Diseño y evaluación experimental de un mecanismo de fuerza constante para máquinas de acondicionamiento muscular.Aplicación al ejercicio de press de banca

Abstract

In strength and endurance training, the patterns of load movement and muscle activation should be adapted to the training objective. Traditionally, machines with moving mass (weights) are used as loading systems. One of the characteristics of this type of machine is the presence of inertial forces due to the acceleration of the moving mass, which means that the force exerted by the athlete can be highly variable throughout the exercise. This Doctoral Thesis proposes the design of a bench press bodybuilding machine so that the force pattern to be exerted by the athlete is as constant as possible during the execution of the exercise, regardless of the execution speed, avoiding the presence of high inertial forces. To this end, the use of a constant force mechanism as the loading device in the bodybuilding machine is proposed. Constant force mechanisms are designed to maintain a constant force value at the input link throughout a wide range of the mechanism stroke. This work details the development of a constant force mechanism patented by the University of Almer´ıa which is based on the use of cams, springs and rollers to achieve the constant force characteristic. A prototype of the mechanism has been designed, manufactured and experimentally evaluated on the basis of the conceptual design described in the referred patent. Once the prototype was validated, two units of the constant force mechanism were sized and manufactured for implementation in a Smith-type bodybuilding machine. This modified Smith machine has been used to evaluate and compare the bench press exercise performed with the two loading systems: free weight and constant force mechanism. The results show significant differences in the force curves between both loading systems. With the proposed loading system, the force curve remains more constant than in the free weight system, substantially reducing the force drop in the braking phase. For the study of the bench press exercise, a multibody model is also proposed, which provided the value of the most significant parameters of the exercise, such as the joint torques, the resultant force exerted by the athlete, and the required power. These parameters have been compared in the performance of the exercise using the traditional loading system (free weight) and using the new proposed loading system. To further study the bench press exercise, a parametric optimization model was proposed in order to determine the optimal distribution of joint torques (shoulder and elbow) during the performance of the exercise together with the assumption of certain execution requirements. Specifically, this research focuses on two different training objectives: to gain resistance, and to perform the exercise in the safest possible way, avoiding overloads.