Ian N. Bezodis

Lower-Limb Mechanics during the Support Phase of Maximum-Velocity Sprint Running

INTRODUCTION The forces produced by an athlete during the support phase of a sprint run are a vital determinant of the outcome of the performance. The purpose of this study was to improve the understanding of sprint technique in well-trained sprinters through the comprehensive analysis of joint kinetics during the support phase of a maximum-velocity sprint. METHODS Four well-trained sprinters performed maximum-effort 60-m sprints. Two-dimensional high-speed video (200 Hz) and ground-reaction force (1000 Hz) data were collected at the 45-m mark. Horizontal velocity, step length, step frequency, and normalized moment, power, and work, via inverse dynamics, were calculated for two trials in each athlete. RESULTS The hip extensors performed positive work in early stance (normalized value = 0.063 +/- 0.017), and the plantar flexors performed positive work in late stance (normalized value = 0.053 +/- 0.010). The knee extensors played a negligible role in positive work generation throughout stance. CONCLUSIONS In contrast to previous findings, the knee moment did not contribute substantially to power generation during the latter part of the support phase. This may be explained in part by the specific technical requirements of the maximum-velocity phase of the sprint. However, major periods of power generation of the hip extensors in early stance and of the plantar flexors in late stance were observed. The action of the knee joint during the support phase may therefore have been more of a facilitator for the radial transfer of power from the hip through the ankle on to the track.

Elite sprinting: Are athletes individually step frequency or step length reliant?

PURPOSE The aim of this study was to investigate the step characteristics among the very best 100-m sprinters in the world to understand whether the elite athletes are individually more reliant on step frequency (SF) or step length (SL). METHODS A total of 52 male elite-level 100-m races were recorded from publicly available television broadcasts, with 11 analyzed athletes performing in 10 or more races. For each run of each athlete, the average SF and SL over the whole 100-m distance was analyzed. To determine any SF or SL reliance for an individual athlete, the 90% confidence interval (CI) for the difference between the SF-time versus SL-time relationships was derived using a criterion nonparametric bootstrapping technique. RESULTS Athletes performed these races with various combinations of SF and SL reliance. Athlete A10 yielded the highest positive CI difference (SL reliance), with a value of 1.05 (CI = 0.50-1.53). The largest negative difference (SF reliance) occurred for athlete A11 as -0.60, with the CI range of -1.20 to 0.03. CONCLUSIONS Previous studies have generally identified only one of these variables to be the main reason for faster running velocities. However, this study showed that there is a large variation of performance patterns among the elite athletes and, overall, SF or SL reliance is a highly individual occurrence. It is proposed that athletes should take this reliance into account in their training, with SF-reliant athletes needing to keep their neural system ready for fast leg turnover and SL-reliant athletes requiring more concentration on maintaining strength levels.

Lower limb joint kinetics and ankle joint stiffness in the sprint start push-off

Abstract Sprint push-off technique is fundamental to sprint performance and joint stiffness has been identified as a performance-related variable during dynamic movements. However, joint stiffness for the push-off and its relationship with performance (times and velocities) has not been reported. The aim of this study was to quantify and explain lower limb net joint moments and mechanical powers, and ankle stiffness during the first stance phase of the push-off. One elite sprinter performed 10 maximal sprint starts. An automatic motion analysis system (CODA, 200 Hz) with synchronized force plates (Kistler, 1000 Hz) collected kinematic profiles at the hip, knee, and ankle and ground reaction forces, providing input for inverse dynamics analyses. The lower-limb joints predominately extended and revealed a proximal-to-distal sequential pattern of maximal extensor angular velocity and positive power production. Pearson correlations revealed relationships (P < 0.05) between ankle stiffness (5.93 ± 0.75 N · m · deg−1) and selected performance variables. Relationships between negative power phase ankle stiffness and horizontal (r = −0.79) and vertical (r = 0.74) centre of mass velocities were opposite in direction to the positive power phase ankle stiffness (horizontal: r = 0.85; vertical: r = −0.54). Thus ankle stiffness may affect the goals of the sprint push-off in different ways, depending on the phase of stance considered.

Which starting style is faster in sprint running--standing or crouch start?

The purpose of this study was to further understand the biomechanical differences between the standing and crouch starting methods, and to investigate whether one of the starting styles provides better acceleration and proves to be faster. Six university track team sprinters performed 2 x 3 x 50 m trials. Digitised video, photocell timing, and velocity data revealed that during the first steps of the performance the standing start produced higher body centre of mass horizontal velocity than the crouch start. This may be due to the longer distance between the feet in the standing start, which caused longer push-off phases, and the work against gravity in the crouch start. However, this advantage in horizontal velocity disappeared by the 10 m mark, where similar velocities were recorded with both start styles. Further, there was no statistically significant difference between the two starting styles in horizontal velocity at the 25 m mark, nor in the time to reach the 25 m or 50 m mark. Regarding relay running, where athletes need to decide to adopt either a crouch start without starting blocks or a standing start, there seems to be no specific reason for outgoing athletes to use a crouch start, although this area warrants further investigation.Abstract The purpose of this study was to further understand the biomechanical differences between the standing and crouch starting methods, and to investigate whether one of the starting styles provides better acceleration and proves to be faster. Six university track team sprinters performed 2 x 3 x 50m trials. Digitised video, photocell timing, and velocity data revealed that during the first steps of the performance the standing start produced higher body centre of mass horizontal velocity than the crouch start. This may be due to the longer distance between the feet in the standing start, which caused longer push‐off phases, and the work against gravity in the crouch start. However, this advantage in horizontal velocity disappeared by the 10m mark, where similar velocities were recorded with both start styles. Further, there was no statistically significant difference between the two starting styles in horizontal velocity at the 25 m mark, nor in the time to reach the 25m or 50m mark. Regarding relay running, where athletes need to decide to adopt either a crouch start without starting blocks or a standing start, there seems to be no specific reason for outgoing athletes to use a crouch start, although this area warrants further investigation.

Towards real-time profiling of sprints using wearable pressure sensors

On-body sensor systems for sport are challenging since the sensors must be lightweight and small to avoid discomfort, and yet robust and highly accurate to withstand and capture the fast movements associated with sport. In this work, we detail our experience of building such an on-body system for track athletes. The paper describes the design, implementation and deployment of an on-body sensor system for sprint training sessions. We autonomously profile sprints to derive quantitative metrics to improve training sessions. Inexpensive Force Sensitive Resistors (FSRs) are used to capture foot events that are subsequently analysed and presented back to the coach. We show how to identify periods of sprinting from the FSR data and how to compute metrics such as ground contact time. We evaluate our system using force plates and show that millisecond-level accuracy is achievable when estimating contact times.

A low-cost accurate speed-tracking system for supporting sprint coaching:

Accurate speed and split-time information on sprinters is crucial in coaching support. Furthermore, speed and stride parameters (i.e. contact time, stride frequency, and stride length) are important in research on the biomechanics of running. Existing speed-tracking systems for sprinting are expensive, unable to support multiple competing athletes, involve a complicated set-up procedure, or are not sufficiently accurate. This paper describes the design, evaluation results, and application scenarios of a novel, practical, and cost-effective light-sensor network system (commissioned at the National Indoor Athletics Centre, Cardiff, UK) that is capable of capturing criterion-comparable split-time information on simultaneously competing sprinters for long- and short-term coaching support and for biomechanics and sports science research purposes. This unique system is specifically designed to support coaching activities on a daily basis. It was also shown that the light-sensor network system can be integrated with other body-attached measurement systems to achieve continuous tracking of position, speed, and stride parameters of a race.

Left ventricular energetics: new insight into the plasticity of regional contributions at rest and during exercise

Although the human left ventricle (LV) operates as a functional syncytium and previous studies have reported a single value for LV stroke work at rest, more intricate plasticity of regional LV energetics may be required during enhanced cardiovascular demand. We compared kinetic energy of the LV base and apex, respectively, during ventricular contraction and relaxation at rest and during continuous and discontinuous incremental exercise. At rest, prior to both exercise trials, the accumulated kinetic energy during contraction and relaxation was significantly higher at the LV base compared with the apex (P ≤ 0.05). With increasing exercise intensity, kinetic energy during contraction increased significantly more at the LV base (interaction effect: P < 0.0001), while kinetic energy during relaxation increased significantly more at the apex during high-intensity exercise (interaction effect: P < 0.001). Total kinetic energy produced over the entire cardiac cycle was significantly greater at the LV apex during high exercise intensities (P < 0.05). We further show that the region-specific differences in kinetic energy at rest and during exercise are explained by significantly different wall mechanics, showing heterogenic contributions from radial, circumferential, and angular components at the base and apex, respectively. In conclusion, the present findings provide unique insight into human LV function by demonstrating that within this functional syncytium, significant differences in the regional contributions of kinetic energy to overall LV work exist. Importantly, regional contributions are not fixed but highly plastic and the underpinning LV wall energetics adjust according to the prevailing cardiovascular demand.

Force production during maximal effort bend sprinting: Theory vs reality.

This study investigated whether the “constant limb force” hypothesis can be applied to bend sprinting on an athletics track and to understand how force production influences performance on the bend compared with the straight. Force and three‐dimensional video analyses were conducted on seven competitive athletes during maximal effort sprinting on the bend (radius 37.72 m) and straight. Left step mean peak vertical and resultant force decreased significantly by 0.37 body weight (BW) and 0.21 BW, respectively, on the bend compared with the straight. Right step force production was not compromised in the same way, and some athletes demonstrated substantial increases in these variables on the bend. More inward impulse during left (39.9 ± 6.5 Ns) than right foot contact (24.7 ± 5.8 Ns) resulted in 1.6° more turning during the left step on the bend. There was a 2.3% decrease in velocity from straight to bend for both steps. The constant limb force hypothesis is not entirely valid for maximal effort sprinting on the bend. Also, the force requirements of bend sprinting are considerably different to straight‐line sprinting and are asymmetrical in nature. Overall, bend‐specific strength and technique training may improve performance during this portion of 200‐ and 400‐m races.

Profiling sprints using on-body sensors

This paper describes the design, implementation and deployment of a wireless sensor system for athletes. The system is designed to profile sprints based on input from on-body sensors that are wirelessly connected to a nearby infrastructure. We discuss the choice and use of inexpensive Force Sensitive Resistors (FSRs) to measure foot event timings and provide a detailed analysis of the profiling method used to represent high-level information to the coaches and athletes. In this profiling method, we detect sprinting intervals from high-resolution sensor data, and compute the ground contact times for sprinting performances. We validate our results using force plates and show that the system achieves comparable accuracy in measuring the foot contact times (millisecond accuracy) without the limitations of one or few steps.

Sensing for stride information of sprinters

Accurate sprint-related information, such as stride times, stance times, stride lengths, continuous Centre-of-Mass (CoM) displacements and split times of sprinters are important to both sprint coaches and biomechanics researchers. These information are traditionally captured using camera-based systems which are very expensive and time-consuming to setup. This paper investigates - through a series of experiments - whether an integrated sensing system would provide a practical, cost-effective alternative to measuring stride-related information of sprinters. The results show that the system achieves an accuracy within 5ms for stance time and stride time measurements, and ~10cm for localisation-related information such as CoM forward displacement and CoM stride displacement (i.e. stride length).