Elsevier

Journal of Biomechanics

Volume 43, Issue 7, 7 May 2010, Pages 1432-1436
Journal of Biomechanics

Short communication
Bottom-up estimation of joint moments during manual lifting using orientation sensors instead of position sensors

https://doi.org/10.1016/j.jbiomech.2010.01.019Get rights and content

Abstract

L5/S1, hip and knee moments during manual lifting tasks are, in a laboratory environment, frequently established by bottom-up inverse dynamics, using force plates to measure ground reaction forces (GRFs) and an optoelectronic system to measure segment positions and orientations. For field measurements, alternative measurement systems are being developed. One alternative is the use of small body-mounted inertial/magnetic sensors (IMSs) and instrumented force shoes to measure segment orientation and GRFs, respectively. However, because IMSs measure segment orientations only, the positions of segments relative to each other and relative to the GRFs have to be determined by linking them, assuming fixed segment lengths and zero joint translation. This will affect the estimated joint positions and joint moments. This study investigated the effect of using segment orientations only (orientation-based method) instead of using orientations and positions (reference method) on three-dimensional joint moments. To compare analysis methods (and not measurement methods), GRFs were measured with a force plate and segment positions and/or orientations were measured using optoelectronic marker clusters for both analysis methods. Eleven male subjects lifted a box from floor level using three lifting techniques: a stoop, a semi-squat and a squat technique. The difference between the two analysis methods remained small for the knee moments: <4%. For the hip and L5/S1 moments, the differences were more substantial: up to 8% for the stoop and semi-squat techniques and up to 14% for the squat technique. In conclusion, joint moments during lifting can be estimated with good accuracy at the knee joint and with reasonable accuracy at the hip and L5/S1 joints using segment orientation and GRF data only.

Introduction

L5/S1, hip and knee moments during manual lifting tasks have often been assessed in the laboratory using one or more force plates measuring ground reaction forces (GRFs) and an optoelectronic system measuring positions and orientations of body segments (e.g. Faber et al., 2007). The usability of such systems for field measurements is limited by the restricted measurement volume of the optoelectronic system and the constraints on foot placement imposed by the force plates.

A possible alternative measurement system that does not have these limitations consists of instrumented shoes measuring GRFs (Schepers et al., 2007) and small body-mounted inertial/magnetic sensors (IMSs) measuring segment kinematics (Luinge and Veltink, 2005). However, IMSs measure only the orientations and not the positions of segments. Therefore, the positions of the segments relative to each other and relative to the GRFs have to be determined by linking the segments to each other, assuming fixed segment lengths and joints that do not allow translation.

Since in reality joint rotation centres are not fixed (van den Bogert et al., 2008), and because of soft-tissue artefacts (Leardini et al., 2005), errors in estimated joint positions are inevitable. These errors are also present when measuring with an optoelectronic system. However, when only orientations of segments are available, position errors present in each segment will add up when linking segments to each other (Fig. 1).

The present study investigated the effect of the additional position error resulting from using only segment orientation (linking the segments using fixed segment lengths) instead of segment position and orientation on the estimated knee, hip and L5/S1 moments during manual lifting. Three different lifting techniques were studied to test whether the amount of knee flexion affects the discrepancy between the two analysis methods. To purely test the effect of analysis method (and not of measurement method), GRFs were measured with a force plate and segment positions and/or orientations were measured using optoelectronic marker clusters for both analysis methods.

Section snippets

Subjects and procedure

Since the current study was the first to investigate the effects of using orientation instead of position sensors on bottom-up estimation of joint moments, a relatively lean male subject population was chosen to minimize the interference of large soft-tissue artefacts and errors in joint centre localization. After signing an informed consent, 11 healthy male subjects (age: 27.4±4.3 years; mass: 73.8±8.9 kg, height: 181.0±5.2 cm) participated in the experiment, which was approved by the local

Results

Fig. 2 shows average stick figures at lift-off for the reference method and for the orientation-based method. The differences and absolute differences in knee joint positions and moments between the reference method and orientation-based method were generally small. Relative to the highest peak total moment, the moment differences and absolute differences remained below 2% for the stoop and semi-squat techniques and below 4% for the squat technique (Fig. 3 and Table 1).

For the hip and L5/S1

Discussion

The present study investigated how accurate knee, hip and L5/S1 moments can be estimated, when only orientations of segments are available. For the knee moment, only small differences with the reference method were found, which can be explained by the small differences between the ankle joint centre positions estimated from foot and calf coordinate systems (Fig. 2). Because the orientation-based hip and L5/S1 positions and moments were not only affected by position differences in the ankle

Conclusion

The present study showed that bottom-up calculated knee moments could be estimated with good accuracy during manual lifting when using only the orientation instead of the orientation and position of segments. Hip and L5/S1 moments could be estimated with less accuracy, especially when subjects used a squat technique.

Conflict of interest statement

There are no conflicts of interest.

Acknowledgment

This study is part of the FreeMotion project, funded by the Dutch Ministry of Economic Affairs and SenterNovem.

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