Study of Force Production in Quick Lift Training Motion by Comparison between Skilled and Unskilled Subjects

Abstract

The purpose of the research is to investigate power clean (PC) motion, a quick lift training aimed at improving explosive force generation, to clarify its mechanism from a biomechanical point of view. In the first experiments, the subjects were divided into two groups, 6 in a “skilled” group and 6 in an “unskilled” group, according to their lifting weight limits. The trajectory of lower limb joints during PC was recorded by a motion capture system and joint kinetics data were calculated from the obtained data. The results of the first experiments indicated that all skilled subjects showed greater peak joint torque, and greater torque power than those of the unskilled subjects. They also displayed a counter movement called the short stretching cycle (SSC) typically shown in skilled PC motion. The investigations of joint torque power and joint force power in phase transition during PC suggested some energy flow between adjacent segments must occur to effectively utilize the target areas. The second experiment was performed with different subjects ; 10 skilled and 10 unskilled, in which the joint torque and its rate of torque development (RTD) were calculated from the obtained data. Joint stiffness was evaluated from a newly established joint stiffness index (JSI) calculated from EMG of agonist and antagonist muscle pairs. Experimental results indicated that skilled subjects showed large muscle activity in flexor muscles but extensor activity was almost equivalent to that of the unskilled. Skilled JSI and RTD were larger than those of unskilled subjects at the time of the counter movement phase. These results suggest that the skilled subjects conducted a dexterous control of joint stiffness during SSC to effectively transmit a large amount of energy to exert a large lifting force during PC.