RELATIONS BETWEEN COMPRESSIVE AXIAL FORCES IN AN INSTRUMENTED MASSIVEFEMORAL IMPLANT, GROUND REACTION FORCES, AND INTEGRATED ELECTROMYOGRAPHS FROM VASTUS LATERALIS DURING VARIOUS OSTEOGENIC EXERCISES

A subject, who had undergone surgery to replace one hip joint and the proximal half of the femur with an instrumented titanium implant, perf ormed brief exercises whilst simultaneous measurements were made of co mpressive axial force in the implant using short-range wireless teleme try, ground reactions using a Kistler force plate, and electromyograph ic activity of the vastus lateralis (VL) and erector spinae (ES) muscl es using surface electrodes. Recordings were made barefoot and a earin g 'trainers'. The exercises (slow jumping in counter movement style, f ast continuous jumping, and jogging on the spot) have been found effec tive in controlled interventions for increasing bone mineral density i n women. The implant forces were 250-400% BW. The values were about tw ice the magnitude of the ground reaction forces and significantly corr elated with them for both peak force and its rate of rise but their re altive magnitudes varied depending on mode of activity (jumping or jog ging). Implant forces were significantly related to the muscle activit y; in multiple regression analysis implant forces during take off from slow jumps VL contributed significantly in addition to the ground rea ction (98% total explained variance). There was more activity in VL du ring jumping than jogging for the same implant force which may explain why jumping appears to be more osteogenic than jogging for the femur. For the same ground reaction, wearing trainers increased both the mag nitude of the compressive loading of the femur and its rate of rise. ( C) 1997 Published by Elsevier Science Ltd.