Trunk stiffness increases with steady-state effort

Trunk stiffness was measured in healthy human subjects as a function of ste ady-state preload efforts in different horizontal loading directions. Since muscle stiffness increases with increased muscle activation associated wit h increasing effort, it is believed that coactivation of muscles helps to s tiffen and stabilize the trunk. This paper tested whether increased steady- state preload effort increases trunk stiffness. Fourteen young healthy subj ects each stood in an apparatus with the pelvis immobilized. They were load ed horizontally at directions of 0, 45, 90, 135 and 180 degrees to the forw ard direction via a thoracic harness. Subjects first equilibrated with a st eady-state load of 20 or 40% of their maximum extension effort. Then a sine -wave force perturbation of nominal amplitude of 7.5 or 15% of maximum effo rt and nominal period of 250 ms was applied. Both the applied force and sub sequent motion were recorded. Effective trunk mass and trunk-driving point stiffness were estimated by fitting the experimental data to a second-order differential equation of the trunk dynamic behavior. The mean effective tr unk mass was 14.1 kg (s.d. = 4.7). The trunk-driving point stiffness increa sed on average 36.8% (from 14.5 to 19.8 N/mm) with an increase in the nomin al steady-state preload effort from 20 to 40% (F-1,F-13 = 204.96, p < 0.001 ). There was a smaller, but significant variation in trunk stiffness with l oading direction. The measured increase in trunk stiffness probably results from increased muscle stiffness with increased muscle activation at higher steady-state efforts. (C) 2001 Elsevier Science Ltd. All rights reserved.