The force exerted by a muscle is a function of the activation level and the maximum (tetanic) muscle force. In "maximum" voluntary knee extensions muscle activation is lower for eccentric muscle velocities than for concentric velocities. The aim of this study was to model this "differential activation" in order to calculate the maximum voluntary knee extensor torque as a function of knee angular velocity. Torque data were collected on two subjects during maximal eccentric-concentric knee extensions using an isovelocity dynamometer with crank angular velocities ranging from 50 to 450 degrees s(-1). The theoretical tetanic torque/angular velocity relationship was modelled using a four parameter function comprising two rectangular hyperbolas while the activation/angular velocity relationship was modelled using a three parameter function that rose from submaximal activation for eccentric velocities to full activation for high concentric velocities. The product of these two functions gave a seven parameter function which was fitted to the joint torque/angular velocity data, giving unbiased root mean square differences of 1.9% and 3.3% of the maximum torques achieved. Differential activation accounts for the non-hyperbolic behaviour of the torque/angular velocity data for low concentric velocities. The maximum voluntary knee extensor torque that can be exerted may be modelled accurately as the product of functions defining the maximum torque and the maximum voluntary activation level. Failure to include differential activation considerations when modelling maximal movements will lead to errors in the estimation of joint torque in the eccentric phase and low velocity concentric phase.

Over the past fifty years there has been considerable development in motion analysis systems and in computer simulation modelling of sports movements while the relevance and importance of functional variability of sports technique has become increasingly recognised. Technical developments for experimental work have led to increased, and still increasing, subject numbers. Increased subjects per study give better statistical power, the ability to utilise different data analyses, and thus the determination of more subtle and nuanced factors. The overall number of studies has also increased massively. Most actions in sport can, and have, been studied at some level with even the more challenging ones, such as player on player impacts, having some developing research. Computer simulation models of sports movements have ranged from simple (one or two segment) models to very complex musculoskeletal models and have used parameters ranging from the generic to individual-specific. Simple models have given insights into the key mechanics of movement while individual-specific model optimisations have been used to improve athlete performance. Our depth of understanding of the mechanics of sports techniques has increased across a wide range of sports. In the future there is likely to be more development and use of markerless motion capture, individual-specific model parameters, and more consideration of motor control aspects in the analysis of sports technique.

In landings from a flight phase the mass centre of an athlete experiences rapid decelerations. This study investigated the extent to which co-contraction is beneficial or necessary in drop landings, using both experimental data and computer simulations. High speed video and force recordings were made of an elite martial artist performing drop landings onto a force plate from heights of 1.2, 1.5 and 1.8m. Matching simulations of these landings were produced using a planar 8-segment torque-driven subject-specific computer simulation model. It was found that there was substantial co-activation of joint flexor and extensor torques at touchdown in all three landings. Optimisations were carried out to determine whether landings could be effected without any co-contraction at touchdown. The model was not capable of landing from higher than 1.05m with no initial flexor or extensor activations. Due to the force-velocity properties of muscle, co-contraction with net zero joint torque at touchdown leads to increased extensor torque and decreased flexor torque as joint flexion velocity increases. The same considerations apply in any activity where rapid changes in net joint torque are required, as for example in jumps from a running approach.

This paper presents a general method for synchronising digitised video data using a mathematical approach based upon the direct linear transformation reconstruction technique. The method was tested using digitised data from genlocked video recordings of gymnastic vaulting, tumbling, high bar and rings. The mean synchronisation error was less than 0.002 s for vaulting and less than 0.001 s for the other activities.

This article was accepted for publication in the Journal of Biomechanics [© Elsevier] and the definitive version is available at: www.elsevier.com/locate/jbiomechThe determination of subject-specific model parameter values is necessary in order for a computer\ud simulation model of human motion to be evaluated quantitatively. This study used an optimisation\ud procedure along with a kinematically-driven simulation model of the contact phase in running\ud jumps to determine the elastic parameters of segmental wobbling masses and the foot-ground\ud interface. Kinetic and kinematic data were obtained on a running jump for height and a running\ud jump for distance performed by an elite male high jumper. Stiffness and damping coefficients of\ud the visco-elastic elements in the model were varied until the difference between simulation and\ud performance was minimised. Percentage differences of 6% and 9% between the simulated and\ud recorded performances were obtained in the jumps for height and distance respectively. When the\ud parameters obtained from the jump for height were used in a simulation of the jump for distance\ud (and vice versa) there was poor agreement with the recorded jump. On the other hand a common\ud set of visco-elastic parameters were obtained using the data from both recorded jumps resulting in\ud a mean difference of only 8% (made up of 7% and 10%) between simulation and performance that\ud was almost as good as the individual matches. Simulations were not overly sensitive to\ud perturbations of the common set of visco-elastic parameters. It is concluded that subject-specific\ud elastic parameters should be calculated from more than a single jump in order to provide a robust\ud set of values that can be used in different simulations