Hip muscle co-contraction: evidence from concurrent in vivo pressure measurement and force estimation.

The biomechanical literature over the past three decades reports direct measurements of human hip joint contact forces from instrumented implants which in general are quite different than estimates of contact forces based on external kinematic-ground reaction force data and inverse Newtonian analyses. Because direct physical measurement establishes veridical values in science, the higher analytical estimates may overestimate the balanced muscle force increments in agonist-antagonistic muscles about the joint (called co-contraction) which control joint impedance and contribute to joint stability. We studied the extent of muscle co-contraction by comparing in vivo endoprosthesis pressure measurements on hip articular cartilage and intersegmental force estimations from concurrent kinetic-kinematic data. Muscle co-contraction was evident from pressure magnitudes higher than those consistent with external data, from pressure rises before foot-floor contact, and substantial differences in the locations on the acetabulum of the highest pressures compared with the corresponding force vectors estimated from external data. Therefore, joint force and pressure inferences from external kinematic and kinetic data, without corroborating direct, internal measurements, should be made with more caution than is evident in the current literature.