes jumping performance, a F-v imbalance being associated to a lower performance. These results have potential strong applications in the field of strength and conditioning.

Injuries in Youth and National Combined Events Championships.

In major track and field competitions, the most risky discipline is the combined event. Therefore, we aimed to record and analyze the incidence and characteristics of sports injuries incurred during the Youth and National Combined Events Championships. During the French Athletics Combined Events Championships in 2010, all newly occurred injuries were prospectively recorded by the local organising committee of physicians and physiotherapists working in the medical centres at the stadium, in order to determine incidence and characteristics of newly occurred injuries. In total, 51 injuries and 9 time-loss injuries were reported among 107 registered athletes, resulting in an incidence of 477 injuries and 84 time-loss injuries per 1,000 registered athletes. Approximately 72% of injuries affected lower limbs and 60% were caused by overuse. Thigh strain (17.6%) was the most common diagnosis. 14 dropouts were recorded, 8 were caused by an injury (57.1%). During the National and Youth Combined Events Championships, over one third of the registered athletes incurred an injury, with an injury incidence higher than in international elite track and field competitions. Interestingly, this higher injury risk concerned the younger population affecting immature musculoskeletal structures. In combined events, preventive interventions should mainly focus on overuse and thigh injuries.