PERFORMANCE SENSITIVITY TO PERTURBATIONS IN ACTIVATION TIMING

This study investigated the sensitivity of optimum jumping performances to perturbations in activation timing. A planar eight-segment computer simulation model was used to simulate the takeoff phase in a high jumping performance. The model was evaluated and subsequently used to produce an optimum performance with a jump height of 2.63 m. The mLJscle activation onset timings at the knee were then varied by ± 5 ms and the effect on the simulated performance was determined. By simply varying the knee activation onset timings the performance did not change in terms of jump height, but the simulations included penalties which indicated that anatomical constraints had been violated. Reoptimisation with a measure of robustness included resulted in an optimum simulated jump of 2.32 m with no penalties which was unaffected by 5 ms perturbations

COORDINATION IN DYNAMIC JUMPING

This study investigated coordination in dynamic jumping using a forward dynamics computer simulation model. A planar eight-segment torque-driven model was used to match the takeoff phase in a recorded running jump for height and recorded jump for distance by varying the torque generator activation timings. Two optimisations were then carried out to maximise height reached and distance travelled for each set of initial conditions used in the matching simulations. Although for each set of initial conditions, the order of activation onset timing was different for the two optimisations, the timing of activation onset in the optimisations for height and distance using the same initial conditions was very similar. This study has shown that the optimal activations are more a function of the initial conditions than the selection of maximal height or maximal distance

OPTIMISATION OF PERFORMANCE IN RUNNING JUMPS FOR HEIGHT

This study investigates the effect of approach conditions and takeoff technique on optimum performance. A planar eight-segment computer simulation model was used to simulate the takeoff phase in high jumping. Optimisations based on performances in the laboratory and at an athletics track were carried out to maximise the height reached by the mass centre in the flight phase. Three pairs of optimisations were performed: (i) optimisation of technique, (ii) optimisation of technique and initial conditions, (iii) optimisation of technique, initial conditions and approach velocity. In the first pair of optimisations the increases in height were 0.12 m and 0.17 m respectively. In the second pair of optimisations the additional increases in height were 0.09 m and 0.19 m and in the third pair further increases of 0.42 m and 0.02 m were obtained

Within-day repeatability of coordination variability measures across the running gait cycle

The purpose of this study was to identify the within-day repeatability of coordination variability calculated using a velocity ellipse area method. Twenty participants attended two data collection sessions within 6 hours. At each session, a marker based motion capture system measured kinematics whilst participants ran at 12 km/h on a treadmill. The minimum detectable change in coordination variability was calculated for four commonly researched joint/segment couplings. Of the couplings investigated, thigh flexion/extension – shank flexion/extension and hip flexion/extension – knee flexion/extension were most repeatable. But in the most repeatable coupling, an average change of 75% across the gait cycle would be required between sessions to detect a meaningful change. This indicated poor repeatability and possible causes are discussed

Sagittal hip-knee coordination during a 45 degree cutting task

Background Hashemi et al. (2011) put forward a new theory of ACL injury mechanism, named the ‘hip extension knee flexion paradox’ that attempted to integrate itself with the most convincing findings in ACL research. This theory of mechanism stated that non-contact ACL injury would occur when the following four criteria are simultaneously met: 1) the tibial plateau has a posterior slope 2) the knee is near full extension upon application of a dynamic ground reaction force 3) activation of musculature about the knee is delayed and 4) hip extension and knee flexion occur concurrently. In their paper, Hashemi et al. (2011) also suggest that fatigue may play an important role in increasing the risk of non-contact ACL injury by delaying activation of musculature about the knee joint. Hip-knee coordination and the effect of fatigue on it, have however not previously been investigated. Purpose The purpose of this study was to quantify hip – knee coordination in the sagittal plane during a 45° cutting movement. Methods Nine female team sports players with experience at international or national level and no history of ACL injury were recruited. Written consent was obtained at the time of testing. Cutting was assessed under pre and post fatigue conditions, whilst motion capture data were collected for the lower limbs using QTM (Qualysis AB, Göteburg, Sweden). In each condition, ten successful 45° cuts were captured for the dominant leg. The fatiguing protocol comprised drop maximal vertical jumps and a multi-directional short sprint exercise. Hip and knee sagittal joint angles were extracted between the time of foot contact to 40 ms, the window in which ACL injury is believed to occur. A coordination pattern of 1) hip flexion – knee flexion, 2) hip extension – knee flexion, 3) hip extension – knee extension, 4) hip extension knee flexion was determined for each time point. The average time that each participant spent in each coordination pattern was calculated and the median coordination pattern was determined for each participant at each time point. Wilcoxon tests were executed in SPSS (IBM SPSS Version 22, Armonk, NY) to determine the effect of fatigue on the percentage time spent in hip extension – knee flexion and hip extension – knee extension. Results The average modal coordination pattern of the group, both pre- and post-fatigue, was to extend the hip and knee for the first 10 ms after foot contact. By 25 ms, all participants had changed coordination pattern to flex at the knee whilst continuing to extend at the hip. Both pre- and post-fatigue, a greater percentage of time (p<0.05) was spent in hip extension – knee flexion than in hip extension – knee extension. No change was seen in the percentage time spent in hip extension – knee flexion after fatigue. Practical Applications Most players adopted a hip extension – knee extension pattern at foot contact before switching to hip extension – knee flexion. The initial observation of hip extension – knee extension was interesting as it represents stiffness at both the hip and knee joint, which has been suggested as high risk for non-contact ACL injury (Pollard et al., 2010). The latter coordination pattern has also been hypothesised as high risk for non-contact ACL injury and was dominant both pre- and post-fatigue. The absence of change in coordination patterns after fatigue suggests this risk factor was not accentuated as a result of a short duration fatiguing protocol in an elite, healthy population

Determinants of Time to Fatigue during Non-Motorized Treadmill Exercise

Treadmill exercise is commonly used for aerobic and anaerobic conditioning. During non-motorized treadmill exercise, the subject must provide the power necessary to drive the treadmill belt. The purpose of this study was to determine what factors affected the time to fatigue on a pair of non-motorized treadmills. Twenty subjects (10 males/10 females) attempted to complete five minutes of locomotion during separate trials at 3.22, 4.83, 6.44, 8.05, 9.66, and 11.27 km (raised dot) h(sup -1). Total exercise time (less than or equal to 5 min) was recorded. Exercise time was converted to the amount of 15 second intervals completed. Peak oxygen uptake (VO2) was measured using a graded exercise test on a standard treadmill, and anthropometric measures were collected from each subject before entering into the study. A Cox proportional hazards regression model was used to determine significant predictive factors in a multivariate analysis. Non-motorized treadmill speed and absolute peak VO2 were found to be significant predictors of exercise time, but there was no effect of anthropometric characteristics. Gender was found to be a predictor of treadmill time, but this was likely due to a higher peak VO2 in males than in females. These results were not affected by the type of treadmill tested in this study. Coaches and therapists should consider the cardiovascular fitness of an athlete or client when prescribing target speed since these factors are related to the total exercise time than can be achieved on a non-motorized treadmill

The effects of initial conditions and takeoff technique on running jumps for height and distance

This study used a subject-specific model with eight segments driven by joint torques for forward dynamics simulation to investigate the effects of initial conditions and takeoff technique on the performance of running jumps for height and distance. The torque activation profiles were varied in order to obtain matching simulations for two jumping performances (one for height and one for distance) by an elite male high jumper, resulting in a simulated peak height of 1.98 m and a simulated horizontal distance of 4.38 m. The peak height reached / horizontal distance travelled by the mass centre for the same corresponding initial conditions were then maximized by varying the activation timings resulting in a peak height of 2.09 m and a horizontal distance of 4.67 m. In a further two optimizations the initial conditions were interchanged giving a peak height of 1.78 m and a horizontal distance of 4.03 m. The four optimized simulations show that even with similar approach speeds the initial conditions at touchdown have a substantial effect on the resulting performance. Whilst the takeoff phase is clearly important, unless the approach phase and the subsequent touchdown conditions are close to optimal then a jumper will be unable to compensate for touchdown condition shortcomings during the short takeoff phase to achieve a performance close to optimum

A biomechanical comparison of initial sprint acceleration performance and technique in an elite athlete with cerebral palsy and able-bodied sprinters

Cerebral palsy is known to generally limit range of motion and force producing capability during movement. It also limits sprint performance, but the exact mechanisms underpinning this are not well known. One elite male T36 multiple-Paralympic sprint medallist (T36) and 16 well-trained able-bodied (AB) sprinters each performed 5–6 maximal sprints from starting blocks. Whole-body kinematics (250 Hz) in the block phase and first two steps, and synchronised external forces (1,000 Hz) in the first stance phase after block exit were combined to quantify lower limb joint kinetics. Sprint performance (normalised average horizontal external power in the first stance after block exit) was lower in T36 compared to AB. T36 had lower extensor range of motion and peak extensor angular velocity at all lower limb joints in the first stance after block exit. Positive work produced at the knee and hip joints in the first stance was lower in T36 than AB, and the ratio of positive:negative ankle work produced was lower in T36 than AB. These novel results directly demonstrate the manner in which cerebral palsy limits performance in a competition-specific sprint acceleration movement, thereby improving understanding of the factors that may limit performance in elite sprinters with cerebral palsy