Influencing factors on lower-limb stiffness in long-distance running

Résumé

The study of the spring-mass model variables while running resulted in a great contribution to the understanding of the behaviour of such model not only humans, but in animals as well. Although the study of the running spatiotemporal parameters has contributed to obtain a deeper knowledge about the spring-mass model and its capacity to estimate and predict kinematic variables, the contribution of lower-limb stiffness to this model needed further research. The main aim of the present PhD Thesis was to determine the effect of various influential factors on lower-limb stiffness while treadmill running in healthy adults. Different studies were executed to accomplish the main aim of this PhD Thesis: A unilateral cross-over study aiming at examining the test-retest reliability of the OptoGait photoelectric system for spatiotemporal parameters and lower-body stiffness analysis while treadmill running in healthy adults (Study 1). This first study is key as the entire development of this PhD Thesis has been based on the material and methods implemented and the findings reported; A unilateral cross-over study to clarify the likely relationship between reactive strength index while jumping and lower-limb stiffness while treadmill running in amateur endurance runners as well as sex differences (Study 2); and, ultimately, a unilateral cross-over study to identify the effects of footwear, foot-strike pattern, and step frequency on spatiotemporal parameters and lower-body stiffness (Study 3). The main findings derived from this PhD Thesis suggest that: the OptoGait system can be used confidently for running spatiotemporal parameters analysis and lower body stiffness at a constant velocity for healthy adults. The spring-mass model reacts differently to tasks based on their specificity principle. Additionally, sex-related differences must be considered when assessing the stretch-shortening cycle. Lower-limb stiffness responds differently to changes in footwear condition, foot-strike pattern, and step frequency. The findings reported here update the knowledge of lower-body stiffness while running and offer new scopes of action. A reliable and user-friendly system for running spatiotemporal parameters and lower-body stiffness analysis has been provided. Moreover, although both the SSC and lower-limb stiffness are key within the neuromuscular behaviour when elastic energy is used in sport, the specificity principle of each individual sporting task may make them behave differently; additionally, the menstrual cycle should be considered when working with female athletes since musculotendinous properties change over it. Ultimately, it is highly recommended to avoid measuring the effect of different variables on lower-limb stiffness individually as it has been shown that they influence each another, therefore, the behaviour of the spring-mass model when altering variables such as footwear, FSP, and SF needs to be examined should be analysed attentively.