Acute effects of caffeine supplementation on kinematics and kinetics of sprinting.

We investigated the acute effects of caffeine supplementation (6 mgï½¥kg-1 ) on 60-m sprint performance and underlying components with a step-to-step ground reaction force measurement in 13 male sprinters. After the first round sprint as a control, caffeine supplementation-induced improvement in 60-m sprint times (7.811 s at the first versus 7.648 s at the second round, 2.05%) were greater compared with the placebo condition (7.769 s at the first versus 7.768 s at the second round, 0.02%). Using average values for every four steps, in the caffeine condition, higher running speed (all six step groups), higher step frequency (5th-16th and 21st-24th step groups), shorter support time (all the step groups except for 13th-16th step) and shorter braking time (9th-24th step groups) were found. Regarding ground reaction forces variables, greater braking mean force (13th-19th step group), propulsive mean force (1st-12th and 17th-20th step groups), and effective vertical mean force (9th-12th step group) were found in the caffeine condition. For the block clearance phase at the sprint start, push-off and reaction times did not change, while higher total anteroposterior mean force, average horizontal external power, and ratio of force were found in the caffeine condition. These results indicate that, compared with placebo, acute caffeine supplementation improved sprint performance regardless of sprint sections during the entire acceleration phase from the start through increases in step frequency with decreases in support time. Moreover, acute caffeine supplementation promoted increases in the propulsive mean force, resulting in the improvement of sprint performance.

Normative spatiotemporal and ground reaction force data for female and male sprinting.

This study aimed to demonstrate the differences in spatiotemporal and ground reaction force (GRF) variables during overground sprinting between performance levels for female and male sprinters with providing normative data during the entire acceleration phase. Forty-four female and 102 male sprinters performed 60-m sprints, during which the spatiotemporal and GRF variables were obtained using a long force platform system. Female and male sprinters were each allocated into four groups based on their maximal speed (7.5-9.5 m/s and 8.5-10.5 m/s, respectively) with 0.5 m/s intervals, and average values for 50-m distance were calculated. Using the GRF data, normative data for four groups of female and male sprinters were successfully obtained. For female sprinters using average values of all steps, there were differences between performance levels for step frequency (SF) and support time (ST), all impulses, and all mean forces. For male sprinters using average values of all steps, there were differences between performance levels for SF, ST and flight time, all impulses except for braking impulse, and all of the mean forces. The normative data indicate that most of the spatiotemporal and GRF variables may be changed, particularly increasing SF and propulsive force, when sprint performance is improved.

Comparison of kinematics and kinetics between unassisted and assisted maximum speed sprinting.

Producing comparable/greater ground reaction forces (GRFs) at faster running speeds is beneficial for sprint performance, and assisted sprint training is used to induce faster running speed conditions. This study aimed to demonstrate the characteristics of assisted sprinting at the maximal speed phase and investigate acute differences to control sprinting. Fifteen sprinters completed control and assisted (5 kg) sprints over force platforms. Assisted sprinting increased running speed (9.3% mean difference), while propulsive mean force (-4.3%) and impulse (-12.4%) decreased, suggesting that running speed improvements were caused primarily by assisted pulling force rather than improvements in anteroposterior force production of athletes. In addition, vertical mean force increased (4.2%), probably due to braking mean force (34.2%) and impulse (32.5%) increases. Magnitude of control trial maximum speed was achieved earlier (during acceleration) in assisted trials, and net anteroposterior (includes both braking and propulsive components) mean force (67.2%) and impulse (67.9%) increased at this matched speed, suggesting that assisted sprints could be used to practice producing greater GRFs at comparable speeds. Running speed improvement by pulling force was associated with contact time decreases (r = -.565), suggesting that shortening contact time may be important for effective assisted sprinting.

Lower-limb wearable resistance overloads joint angular velocity during early acceleration sprint running.

Lower-limb wearable resistance (WR) facilitates targeted resistance-based training during sports-specific movement tasks. The purpose of this study was to determine the effect of two different WR placements (thigh and shank) on joint kinematics during the acceleration phase of sprint running. Eighteen participants completed maximal effort sprints while unloaded and with 2% body mass thigh- or shank-placed WR. The main findings were as follows: 1) the increase to 10 m sprint time was small with thigh WR (effect size [ES] = 0.24), and with shank WR, the increase was also small but significant (ES = 0.33); 2) significant differences in peak joint angles between the unloaded and WR conditions were small (ES = 0.23-0.38), limited to the hip and knee joints, and <2° on average; 3) aside from peak hip flexion angles, no clear trends were observed in individual difference scores; and, 4) thigh and shank WR produced similar reductions in average hip flexion and extension angular velocities. The significant overload to hip flexion and extension velocity with both thigh- and shank-placed WR may be beneficial to target the flexion and extension actions associated with fast sprint running.

Optimal mechanical force-velocity profile for sprint acceleration performance.

The aim was to determine the respective influences of sprinting maximal power output ( P H max ) and mechanical Force-velocity (F-v) profile (ie, ratio between horizontal force production capacities at low and high velocities) on sprint acceleration performance. A macroscopic biomechanical model using an inverse dynamics approach applied to the athlete's center of mass during running acceleration was developed to express the time to cover a given distance as a mathematical function of P H max and F-v profile. Simulations showed that sprint acceleration performance depends mainly on P H max , but also on the F-v profile, with the existence of an individual optimal F-v profile corresponding, for a given P H max , to the best balance between force production capacities at low and high velocities. This individual optimal profile depends on P H max and sprint distance: the lower the sprint distance, the more the optimal F-v profile is oriented to force capabilities and vice versa. When applying this model to the data of 231 athletes from very different sports, differences between optimal and actual F-v profile were observed and depend more on the variability in the optimal F-v profile between sprint distances than on the interindividual variability in F-v profiles. For a given sprint distance, acceleration performance (<30 m) mainly depends on P H max and slightly on the difference between optimal and actual F-v profile, the weight of each variable changing with sprint distance. Sprint acceleration performance is determined by both maximization of the horizontal power output capabilities and the optimization of the mechanical F-v profile of sprint propulsion.

Relationships between kinematic characteristics and ratio of forces during initial sprint acceleration.

In track sprinting, acceleration performance is largely determined by the ability to generate a high ratio of forces (RF), but the technical features associated with this remain unknown. This study therefore investigated the relationships between selected kinematic characteristics and RF during the initial acceleration phase. Fourteen male sprinters completed two maximal 60 m sprints from a block start. Full-body kinematic and external kinetic data were obtained from the first four steps, and the relationships between selected kinematic characteristics and mean RF over the first four steps were determined. Placing the stance foot further behind (or less far in front of) the whole-body centre of mass at touchdown was significantly related to greater RF (r = -0.672), and more anterior orientation of the proximal end of the foot (r = -0.724) and shank (r = -0.764) segments at touchdown were also significantly related to greater RF. Following touchdown, greater ankle dorsiflexion range of motion during early stance was significantly related to greater RF (r = 0.728). When aiming to enhance RF during initial acceleration, practitioners should be encouraged to focus on lower leg configurations when manipulating touchdown distance, and the role of dorsiflexion during early stance is also an important consideration.

Alterations of spatiotemporal and ground reaction force variables during decelerated sprinting.

This study aimed to elucidate changes in spatiotemporal and ground reaction force (GRF) variables during 90-m overground decelerated sprinting and determinants of the decrease in running speed. In 14 sub-elite male sprinters, a virtual 90-m sprint was reconstructed during which spatiotemporal and GRF variables were averaged for four steps in maximal speed (45.8-m mark) and deceleration (76.5-m mark) phases. With decreases in running speed (3.5 ± 1.1%) from the maximal speed to deceleration phases, step frequency (SF) (3.5 ± 1.9%), net anteroposterior mean force (64.4 ± 15.9%), and propulsive and vertical mean forces during the propulsive phase (3.5 ± 3.8% and 5.3 ± 3.3%) decreased, and support (ST) (2.9 ± 2.5%) and flight times (FT) (4.3 ± 3.3%), braking mean force (7.3 ± 4.0%), and effective vertical impulse during the entire support (5.1 ± 3.4%) and braking phases (20.6 ± 11.2%) increased. In addition, the decrease in running speed was associated with changes in SF, ST, and net anteroposterior mean force (r = .667, -.713, and .524, respectively). The current results demonstrate that decreases in running speed during short-distance overground sprinting are probably caused by decreases in SF through increases in ST and FT, as well as impairment of the ability to minimize braking force and maintaining propulsive force. A compromised ability to maintain the magnitude of applied force during the propulsive phase and the necessity for lengthening FT may cause greater braking force, which increases effective vertical impulse during the braking and entire support phases. The SF, ST, and net anteroposterior mean force are determinants of the magnitudes of decreases in running speed during short-distance overground sprinting.

Kinetic and kinematic synchronization between blind and guide sprinters.

This study aimed to evaluate the magnitude of synchronization and symmetry between blind and guide sprinters. Elite and sub-elite pairs of male blind sprinters and guide sprinters performed maximal effort 60-m sprints, during which ground reaction force (GRF) for a 50-m distance and sprinting motion during the initial acceleration and maximal speed phases were measured. While there were no significant differences in spatiotemporal and GRF variables between the sprinters of the elite pair, flight time and braking, propulsive and vertical forces in the sub-elite pair showed a significant difference between the sprinters. During the initial acceleration phase, although thigh segment angles in the sagittal plane between the blind and guide sprinters showed obvious phase shifting (lag = -0.078 and -0.088) for the sub-elite pair, the elite pair showed no phase shift and high cross-correlation coefficients (0.96 and 0.83) for the corresponding variable. During the maximal speed phase, for both the elite and sub-elite pairs, there were trivial lags (-0.004 to 0.008) and high cross-correlation coefficients (>0.98) between the thighs of the blind and guide sprinters for both legs. The results demonstrate that a higher magnitude of synchronization between blind and guide sprinters is possibly important for better blind sprint performance.

Kinetic and kinematic determinants of female sprint performance.

This study elucidated spatiotemporal and ground reaction force determinants of running speed and acceleration for female sprinters during the entire sprinting. Fifteen female sprinters completed 60 m sprints. Kinematic and kinetic variables were measured using force platforms over a 50 m distance from the start. Results demonstrated that higher step frequency (11th-27th steps, r = 0.517-0.717) through shorter support time (12th-27th steps, r = -0.535 to -0.634) could be determinants of running speed. Moreover, increasing step length (1st-10th steps, r = 0.550-0.938), suppressing increases in step frequency (2nd-7th steps, r = -0.639 to -0.870), suppressing decreases in support time (1st-5th steps, r = 0.599-0.709) and increases in flight time (4th-7th steps, r = 0.523-0.649) can be essential for greater running acceleration. Propulsive mean force (1st-5th steps, r = 0.663-0.876) and anteroposterior net mean force (all steps, r = 0.697-0.894) are likely determinants of greater running acceleration. At the maximal speed phase there was no correlation between running speed and the other variables. Differences with previously found male sprint determinants suggest that training targets specific to female sprinters are necessary for improving training designs and race strategy.

Ground reaction forces during sprint hurdles.

This study aimed to demonstrate ground reaction forces (GRFs) during sprint hurdles and to clarify determinants of faster sprint hurdlers. Eleven male hurdlers performed 60-m sprint hurdle trials, clearing five hurdles, during which step-to-step spatiotemporal and GRF variables were measured. The preparatory step showed smaller braking and effective vertical impulses compared with the other steps, possibly lowering the centre of mass (CM). The greater braking and smaller propulsive impulses, which result in negative net anteroposterior impulse, were characteristics of the hurdle step. This deceleration may be due to producing a large elevation of CM for clearing the hurdle through large vertical GRF production. Compared with the other steps, the second greatest braking mean force and relatively small propulsive impulse, and large propulsive impulse through long propulsive time were shown at the landing and recovery steps, respectively. The results showed better sprint hurdle performance could be achieved by minimizing braking impulse through suppressing braking time, and increasing propulsive impulse through maximizing propulsive mean force at the hurdle step; suppressing braking and propulsive times at the landing step; minimizing propulsive time, increasing effective vertical mean force, and maximizing anteroposterior net mean force through increasing propulsive mean force at the recovery step.

Acceleration mechanics during forward and backward running: A comparison of step kinematics and kinetics over the first 20 m.

Backward running (BR) and forward running (FR) are unique movements utilized by athletes in many sports. Importantly, this investigation provides further insights on BR and benchmarking against more commonly researched FR capacity. Twenty-one collegiate soccer players (age 20.0 ± 0.8 years, body mass 65.6 ± 7.7 kg, body height 1.70 ± 0.07 m) performed maximal effort BR and FR along 20 m of in-ground force platforms. Step kinematics and kinetics were compared between BR and FR over four relative acceleration phases (BR = steps 1-6, 7-12, 13-18 and 19-23; FR = steps 1-4, 5-8, 9-12, 13-15). The primary findings of this study were that BR speeds were 29% slower than FR (p < 0.001), all step kinematics differed between BR and FR (p < 0.01), except contact time from the second to fourth step phases (p > 0.05), and most step kinetics were lower during BR (p < 0.05), with the exceptions of peak vertical force (p > 0.05). These findings indicate that lower running speeds over the acceleration phases of BR appear to be primarily due to lower horizontal ground reaction force application, resulting in shorter stride lengths and decreased flight times compared to FR.

Effects of forearm wearable resistance during accelerated sprints: From a standing start position.

Fusiform weighted garments enable specific loading strategies during sport-specific movements. Loading the arms over during accelerated sprinting from a 2-point start position is pertinent to a variety of sporting performances. Fourteen sprint-trained individuals (age = 20.61 ± 1.16 years; height = 1.73 m ± 3.85 cm; body mass 65.33 ± 4.86 kg; personal best 100-m race time 11.40 ± 0.39 s) performed unloaded/loaded wearable resistance (WR) sprints. Between-condition step kinematics and kinetics were compared over four acceleration phases: steps 1-4, 5-8, 9-12 and 13-16. Sprint performance did not differ between unloaded and loaded WR at 10-m (-1.41%; ES = -0.32), or 30-m (-0.76%; ES = -0.24). Sprinting with forearm WR significantly decreased step frequency during phase two (p < 0.05, -3.42%; ES = -0.81) and three (-3.60%; ES = -0.86) and step velocity during phase four of the 30 m sprinting task (p < 0.05, -3.61%; ES: 0.91) only. There were no significant differences (p ≤ 0.05) between step kinetics amongst the two conditions. Findings indicate that arm-loaded WR may provide specific sprinting overload for 2-point starting positions. This may be relevant to a wider sporting context such as field and team sport performances.

Waveform analysis of shank loaded wearable resistance during sprint running acceleration.

Lower-limb wearable resistance (WR) provides a specific and targeted overload to the musculature involved in sprint running, however, it is unknown if greater impact forces occur with the additional limb mass. This study compared the contact times and ground reaction force waveforms between sprint running with no load and 2% body mass (BM) shank-positioned WR over 30 m. Fifteen male university-level sprint specialists completed two maximum effort sprints with each condition in a randomized order. Sprint running with shank WR resulted in trivial changes to contact times at 5 m, 10 m, and 20 m (effect size [ES] = <0.20, p > 0.05) and a small, significant increase to contact time at 30 m by 1.94% (ES = 0.25, p = 0.03). Significant differences in ground reaction force between unloaded and shank loaded sprint running were limited to the anterior-posterior direction and occurred between 20% and 30% of ground contact at 10 m, 20 m, and 30 m. Shank WR did not result in greater magnitudes of horizontal or vertical forces during the initial impact portion of ground contact. Practitioners can prescribe shank WR training with loads ≤2% BM without concern for increased risk of injurious impact forces.

Changes to horizontal force-velocity and impulse measures during sprint running acceleration with thigh and shank wearable resistance.

This study determined the effects of two wearable resistance (WR) placements (i.e. thigh and shank) on horizontal force-velocity and impulse measures during sprint running acceleration. Eleven male athletes performed 50 m sprints either unloaded or with WR of 2% body mass attached to the thigh or shank. In-ground force platforms were used to measure ground reaction forces and determine dependent variables of interest. The main findings were: 1) increases in sprint times and reductions in maximum velocity were trivial to small when using thigh WR (0.00-1.93%) and small to moderate with shank WR (1.56-3.33%); 2) athletes maintained or significantly increased horizontal force-velocity mechanical variables with WR (effect size = 0.32-1.23), except for theoretical maximal velocity with thigh WR, and peak power, theoretical maximal velocity and maximal ratio of force with shank WR; 3) greater increases to braking and vertical impulses were observed with shank WR (2.72-26.3% compared to unloaded) than with thigh WR (2.17-12.1% compared to unloaded) when considering the entire acceleration phase; and, 4) no clear trends were observed in many of the individual responses. These findings highlight the velocity-specific nature of this resistance training method and provide insight into what mechanical components are overloaded by lower-limb WR.

Step-to-Step Kinematic Validation between an Inertial Measurement Unit (IMU) 3D System, a Combined Laser+IMU System and Force Plates during a 50 M Sprint in a Cohort of Sprinters.

The purpose was to compare step-by-step kinematics measured using force plates (criterion), an IMU only and a combined laser IMU system in well-trained sprinters. Fourteen male experienced sprinters performed a 50-m sprint. Step-by-step kinematics were measured by 50 force plates and compared with an IMU-3D motion capture system and a combined laser+IMU system attached to each foot. Results showed that step kinematics (step velocity, length, contact and flight times) were different when measured with the IMU-3D system, compared with force plates, while the laser+IMU system, showed in general the same kinematics as measured with force plates without a systematic bias. Based upon the findings it can be concluded that the laser+IMU system is as accurate in measuring step-by-step kinematics as the force plate system. At the moment, the IMU-3D system is only accurate in measuring stride patterns (temporal parameters); it is not accurate enough to measure step lengths (spatial) and velocities due to the inaccuracies in step length, especially at high velocities. It is suggested that this laser+IMU system is valid and accurate, which can be used easily in training and competition to obtain step-by step kinematics and give direct feedback of this information during training and competition.

Ground reaction force across the transition during sprint acceleration.

Abrupt changes in kinematics during sprint acceleration called transitions have previously been observed. This study aimed to examine whether ground reaction force (GRF) variables during sprint acceleration also show specific features of the transitions. Twenty-one male sprinters performed 60-m sprints, during which GRF data were recorded. Step-to-step spatiotemporal and GRF variables were approximated using an exponential function and three straight lines. Moreover, statistical parametric mapping (SPM) was used to test changes in GRF curves across the transitions. For running speed, the exponential approximation resulted in smaller root-mean-square (RMS) of residuals. For the other variables, however, RMS of residuals was smaller when the three lines approximation was adopted. Breakpoints around the 5th and 15th steps were detected using effective vertical impulse during the braking phase with the three lines approximation. Across the breakpoints, SPM showed significant differences in the antero-posterior GRF curves at the next step after the first breakpoint and at the second breakpoint. Moreover, the second braking phase of the antero-posterior GRF appeared at the next step after the first breakpoint, and the corresponding first propulsive phase disappeared at the second breakpoint. Consequently, changes in GRF variables during sprint acceleration are likely accompanied by specific alterations. The breakpoints around the 5th and 15th steps found in an effective vertical impulse during the braking phase can be a criterion indicating transitions in GRF variables during sprint acceleration. The transitions are characterized by an appearance and disappearance of the second braking and first propulsive phases, respectively, of the antero-posterior GRF.

The effect of biological maturity status on ground reaction force production during sprinting.

Sprint ability develops nonlinearly across childhood and adolescence. However, the underpinning ground reaction force (GRF) production is not fully understood. This study aimed to uncover the kinetic factors that explain these maturation-related sprint performance differences in Japanese boys and girls. A total of 153 untrained schoolchildren (80 boys, 73 girls) performed two 50-m maximal effort sprints over a 52-force-platform system embedded in an indoor track. Maturity offset (years from peak height velocity; PHV) was estimated using anthropometric data and used to categorise the children into six-year-long maturation groups (from group 1 [5.5-4.5 years before PHV] to group 6 [0.5 years before to 0.5 years after PHV). Maximum and mean step-averaged velocities across 26 steps were compared across consecutive maturation groups, with further GRF analysis (means and waveforms [statistical parametric mapping]) performed when velocity differences were observed. For boys, higher maximum velocities (effect size ± 90% CI = 1.63 ± 0.69) were observed in maturation group 2 (4.5-3.5 years before PHV) compared to group 1 (5.5-4.5 years before PHV), primarily attributable to higher antero-posterior GRFs across shorter ground contacts. Maximum velocities increased from maturation group 4 (2.5-1.5 years before PHV) to group 5 (1.5-0.5 years before PHV) in the girls (effect size ± 90% CI = 1.00 ± 0.78), due to longer ground contacts rather than higher GRFs per se. Waveform analyses revealed more effective reversal of braking forces and higher propulsive forces (e.g. 14%-77% of stance 4), particularly for comparisons involving boys, which suggested potentially enhanced stretch-shortening ability. Youth sport practitioners should consider these maturation-specific alterations when evaluating young athletes' sprint abilities.

Improvement in sprint start performance by modulating an initial loading location on the starting blocks.

This study examined whether modulation of the centre of pressure (COP) on the starting block surface would improve sprint start performance. Twenty male national-level sprinters performed 15-m sprints from the starting blocks in three different conditions (normal, anterior loading and posterior loading), during which ground reaction forces (GRFs) were recorded. The COP location, 10-m time, average horizontal external power (AHEP), spatiotemporal and GRF variables were calculated. The results demonstrate that, although modulation of COP location may not improve sprint start performance for the entire group, it could improve the corresponding performance for specific individuals. A sprinter who favours the posterior front block location and more to the posterior rear block COP location on the block surface at the set position could possibly improve AHEP using the anterior loading condition. An improvement of AHEP in the anterior loading condition (p =.056, effect size [ES] =.305) would be accomplished by greater rear block anteroposterior mean force (p =.043, ES =.574). Moreover, the posterior loading condition may improve the 10-m time and/or AHEP for some individuals, whereas no specific characteristics of the individuals were found. Finally, an improvement of 10-m time in the posterior loading condition (p =.015, ES =.609) would be accomplished by shorter reaction time (p =.035, ES =.780).

Inertial measurement unit-based hip flexion test as an indicator of sprint performance.

This study aimed 1) to examine the validity of inertial measurement unit (IMU)-based hip flexion strength test, and 2) to investigate the hip flexion strength test as an indicator of sprint performance. Eight males performed five repeated hip flexion-extension, while leg motion was recorded using an IMU and a motion capture system (Mocap). As the second experiment, 24 male athletes performed the IMU-based hip flexion strength test and sprinted 50 m, during which step-to-step ground reaction force (GRF) was recorded. The strength test variables were calculated using IMU and Mocap data including angular impulse, mean moment, and positive and negative work and power. Using GRF data, step-to-step spatiotemporal variables were obtained. The results showed high intra-class correlation coefficient and correlation coefficient (both >0.909) between IMU and Mocap for angular impulse, mean moment, positive work and power. The hip flexion mean moment showed significant correlation with running speed from the 5th-8th step section onwards. The angular impulse, mean moment, positive work and power are recommended to be used for the IMU-based hip flexion strength test variables in terms of accuracy and validity. Moreover, the proposed IMU-based hip flexion strength test can be an indicator for better sprinting performance.

External mechanical work done during the acceleration stage of maximal sprint running and its association with running performance.

This study aimed to elucidate how external mechanical work done during maximal acceleration sprint running changes with increasing running velocity and is associated with running performance. In twelve young males, work done at each step over 50 m from the start was calculated from mechanical energy changes in horizontal anterior-posterior and vertical directions and was divided into braking (-Wkap and -Wv, respectively) and propulsive (+Wkap and +Wv, respectively) phases. The maximal running velocity (Vmax) appeared at 35.87±7.76 m and the time required to run 50 m (T50 m) was 7.11±0.54 s. At 80% Vmax or higher, +Wkap largely decreased and -Wkap abruptly increased. The change in the difference between +Wkap and |-Wkap| (ΔWkap) at every step was relatively small at 70% Vmax or lower. Total work done over 50 m was 82.4±7.5 J kg-1 for +Wkap, 36.2±4.4 J kg-1 for |-Wkap|, 14.3±1.9 J kg-1 for +Wv, and 10.4±1.2 J kg-1 for |-Wv|. The total ΔWkap over 50 m was more strongly correlated with T50 m (r=-0.946, P<0.0001) than the corresponding associations for the other work variables. These results indicate that in maximal sprint running over 50 m, work done during the propulsive phase in the horizontal anterior-posterior direction accounts for the majority of the total external work done during the acceleration stage, and maximizing it while suppressing work done during the braking phase is essential to achieve a high running performance.