Considerations for single and double leg drop jumps: bilateral deficit, standardizing drop height, and equalizing training load

Bilateral deficit is well documented; however, bilateral deficit is not present in all tasks and is more likely in dynamic activities than isometric activities. No definitive mechanism(s) for bilateral deficit is known but an oft cited mechanism is lower activation of fast twitch motor units. The aim of this study was to produce comparable and consistent one and two legged drop jumps to examine bilateral deficit in elite power athletes and elite endurance athletes. Seven power athletes and seven endurance athletes performed single and double leg drop jumps from a range of heights that equalized loading per leg in terms of: height dropped, energy absorbed, and momentum absorbed. Force and motion data were collected at 800 Hz. Bilateral deficit for jump height, peak concentric force, and peak concentric power were calculated. Power athletes had a significantly greater (P < .05) bilateral deficit for jump height and peak power, possibly due to power athletes having more fast twitch motor units, however, endurance athletes generally had a bilateral surfeit which could confound this inference. Results indicate that equalizing loading by impulse per leg is the most appropriate and that a consistent drop height can be obtained with a short 10 minute coaching session

Utilising human performance criteria and computer simulation to design a martial arts kicking robot with increased biofidelity

The rules and regulations of Taekwondo stipulate how the sport must be played and the necessary personal protective equipment. As such, personal protective equipment performance under controlled rigid drop-tests is also outlined. Unfortunately, these impacts do not replicate human loading effectively, making conclusions about their performance unknown. However, it may be possible to use human kinematic data to improve the biofidelity of current impactors, including a current single-segment martial arts kicking robot. Five martial artists performed a series of roundhouse kicks while reflective markers on the kicking leg and pelvis were used to track hip, knee, ankle and foot positions. Using specific single-segment martial arts kicking robot robot parameters, computer simulation was used to model a singlesegment martial arts kicking robot performance (1-SM) and to form a multi-segment, multi-joint model to match human kinematic data (3-SM). The 3-SM was found to produce similar kinematics to human performance while reducing the overall effective mass at impact, motor torque and stress concentration magnitudes in the leg when compared to the 1-SM. This study suggested that human performances could be used to improve current mechanical testing techniques without introducing much complexity to improve the external validity of protective equipment evaluation testing

Sprint starts and the minimum auditory reaction time

The simple auditory reaction time is one of the fastest reaction times and is thought to be rarely less than 100 ms. The current false start criterion in a sprint used by the International Association of Athletics Federations is based on this assumed auditory reaction time of 100 ms. However, there is evidence, both anecdotal and from reflex research, that simple auditory reaction times of less than 100 ms can be achieved. Reaction time in nine athletes performing sprint starts in four conditions was measured using starting blocks instrumented with piezoelectric force transducers in each footplate that were synchronized with the starting signal. Only three conditions were used to calculate reaction times. The pre-motor and pseudo-motor time for two athletes were also measured across 13 muscles using surface electromyography (EMG) synchronized with the rest of the system. Five of the athletes had mean reaction times of less than 100 ms in at least one condition and 20% of all starts in the first two conditions had a reaction time of less than 100 ms. The results demonstrate that the neuromuscular-physiological component of simple auditory reaction times can be under 85 ms and that EMG latencies can be under 60 ms

Modelling suppressed muscle activation by means of an exponential sigmoid function: validation and bounds

The aim of this study was to establish how well a three-parameter sigmoid exponential function, DIFACT, follows experimentally obtained voluntary neural activation-angular velocity profiles and how robust it is to perturbed levels of maximal activation. Six male volunteers (age 26.3±2.73 years) were tested before and after an 8-session, 3-week training protocol. Torque–angular velocity (T–ω) and experimental voluntary neural drive–angular velocity (%VA–ω) datasets, obtained via the interpolated twitch technique, were determined from pre- and post-training testing sessions. Non-linear regression fits of the product of DIFACT and a Hill type tetanic torque function and of the DIFACT function only were performed on the pre- and post-training T–ω and %VA–ω datasets for three different values of the DIFACT upper bound, αmax, 100%, 95% & 90%. The determination coefficients, R2, and the RMS of the fits were compared using a two way mixed ANOVA and results showed that there was no significant difference (p<0.05) due to changing αmax values indicating the DIFACT remains robust to changes in maximal activation. Mean R2 values of 0.95 and 0.96 for pre- and post-training sessions show that the maximal voluntary torque function successfully reproduces the T–ω raw dataset

A combined muscle model and wavelet approach to interpreting the surface EMG signals from maximal dynamic knee extensions

This study aimed to identify areas of reduced surface EMG amplitude and changed frequency across the phase space of a maximal dynamic knee extension task. The hypotheses were: (1) amplitude would be lower for eccentric contractions compared to concentric contractions and unaffected by fibre length; and (2) mean frequency would also be lower for eccentric contractions and unaffected by fibre length. Joint torque and EMG signals from the vastii and rectus femoris were recorded for eight athletic subjects performing maximum knee extensions at thirteen joint velocities spanning ±250° s–1. The instantaneous amplitude and mean frequency were calculated using the continuous wavelet transform time – frequency method, and the fibre dynamics were determined using a muscle model of the knee extensions. The results indicated: (1) only for the rectus femoris were amplitudes significantly lower for eccentric contractions (p = 0.019), for the vastii amplitudes during eccentric contractions were less than maximal, but this was also the case for concentric contractions due to a significant reduction in amplitude towards knee extension (p = 0.023); and (2) mean frequency increased significantly with decreasing fibre length for all knee extensors and contraction velocities (p = 0.029). Using time – frequency processing of the EMG signals and a muscle model allowed the simultaneous assessment of fibre length, velocity and EMG

Predicting maximum eccentric strength from surface EMG measurements

The origin of the well documented discrepancy between maximum voluntary and in vitro tetanic eccentric strength has yet to be fully understood. This study aimed to determine whether surface EMG measurements can be used to reproduce the in vitro tetanic force – velocity relationship from maximum voluntary contractions. Five subjects performed maximal knee extensions over a range of eccentric and concentric velocities on an isovelocity dynamometer whilst EMG from the quadriceps were recorded. Maximum voluntary (MVC) force – length – velocity data were estimated from the dynamometer measurements and a muscle model. Normalised amplitude – length – velocity data were obtained from the EMG signals. Dividing the MVC forces by the normalised amplitudes generated EMG corrected force – length – velocity data. The goodness of fit of the in vitro tetanic force – velocity function to the MVC and EMG corrected forces was assessed. Based on a number of comparative scores the in vitro tetanic force – velocity function provided a significantly better fit to the EMG corrected forces compared to the MVC forces (p ≤ 0.05), Furthermore, the EMG corrected forces generated realistic in vitro tetanic force – velocity profiles. A 58 ± 19% increase in maximum eccentric strength is theoretically achievable through eliminating neural factors. In conclusion, EMG amplitude can be used to estimate in vitro tetanic forces from maximal in vivo force measurements, supporting neural factors as the major contributor to the difference between in vitro and in vivo maximal force

Relative age effect in Spanish association football: its extent and implications for wasted potential

Spain is one of the largest and most successful powers in international youth football, but this success has not extended to the national team. This lack of continued success seems to indicate a loss of potential. Relative age effect has been detected in football across many countries. Understanding the extent of this bias in the youth teams of Spanish elite clubs may help to improve selection processes and reduce the waste of potential. Comparisons between players from: the Spanish Professional Football League, all age categories of these clubs’ youth teams, the U17-U21 national teams, the national team and the Spanish population, show a constant tendency to under-represent players from the later months of the selection year at all age groups of youth and U17 to U21 national teams. Professional and national team players show a similar but diminished behaviour that weakens with aging, which suggests that talent identification and selection processes can be improved to better identify potential talent early on and minimise wasted potential

Wobbling mass influence on impact ground reaction forces: A simulation model sensitivity analysis

To gain insight into joint loadings during impacts, wobbling mass models have been used. The aim of this study was to investigate the sensitivity of a wobbling mass model, of landing from a drop, to the model's parameters. A two-dimensional wobbling mass model was developed. Three rigid linked segments designed to represent the skeleton each had a second mass attached to them, via two translational non-linear spring dampers, representing the soft tissue. Model parameters were systematically varied one at a time and the effect this had on the peak vertical ground reaction force and segment kinematics was examined. Model output showed low sensitivity to most model parameters but was sensitive to the timing of joint torque initiation. Varying the heel pad stiffness in the range of stiffness values reported in the literature had the largest influence on the peak vertical ground reaction force. The analysis indicated that the more proximal body segments had a lower influence on peak vertical ground reaction force per unit mass than the segments nearer the contact point, 340 N/kg, 157 N/kg and 24 N/kg for the shank, thigh and trunk respectively. Model simulations were relatively insensitive to variations in the properties of the connection between the wobbling masses and the skeleton. Given the proviso that estimates for the other model parameters and joint torque activation timings lie in a realistic range, then if the goal is to examine the effects of the wobbling mass on the system this insensitivity is an advantage. If precise knowledge about the motion of the wobbling mass is of interest, however, more experimental work is required to determine precisely these model parameters

Maximum velocities in flexion and extension actions for sport

Speed of movement is fundamental to the outcome of many human actions. A variety of techniques can be implemented in order to maximise movement speed depending on the goal of the movement, constraints, and the time available. Knowing maximum movement velocities is therefore useful for developing movement strategies but also as input into muscle models. The aim of this study was to determine maximum flexion and extension velocities about the major joints in upper and lower limbs. Seven university to international level male competitors performed flexion/extension at each of the major joints in the upper and lower limbs under three conditions: isolated; isolated with a countermovement; involvement of proximal segments. 500 Hz planar high speed video was used to calculate velocities. The highest angular velocities in the upper and lower limb were 50.0 rad·s-1 and 28.4 rad·s-1, at the wrist and knee, respectively. As was true for most joints, these were achieved with the involvement of proximal segments, however, ANOVA analysis showed few significant differences (p<0.05) between conditions. Different segment masses, structures and locations produced differing results, in the upper and lower limbs, highlighting the requirement of segment specific strategies for maximal movements

Soft tissue motion influences skeletal loads during impacts

Soft tissue motion occurs as impulsive loads are applied to the skeletal system. It has been demonstrated that the wave like motion of these wobbling masses can reduce the loads acting on the musculoskeletal system. This is an important concept to consider, whether the loads acting on the musculoskeletal system are being determined using either inverse or direct dynamics