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.

Association of multi-phase rates of force development during an isometric leg press with vertical jump performances.

PURPOSE: This study aimed to elucidate characteristics of explosive force-production capabilities represented by multi-phase rate of force developments (IRFDs) during isometric single-leg press (ISLP) through investigating relationships with countermovement (CMJ) and rebound continuous jump (RJ) performances. METHODS: Two-hundred-and-thirty male athletes performed ISLP, CMJ with an arm swing (CMJAS), and RJ with an arm swing (RJAS). IRFDs were measured during ISLP using a custom-built dynamometer, while CMJAS and RJAS were measured on force platforms. The IRFDs were obtained as rates of increase in force across 50 ms in the interval from the onset to 250 ms. Jump height (JH) was obtained from CMJAS, while RJAS provided JH, contact time (CT), and reactive strength index (RSI) values. RESULTS: All IRFDs were correlated with CMJAS-JH (ρ = 0.20-0.45, p ≤ 0.003), RJAS-JH (ρ = 0.22-0.46, p ≤ 0.001), RJAS-RSI (ρ = 0.29-0.48, p < 0.001) and RJAS-CT (ρ = -0.29 to -0.25, p ≤ 0.025). When an influence of peak force was considered using partial rank correlation analysis, IRFDs during onset to 150 ms were correlated with CMJAS-JH (ρxy/z = 0.19-0.36, p ≤ 0.004), IRFDs during onset to 100 ms were correlated with RJAS-JH and RJAS-RSI (ρxy/z = 0.33-0.36, p < 0.001), and IRFD during onset to 50 ms was only correlated with RJAS-CT (ρxy/z = -0.23, p < 0.001). CONCLUSION: The early phase (onset to 150 ms) IRFDs measured using ISLP enabled the assessment of multiple aspects of leg-extension strength characteristics that differ from maximal strength; these insights might be useful in the assessment of the athletes' leg-extension strength capabilities.

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.

A Load-Velocity Relationship in Sprint?

The aims were to compare predicted maximal velocity from load-velocity relationships established with different resisted and assisted loads by different regression analyses to the measured maximal velocity during sprint running, and to compare maximal velocity measured between a robotic pulley system and laser gun. Sixteen experienced male sprinters performed regular 50 m sprints, a 50 m with 5-kilogram-assisted sprint, and 10, 20, 30, and 30 m resisted sprints with, respectively, 65, 50, 25, and 10% calculated reduction in maximal velocity. Maximal velocity obtained by laser gun during the regular sprint was compared with predicted maximal velocity calculated from four trendlines (linear and polynomial based upon four resisted loads, and linear and polynomial based upon four resisted and one assisted load). Main findings demonstrate that the robotic pulley system and laser measure similar maximal velocities at all loads except at the load of 10% velocity reduction. Theoretical maximal velocity based upon calculated predictions were underestimated by 0.62-0.22 m/s (2.2-0.78 km/h; 6.7-2.3%) compared to measured maximal velocity. It was concluded that different regression analyses underestimated measured maximal velocity in regular sprinting and polynomial regression analysis (with resisted and assisted loads) estimation was closest to measured velocity (2.3%).

Asymmetries of kinematics and kinetics in female and male sprinting.

BACKGROUND: This study examined asymmetries in spatiotemporal (e.g. step length and frequency) and kinetic variables during maximal speed sprinting and aimed to determine differences in the asymmetries between female and male sprinters, and to examine relationships between magnitudes of asymmetries and sprint performance. METHODS: Thirty-two female and 32 male sprinters, who were of comparable performance levels, performed 60-m sprints. Spatiotemporal and ground reaction force variables during maximal speed phase were measured using a long force platform system. The asymmetry was calculated as a difference between values obtained from right and left sides divided by the mean of the two sides. RESULTS: The magnitudes of asymmetries in step length (4.60% vs. 3.08%), step frequency (4.70% vs. 3.11%), stance time (3.81% vs. 2.12%), vertical impulse (8.41% vs. 5.30%) and braking mean force (13.32% vs. 8.55%) for male sprinters were greater than those for female sprinters. No significant correlation was found between maximal running speed and magnitudes of asymmetries for female or male sprinters. CONCLUSIONS: The results demonstrate that the magnitudes of asymmetries in step length, step frequency, stance time, vertical impulse and braking mean force during sprinting could be greater in male sprinters. Moreover, no magnitude of asymmetries could be associated with sprint performance for female and male sprinters. Although it is likely beneficial to be aware that there are greater asymmetries in male sprinting during the maximal speed phase compared to female sprinting as a baseline, the magnitudes of asymmetries can be individually different regardless of performance level.

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.

Kinetic and kinematic changes during resisted sprinting due to towing three common parachute sizes.

BACKGROUND: Different sized parachutes may alter applied resistance during parachute towing (PT), changing results of resisted sprint training interventions. Thus, it was hypothesized that there may be significant net anteroposterior ground reaction force and impulse differences due to parachute size towed. METHODS: Fifteen male sprinters completed control and PT sprints over a 50 m force platform system. Estimated aerodynamic drag, ground reaction forces and kinematic differences during the maximum speed phase were compared between control (no parachute) and PT trials with small (0.39 m2), medium (0.54 m2) and large (0.72 m2) parachutes, using One-way ANOVA (significance set at P<0.050) with Tukey's HSD post-hoc (critical Q value = 3.746) tests. RESULTS: No significant (P>0.050) step length, step frequency, propulsive mean force, vertical mean force, or vertical impulse differences between trials. There was a significant anteroposterior impulse difference (P<0.001, Q=4.574) between small and medium PT, but no differences between medium and large PT. Compared to the control trial, all PT trials increased anteroposterior net mean force (P<0.001), anteroposterior net impulse (P<0.001), and propulsive impulse (P<0.001). However, only PT with the large parachute significantly reduced running speed (P<0.050, Q=3.792), braking mean force (P<0.050, Q=4.130) and braking impulse (P<0.001, Q=5.987), compared to the control trial. CONCLUSIONS: A large parachute may be most effective for PT (compared to control trial) to overload the body during the maximum speed phase in a single session.

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.

Thigh loaded wearable resistance increases sagittal plane rotational work of the thigh resulting in slower 50-m sprint times.

This study determined the acute changes in rotational work with thigh attached wearable resistance (WR) of 2% body mass during 50-m sprint-running. Fourteen athletes completed sprints with, and without, WR in a randomised order. Sprint times were measured via timing gates at 10-m and 50-m. Rotational kinematics were obtained over three phases (steps 1-2, 3-6 and 7-10) via inertial measurement unit attached to the left thigh. Quantification of thigh angular displacement and peak thigh angular velocity was subsequently derived to measure rotational work. The WR condition was found to increase sprint times at 10-m (1.4%, effect size [ES] 0.38, p 0.06) and 50-m (1.9%, ES 0.55, p 0.04). The WR condition resulted in trivial to small increases in angular displacement of the thigh during all phases (0.6-3.4%, ES 0.04-0.26, p 0.09-0.91). A significant decrease in angular velocity of the thigh was found in all step phases (-2.5% to -8.0%, ES 0.17-0.51, p < 0.001-0.04), except extension in step phase 1 with the WR. Rotational work was increased (9.8-18.8%, ES 0.35-0.53, p < 0.001) with WR in all phases of the sprint. Thigh attached WR provides a means to significantly increase rotational work specific to sprinting.

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.

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.

Kinetic and kinematic characteristics of sprint running with a weighted vest.

This study elucidated kinetic and kinematic changes between control and weighted vest sprinting with a load of 7% body mass. Fourteen male sprinters completed 60 m control and vest sprints over a long force platform system. Step-to-step ground reaction force and spatiotemporal variables were grouped, representing the initial acceleration (1st-4th steps), middle acceleration (5th-14th steps), later acceleration (15th step-step before maximum velocity reached) and maximum velocity (stride where maximum velocity reached) phase during each trial. Two-way ANOVA with post hoc Tukey HSD and a Cohen's d effect size with 95% confidence intervals elucidated the difference between trials and phases. Between control and vest trials the velocity decreased (3.41-3.78%) through trivial-small step length (1.95-2.72%) and frequency (0.87-1.54%) decreases. Vertical impulse increased (6.46-6.78%) through moderate support time increases (4.84-6.00%), coupled with no effective vertical mean force differences during the vest trial, compared to the control. There was no significant interaction between trials and phases. Therefore, although weighted vest trials did not increase vertical mean force production, vests did induce an increased vertical force application duration during the support phase step-to-step while supporting a larger total load (body mass plus vest mass).

Low Horizontal Force Production Capacity during Sprinting as a Potential Risk Factor of Hamstring Injury in Football.

Clear decreases in horizontal force production capacity during sprint acceleration have been reported after hamstring injuries (HI) in football players. We hypothesized that lower FH0 is associated with a higher HI occurrence in football players. We aimed to analyze the association between sprint running horizontal force production capacities at low (FH0) and high (V0) velocities, and HI occurrence in football. This prospective cohort study included 284 football players over one season. All players performed 30 m field sprints at the beginning and different times during the season. Sprint velocity data were used to compute sprint mechanical properties. Players' injury data were prospectively collected during the entire season. Cox regression analyses were performed using new HI as the outcome, and horizontal force production capacity (FH0 and V0) was used at the start of the season (model 1) and at each measurement time point within the season (model 2) as explanatory variables, adjusted for individual players' (model 2) age, geographical group of players, height, body mass, and previous HI, with cumulative hours of football practice as the time scale. A total of 47 new HI (20% of all injuries) were observed in 38 out of 284 players (13%). There were no associations between FH0 and/or V0 values at the start of the season and new HI occurrence during the season (model 1). During the season, a total of 801 measurements were performed, from one to six per player. Lower measured FH0 values were significantly associated with a higher risk of sustaining HI within the weeks following sprint measurement (HR = 2.67 (95% CI: 1.51 to 4.73), p < 0.001) (model 2). In conclusion, low horizontal force production capacities at low velocity during early sprint acceleration (FH0) may be considered as a potential additional factor associated with HI risk in a comprehensive, multifactorial, and individualized approach.

Ratio of forces during sprint acceleration: A comparison of different calculation methods.

The orientation of the ground reaction force (GRF) vector is a key determinant of human sprint acceleration performance and has been described using ratio of forces (RF) which quantifies the ratio of the antero-posterior component to the resultant GRF. Different methods have previously been used to calculate step-averaged RF, and this study therefore aimed to compare the effects of three calculation methods on two key "technical" ability measures: decline in ratio of forces (DRF) and theoretical maximal RF at null velocity (RF0). Twenty-four male sprinters completed maximal effort 60 m sprints from block and standing starts on a fully instrumented track (force platforms in series). RF-horizontal velocity profiles were determined from the measured GRFs over the entire acceleration phase using three different calculation methods for obtaining an RF value for each step: A) the mean of instantaneous RF during stance, B) the step-averaged antero-posterior component divided by the step-averaged resultant GRF, C) the step-averaged antero-posterior component divided by the resultant of the step-averaged antero-posterior and vertical components. Method A led to significantly greater RF0 and shallower DRF slopes than Methods B and C. These differences were very large (Effect size Cohen's d = 2.06 - 4.04) and varied between individuals due to differences in the GRF profiles, particularly during late stance as the acceleration phase progressed. Method B provides RF values which most closely approximate the mechanical reality of step averaged accelerations progressively approaching zero and it is recommended for future analyses although it should be considered a ratio of impulses.

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.

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.

Innovative alignment of sprinting prostheses for persons with transfemoral amputation: Exploratory study on a gold medal Paralympic athlete.

BACKGROUND: Recommendations for the alignment of the socket and foot in the sprinting prosthesis of athletes with transfemoral amputation are either based on walking biomechanics or lack public scientific evidence. OBJECTIVES: To explore the biomechanical changes and the sensations of a gold medal Paralympic sprinter, while running with three bench alignments: a conventional reference (A0), an innovative alignment based on the biomechanics of elite able-bodied sprinters (A2), and an intermediate alignment (A1). STUDY DESIGN: Single subject with repeated measures. METHODS: A1 and A2 feature a progressively greater socket tilt and a plantar-flexed foot compared to A0. The 30-year-old female athlete trained with three prostheses, one per alignment, for at least 2 months. We administered a questionnaire to collect her impressions. Then, she ran on a treadmill at full speed (5.5 m/s). We measured the kinematics and moments of the prosthetic side, and the ground reaction forces of both sides. RESULTS: A2 reduced the prosthetic side hip extension at foot-off while preserving hip range of motion, decreased the impulse of the hip moment, and increased the horizontal propulsion, leaving sufficient margin to prevent knee buckling without increasing sound side braking forces. Biomechanical outcomes matched well with subjective impressions. CONCLUSIONS: A2 appears promising to improve the performance and comfort of sprinters with transfemoral amputation, without compromising safety. CLINICAL RELEVANCE: Observation of elite able-bodied sprinters led to the definition of a new specific alignment for the sprinting prosthesis of athletes with transfemoral amputation, which appears promising to improve performance and comfort, without compromising safety. This may constitute a major improvement compared to alignments based on walking biomechanics.

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.

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.