Muscle-tendon unit length changes differ between young and adult sprinters in the first stance phase of sprint running.

The aim of this study was to compare young and adult sprinters on several biomechanical parameters that were previously highlighted as performance-related and to determine the behaviour of several muscle-tendon units (MTU) in the first stance phase following a block start in sprint running. The ground reaction force (GRF) and kinematic data were collected from 16 adult and 21 young well-trained sprinters. No difference between the groups was found in some of the previously highlighted performance-related parameters (ankle joint stiffness, the range of dorsiflexion and plantar flexor moment). Interestingly, the young sprinters showed a greater maximal and mean ratio of horizontal to total GRF, which was mainly attributed to a greater horizontal GRF relative to body mass and resulted in a greater change in horizontal centre of mass (COM) velocity during the stance phase in the young compared with the adult sprinters. Results from the MTU length analyses showed that adult sprinters had more MTU shortening and higher maximal MTU shortening velocities in all plantar flexors and the rectus femoris. Although previously highlighted performance-related parameters could not explain the greater 100 m sprinting times in the adult sprinters, differences were found in the behaviour of the MTU of the plantar flexors and rectus femoris during the first stance phase. The pattern of length changes in these MTUs provides ideal conditions for the use of elastic energy storage and release for power enhancement.

Joint power generation differentiates young and adult sprinters during the transition from block start into acceleration: a cross-sectional study.

The aim of this study was to investigate differences in joint power generation between well-trained adult athletes and young sprinters from block clearance to initial contact of second stance. Eleven under 16 (U16) and 18 under 18 (U18) promising sprinters executed an explosive start action. Fourteen well-trained adult sprinters completed the exact same protocol. All athletes were equipped with 74 spherical reflective markers, while an opto-electronic motion analysis system consisting of 12 infrared cameras (250 Hz, MX3, Vicon, Oxford Metrics, UK) and 2 Kistler force plates (1,000 Hz) was used to collect the three-dimensional marker trajectories and ground reaction forces (Nexus, Vicon). Three-dimensional kinematics, kinetics, and power were calculated (Opensim) and time normalised from the first action after gunshot until initial contact of second stance after block clearance. This study showed that adult athletes rely on higher knee power generation during the first stance to induce longer step length and therefore higher velocity. In younger athletes, power generation of hip was more dominant.

Control of propulsion and body lift during the first two stances of sprint running: a simulation study.

The aim of this study was to relate the contribution of lower limb joint moments and individual muscle forces to the body centre of mass (COM) vertical and horizontal acceleration during the initial two steps of sprint running. Start performance of seven well-trained sprinters was recorded using an optoelectronic motion analysis system and two force plates. Participant-specific torque-driven and muscle-driven simulations were conducted in OpenSim to quantify, respectively, the contributions of the individual joints and muscles to body propulsion and lift. The ankle is the major contributor to both actions during the first two stances, with an even larger contribution in the second compared to the first stance. Biarticular gastrocnemius is the main muscle contributor to propulsion in the second stance. The contribution of the hip and knee depends highly on the position of the athlete: During the first stance, where the athlete runs in a forward bending position, the knee contributes primarily to body lift and the hip contributes to propulsion and body lift. In conclusion, a small increase in ankle power generation seems to affect the body COM acceleration, whereas increases in hip and knee power generation tend to affect acceleration less.

From block clearance to sprint running: characteristics underlying an effective transition.

The aim of this study was to characterize the specifics of the sprint technique during the transition from start block into sprint running in well-trained sprinters. Twenty-one sprinters (11 men and 10 women), equipped with 74 spherical reflective markers, executed an explosive start action. An opto-electronic motion analysis system consisting of 12 MX3 cameras (250 Hz; 325,000 pixels) and two Kistler force plates (1000 Hz) was used to collect the three-dimensional (3D) marker trajectories and ground reaction forces (Nexus, Vicon). The 3D kinematics, joint kinetics, and power were calculated (Opensim) and were time normalized to 100% from the first action after gunshot until the end of second stance after block clearance (Matlab). The results showed that during the first stance, power generation at the knee plays a significant role in obtaining an effective transition, representing 31% of power generation in the lower limb, in the absence of preceding power absorption. Furthermore, the sprinter actively searches a more forward leaning position to maximize horizontal velocity. Since success during sprinting from the second stance onwards involves high hip and ankle activation, the above-mentioned three characteristics are specific skills required to successfully conclude the transition from start block into sprint running.

The contribution of step characteristics to sprint running performance in high-level male and female athletes.

We investigated the interaction between step length and step rate and its effect on sprint performance in male and female sprinters during initial acceleration (IA) (0-10 m), transition phase (TP) (10-30 m), and at maximal speed (MS). Ten high-level male and female sprinters ran 2 all-out 60-m sprints. Force-time characteristics of start action were recorded by means of instrumented starting blocks. Running speed and acceleration were recorded using a laser system (ULS), whereas step length and step rate were measured for each step (Optojump). Step length was normalized for leg length. Explosive strength of the lower limb muscles was quantified using vertical jump performance, showing a 24.6% higher score in men compared with women. During the 3 phases of sprinting, step rate remained constant and did not differ significantly between men (IA: 4.37 ± 0.21 Hz, TP: 4.47 ± 0.25 Hz, MS: 4.43 ± 0.18 Hz) and women (IA: 4.23 ± 0.18 Hz, TP: 4.34 ± 0.18 Hz, MS: 4.28 ± 0.17 Hz). The data analysis indicates that step characteristics interact differently in men and women across phases. Men do not take full advantage of their higher explosive strength to develop step length and speed during IA, because normalized step length differed only slightly (-4.09%) between men (1.70 ± 0.21) and women (1.66 ± 0.13). However, men outscored women clearly in acceleration (+34.5%) during the TP because they were capable of developing higher step lengths (2.04 ± 0.12 m in men vs. 1.85 ± 0.07 m in women), even when normalized for leg length (2.65 ± 0.12 in men vs. 2.47 ± 0.22 in women). At MS, it was concluded that men and women pursue an optimal balance between step rate and step length because a high negative correlation was found in both sexes (r = -0.94 and r = -0.77). Therefore, training approach needs to be customized to gender-related differences in step length-rate interaction.