Joint moment estimation by a camera-based motion measurement system and a force plate has a limitation of measurement environment and is costly. The purpose of this paper is to evaluate quantitatively inertial sensor-based joint moment estimation methods with five-link, four-link and three-link rigid body models using different trunk segmented models. Joint moments, ground reaction forces (GRF) and center of pressure (CoP) were estimated for squat and sit-to-stand movements in the sagittal plane measured with six healthy subjects. The five-link model and the four-link model that the trunk was divided at the highest point of the iliac crest (four-link-IC model) were appropriate for joint moment estimation with inertial sensors, which showed average RMS values of about 0.1 Nm/kg for all lower limb joints and average correlation coefficients of about 0.98 for hip and knee joints and about 0.80 for ankle joint. Average root mean square (RMS) errors of horizontal and vertical GRFs and CoP were about 10 N, 15 N and 2 cm, respectively. Inertial sensor-based method was suggested to be an option for estimating joint moments of the trunk segments. Inertial sensors were also shown to be useful for the bottom-up estimation method using measured GRFs, in which average RMS values and average correlation coefficients were about 0.06 Nm/kg and larger than about 0.98 for all joints.

The conventional method of estimating joint moments needs kinematic data measured with a 3D optical motion measurement system and ground reaction forces measured with force plate. However, the conventional method is limited generally to laboratory use because of the required measurement systems. Therefore, we proposed a convenient method to estimate joint moments from measurements only with inertial sensors for application to clinical evaluation of motor function of paralyzed and elderly subjects. In this paper, multi-link modeling of the trunk was examined for reliable estimation of joint moments only from measured data with inertial sensors attached on the body. Body segment parameters (segment length and mass, center of mass location and moments of inertia) were calculated from anthropometric data. Experimental test with 3 healthy subjects showed that segmented trunk model estimated joint moments better than a rigid trunk model for squat and sit-to-stand movements. The estimation results were not different largely between the 5-link model that modeled the trunk by 3 segments and the 4-link model that modeled the trunk by 2 segments. However, trunk modeling for 4-link model was suggested to be appropriate when the upper and the middle trunk segments of the 5-link model were modeled as one segment.