Validity and reliability of the 3D motion analyzer in comparison with the Vicon device for biomechanical pedalling analysis

International audienceThe present work aimed to assess the validity and reliability of the 3D motion analyzer (Shimano Dynamics Lab, Sittard, Netherland) during laboratory cycling tests in comparison with the Vicon device (Vicon Motion Systems Ltd. Oxford, UK). Three cyclists were required to complete one laboratory cycling test at three different pedalling cadence and at a constant power output. Kinematic measurements were collected simultaneously from 3D motion analyzer and Vicon devices and performed five times for each pedalling cadence. The two systems showed a high reliability with excellent intraclass correlation coefficients for most kinematic variables. Moreover, this system was considered as valid by considering the error due to the initial markers placement. Experts and scientists should use the Vicon system for the purpose of research whereas the 3D motion analyzer could be used for bike fitting

Assessment of joint kinetics in elite sprint cyclists

Sprint cycling requires the production of explosive muscle power outputs up to very high pedalling rates. The ability to assess muscular function through the course of the sprint would aid training practices for high-level performers. Inverse dynamics provides a non-invasive means of estimating the net muscle actions acting across any joint contributing to movement. However, analysis of joint kinetics requires motion-capture techniques that present some unique challenges for cycling. This thesis presents three studies investigating the application of a custom-designed force pedal system to examine the joint kinetics of elite trained track sprint cyclists. To provide the basis for selecting appropriate testing procedures, study one evaluated differences between two- and three- dimensional techniques while assessing joint kinetics of seated and standing sprint cycling at optimal cadence (the cadence where peak power is delivered). Study two examined the impact of cadence and seating position on joint kinetics, while determining testing reliability using the three-dimensional process. Coefficients of variation were established for between- and within- days repetitions of sprint performance at optimal cadence, and cadences 30% lower and 30% higher, in both seated and standing positions. Study three compared joint kinetics of sprint cycling performance with commonly-applied resistance-training exercises in an elite cycling cohort, in order to better understand training specificity. Joint-specific torque-angular velocity relationships were established from seated and standing sprinting at three cadences and the clean exercise at three loads, with other strength-based exercises examined at maximal load only. Study one determined that flattened projections of the 3D motion into 2D resulted in significant differences in joint powers calculated in the sagittal-plane. When using 2D methods, knee joint power was significantly lower and hip transfer power significantly greater, while hip range of motion was lower and the angle where hip peak power occurred later in the crank cycle. These results indicate that 3D processes should be used where evaluation of absolute values are important, although 2D processes may still be acceptable where relative differences are being assessed. It was observed in Study two that, while crank and total muscle power upheld a quadratic power-cadence relationship, joint-specific powers were uniquely related to cadence and riding position. Crank and joint-specific optimal cadences for power production were distinctly different. The hip displayed a linear maximum power-cadence relationship in seated but quadratic in standing position, with the reverse observed at the knee. Ankle and hip transfer powers both linearly declined with cadence irrespective riding position. In such a case, joint-specific power contribution, hence distribution of muscular effort, cannot be directly inferred from power assessed at the crank. Reliability was highest for crank and total muscle power, particularly at the riders’ optimal cadence. Reliability of joint powers were somewhat lower and uniquely dependent on joint, joint action and trial condition. Results indicate that external power output at the crank is relatively stable across sprints, despite variation in the underlying muscular contributions. Results of study three showed equivalence in the torque-angular velocity relationships at the hip in sprint cycling and different phases of the clean. No such relationship was evident at the knee or ankle. In contrast to the negative linear relationships observed in all other conditions, ankle mechanics in sprinting showed a positive linear relationship highlighting a distinct functional role of this joint. Highest maximal torques at the hip and knee were observed during unilateral single rack pull and step-up exercises, respectively, supporting their efficacy for improving the maximum strength characteristics at these joints. The results of this thesis indicate that joint kinetics are an effective means of assessing muscular performance in highly-trained track sprint cyclists and provide information on the underlying strategies that could not be assessed through conventional testing of power at the crank. The use of 3D processes is recommended where accuracy of assessment and absolute values are important. Flexibility of 2D processes may be advantageous in field-based settings and may be acceptable where only relative change is of interest. High reliability of 3D testing supports its use in monitoring of athletes, with the reliability data presented in this thesis providing an indication of the smallest meaningful changes in various trial conditions. Low coefficients of variation observed in crank and muscle power terms, despite greater variation in joint powers, suggest motor control strategies dynamically respond to task conditions while maintaining a consistent external power. Resistance exercises are seen to display jointspecific profiles that characterise relative hip- or knee- dominance. The comparison of these profiles with those of sprint cycling can help inform exercise selection for strength development of elite riders. The ability to monitor changes and target training intervention at joint level provides a unique approach to athlete development. Outcomes of this thesis support the practical application of joint kinetic assessment in aiding training practices to the highest levels of competition in track sprint cycling. Indeed, the equipment, methods and knowledge obtained from this research is currently applied in the preparation of Australia’s best sprint cyclists

Effects of quadriceps strength asymmetry, ageing and external loading on stair negotiation

Ageing is associated with reduced stability and increased risk of falls. One risk factor for falls is sarcopenia: age-related muscle loss. Exaggeration of age-related muscle loss in one limb over the other results in strength asymmetry. This asymmetry typically increases with age and previous research has identified negative impact on gait and stability. This thesis develops to, firstly, identify a reliable method to assess strength asymmetry levels (chapter 5), then investigate the effects of quadriceps strength asymmetry on the biomechanics of stair negotiation in a young population (chapter 6) and “symmetrical” and “asymmetrical” older adults (chapter 7), before, finally, studying the effects of external loading (chapter 8). In the first study, within- and between-day testing of isometric maximal voluntary contractions of the quadriceps identified this method as a reliable method to assess strength levels in both legs, and the percentage asymmetry between legs. Consequent studies used this method to analyse the effect of age (young adults versus older adults), asymmetry (15%) and load carriage (unloaded versus 5% body weight bilaterally versus 10% body weight unilaterally) on a range of kinetic and kinematics variables during stair ascent and descent. The young healthy population (8.8±8.5% asymmetry) demonstrated only two significant differences in biomechanical variables between the stronger and weaker leg out of 60 variables measured. This was possibly as a result of type I error as absolute differences in mean values were negligible. Older adults (16.6±14.2% asymmetry) demonstrated detrimental effects of ageing regardless of asymmetry level, suggesting the overall loss of quadriceps strength due to age seems to play a key role. Results demonstrated greater effects on centre of mass and centre of pressure inclination angles and knees angles, rather than contact time and ground reaction force, suggesting the loss of strength can be controlled for to a greater extent in contact time and force. Under additional demands of external loading, effects of group (i.e., age and asymmetry) were typically not found and significant effects were instead a result of loading, primarily during unilateral load carriage. Greater ground reaction force and lateral inclination towards the load was demonstrated when stepping on the leg ipsilateral to the load. Overall, while the impact of strength asymmetry was minimal, this thesis highlights the important impact of age-related loss of muscle strength on stability during stair negotiation. Furthermore, the findings suggest splitting loads bilaterally when under the additional demands of loading.N/