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

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.     

Spinal stability and role of passive stiffness in dynamic squat and stoop lifts

The spinal stability and passive-active load partitioning under dynamic squat and stoop lifts were investigated as the ligamentous stiffness in flexion was altered. Measured in vivo kinematics of subjects lifting 180 N at either squat or stoop technique was prescribed in a nonlinear transient finite element model of the spine. The Kinematics-driven approach was utilized for temporal estimation of muscle forces, internal spinal loads and system stability. The finite element model accounted for nonlinear properties of the ligamentous spine, wrapping of thoracic extensor muscles and trunk dynamic characteristics while subject to measured kinematics and gravity/external loads. Alterations in passive properties of spine substantially influenced muscle forces, spinal loads and system stability in both lifting techniques, though more so in stoop than in squat. The squat technique is advocated for resulting in smaller spinal loads. Stability of spine in the sagittal plane substantially improved with greater passive properties, trunk flexion and load. Simulation of global extensor muscles with curved rather than straight courses considerably diminished loads on spine and increased stability throughout the task.     

PLAD (personal lift assistive device) stiffness affects the lumbar flexion/extension moment and the posterior chain EMG during symmetrical lifting tasks

The PLAD (personal lift assistive device) was designed to reduce the lumbar moment during lifting and bending tasks via elastic elements. This investigation examined the effects of modulating the elastic stiffness. Thirteen men completed 90 lifts (15 kg) using 6 different PLAD stiffnesses in stoop, squat and freestyle lifting postures. The activity of 8 muscles were recorded (latissimus dorsi, thoracic and lumbar erector spinae, rectus abdominis, external oblique, gluteus maximus, biceps femoris and rectus femoris), 3D electromagnetic sensors tracked the motion of each segment and strain gauges measured the elastic tension. EMG data were rectified, filtered, normalized and integrated as a percentage of the lifting task. The highest PLAD tension elicited the greatest reduction in erector spinae activity (mean of thoracic and lumbar) in comparison to the no-PLAD condition for the stoop (37%), squat (38%), and freestyle (37%) lifts, while prompting comparable reductions in gluteus maximums and biceps femoris activity. The highest PLAD stiffness also elicited the greatest reduction in the integrated L4/L5 flexion moment for the stoop (19.0%), squat (18.4%) and freestyle (17.4%) lifts without changing peak lumbar flexion. Each increase in PLAD stiffness further reduced the muscle activity of the posterior chain and the dynamic lumbar moment.     

Physiological and subjective responses to maximal repetitive lifting employing stoop and squat technique

To establish safe levels for physical strain in occupational repetitive lifting, it is of interest to know the specific maximal working capacity. Power output, O₂ consumption, heart rate and ventilation were measured in ten experienced forestry workers during maximal squat and stoop repetitive lifting. The two modes of repetitive lifting were also compared with maximal treadmill running. In addition, electromyogram (EMG) activity in four muscles was recorded and perceived central, local low-back and thigh exertion were assessed during the lifting modes. No significant difference was found in power output between the two lifting techniques. Despite this the mean O₂ consumption was significantly greater during maximal squat lifting [38.7 (SD 5.8) ml·kg^–1-·min^–1] than maximal stoop lifting [32.9 (SD 5.7) ml·kg^–1·min^–1] (P<0.001). No significant correlation was found between O₂ consumption (in millilitres per kilogram per minute) during maximal treadmill running and maximal stoop lifting, while O₂ consumption during maximal squat lifting correlated highly with that of maximal treadmill running (r=0.928, P<0.001) and maximal stoop lifting (r=0.808, P<0.01). While maximal heart rates were significantly different among the three types of exercise, no such differences were found in the central rated perceived exertions. Perceived low-back exertion was rated significantly lower during squat lifting than during stoop lifting. The EMG recordings showed a higher activity for the vastus lateralis muscle and lower activity for the biceps femoris muscle during squat lifting than during stoop lifting. Related to the maximal voluntary contraction, the erector spinae muscle showed the highest activity irrespective of lifting technique.     

Motor variability during a repetitive lifting task is impaired by wearing a passive back-support exoskeleton

PurposeEvaluate whether wearing a passive back-support exoskeleton during repetitive lifting impairs motor variability of erector spinae muscle and spine movement and whether this association is influenced by lifting style.Scope: Thirty-six healthy males performed ten lifts in four randomized conditions with exoskeleton (without, with) and lifting style (squat, stoop) as dependent variables. One lifting cycle contained four phases: bending/straighten without/with load. Erector spinae muscular activity, thoracic kyphosis and lumbar lordosis were measured with surface electromyography and gravimetric position sensors, respectively. Absolute and relative cycle-to-cycle variability were calculated. The effects of exoskeleton and exoskeleton × lifting style were assessed on outcomes during the complete lifting cycle and its four phases.ResultsFor the complete lifting cycle, muscular variability and thoracic kyphosis variability decreased whereas lumbar lordosis variability increased with exoskeleton. For lifting phases, effects of exoskeleton were mixed. Absolute and relative muscular variability showed a significant interaction effect for the phase straighten with load; variability decreased with exoskeleton during squat lifting.ConclusionUsing the exoskeleton impaired several motor variability parameters during lifting, supporting previous findings that exoskeletons may limit freedom of movement. The impact of this result on longer-term development of muscular fatigue or musculoskeletal disorders cannot yet be estimated.     

Mathematical and empirical proof of principle for an on-body personal lift augmentation device (PLAD)

In our laboratory, we have developed a prototype of a personal lift augmentation device (PLAD) that can be worn by workers during manual handling tasks involving lifting or lowering or static holding in symmetric and asymmetric postures. Our concept was to develop a human-speed on-body assistive device that would reduce the required lumbar moment by 20-30% without negative consequences on other joints or lifting kinematics. This paper provides mathematical proof using simplified free body diagrams and two-dimensional moment balance equations. Empirical proof is also provided based on lifting trials with nine male subjects who executed sagittal plane lifts using three lifting styles (stoop, squat, free) and three different loads (5, 15, and 25kg) under two conditions (PLAD, No-PLAD). Nine Fastrak sensors and six in-line strap force sensors were used to estimate the reduction of compressive and shear forces on L4/L5 as well as estimate the forces transferred to the shoulders and knees. Depending on lifting technique, the PLAD applied an added 23-36Nm of torque to assist the back muscles during lifting tasks. The peak pelvic girdle contact forces were estimated and their magnitudes ranged from 221.3+/-11.2N for stoop lifting, 324.3+/-17.2N for freestyle lifts to 468.47+/-23.2N for squat lifting. The PLAD was able to reduce the compression and shear forces about 23-29% and 7.9-8.5%, respectively.     

Low-back loading in lifting two loads beside the body compared to lifting one load in front of the body

Low-back load during manual lifting is considered an important risk factor for the occurrence of low-back pain. Splitting a load, so it can be lifted beside the body (one load in each hand), instead of in front of the body, can be expected to reduce low-back load. Twelve healthy young men lifted 10 and 20-kg wide and narrow loads in front of the body (the single-load lifts). These single-load lifts were compared to a lifting condition in which two 10-kg loads (a total of 20 kg) were lifted beside the body (the split-load lift). Lifts were performed from an initial hand height of 29 cm with four different lifting techniques (stoop, squat, straddle and kneeling techniques). Using measured kinematics, ground reaction forces, and electromyography, low-back loading (3D net moments and spinal forces at the L5/S1 joint) was estimated. Lifting a 20-kg split-load instead of a 20-kg single-load resulted in most cases in a reduction (8–32%) of peak L5/S1 compression forces. The magnitude of the reduction was roughly comparable to halving the load mass and depended on lifting technique and load width. The effects of load-splitting could largely be explained by changes in horizontal distance between the load and L5/S1.     

The kinematics of the scapulae and spine during a lifting task

Simultaneous motion of the scapula and humerus is widely accepted as a feature of normal upper limb movement, however this has usually been investigated under conditions in which purposeful, functional tasks were not considered. The aim of this study was to investigate the synchrony and coordination of the constituent 3D movements of the shoulder girdle and trunk, during a functional activity. 45 healthy women, aged between 20 and 80 years, performed a simple lifting task, moving a loaded box from a shelf at waist level to one at shoulder level and then reversed the movement, during which the linear and angular motions of the scapulae, upper and lower thoracic spine and upper limbs were monitored and analysed using cross-correlation techniques. Results indicated a close and consistent set of coordinated movement patterns, which suggest biomechanical invariance in the responses of the structures adjacent to the upper limb during such a lifting task. These scapulohumeral relationships were, however, more constant and phase-locked when there was a specific purpose to the movement than during periods in which the arm was lowered without load. There were no age-related differences in any movement responses.     

The effect of control strategies for an active back-support exoskeleton on spine loading and kinematics during lifting

With mechanical loading as the main risk factor for LBP, exoskeletons (EXO) are designed to reduce the load on the back by taking over part of the moment normally generated by back muscles. The present study investigated the effect of an active exoskeleton, controlled using three different control modes (INCLINATION, EMG & HYBRID), on spinal compression forces during lifting with various techniques.Ten healthy male subjects lifted a 15 kg box, with three lifting techniques (free, squat & stoop), each of which was performed four times, once without EXO and once each with the three different control modes. Using inverse dynamics, we calculated L5/S1 joint moments. Subsequently, we estimated spine forces using an EMG-assisted trunk model.Peak compression forces substantially decreased by 17.8% when wearing the EXO compared to NO EXO. However, this reduction was partly, by about one third, attributable to a reduction of 25% in peak lifting speed when wearing the EXO. While subtle differences in back load patterns were seen between the three control modes, no differences in peak compression forces were found. In part, this may be related to limitations in the torque generating capacity of the EXO. Therefore, with the current limitations of the motors it was impossible to determine which of the control modes was best. Despite these limitations, the EXO still reduced both peak and cumulative compression forces by about 18%.     

A computer vision based method for 3D posture estimation of symmetrical lifting

Work-related musculoskeletal disorders (WMSD) are commonly observed among the workers involved in material handling tasks such as lifting. To improve work place safety, it is necessary to assess musculoskeletal and biomechanical risk exposures associated with these tasks. Such an assessment has been mainly conducted using surface marker-based methods, which is time consuming and tedious. During the past decade, computer vision based pose estimation techniques have gained an increasing interest and may be a viable alternative for surface marker-based human movement analysis. The aim of this study is to develop and validate a computer vision based marker-less motion capture method to assess 3D joint kinematics of lifting tasks. Twelve subjects performing three types of symmetrical lifting tasks were filmed from two views using optical cameras. The joints kinematics were calculated by the proposed computer vision based motion capture method as well as a surface marker-based motion capture method. The joint kinematics estimated from the computer vision based method were practically comparable to the joint kinematics obtained by the surface marker-based method. The mean and standard deviation of the difference between the joint angles estimated by the computer vision based method and these obtained by the surface marker-based method was 2.31 ± 4.00°. One potential application of the proposed computer vision based marker-less method is to noninvasively assess 3D joint kinematics of industrial tasks such as lifting.     

Temporal changes in motor variability during prolonged lifting/lowering and the influence of work experience

Existing research indicates that repetitive motions are strongly correlated with the development of work-related musculoskeletal disorders (WMSDs). Resulting from the redundant degrees-of-freedom in the human body, there are variations in motions that occur while performing a repetitive task. These variations are termed motor variability (MV), and may be beneficial for reducing WMSD risks. To better understand the potential role of MV in preventing injury risk, we evaluated the effects of fatigue on MV using data collected during a lab-based prolonged, repetitive lifting/lowering task. We also investigated whether experienced workers used different motor control strategies than novices to adapt to fatigue. MV of the whole-body center-of-mass (COM) and box trajectory were quantified using cycle-to-cycle standard deviation, sample entropy, and goal equivalent manifold (GEM) methods. In both groups, there were significantly increased variations of the COM with fatigue, and with a more substantial increase in a direction that did not affect task performance. Fatigue deteriorated the task goal and made it more difficult for participants to maintain their performance. Experienced workers also had higher MV than novices. Based on these results, we conclude that flexible motor control strategies are employed to reduce fatigue effects during a prolonged repetitive task.     

Lower extremity biomechanics during a regular and counterbalanced squat

If the efficiency of human movement patterns could be improved using exercise, this could lead to more effective musculoskeletal disease-injury prevention and rehabilitation programs. It has been suggested that an efficient squat movement pattern emphasizes the use of the large hip extensors instead of the smaller knee extensors. The purpose of this study was to determine whether a counterbalanced squat (CBS) could produce a more hip-dominant and less knee-dominant squat movement pattern as compared with a regular squat (RS). There were 31 recreationally trained college-aged participants (15 male, 16 female) who performed 10 squats (5 CBS and 5 RS), while segment kinematics, ground reaction forces, and muscle (gluteus maximus [GM], quadriceps, hamstrings) electromyographic (EMG) activations were recorded. Peak sagittal plane net joint moments and joint ranges of motion at the hip, knee, and ankle joints along with peak and integrated EMG activation levels for all 3 muscles were compared using analysis of variance (squat type × sex). The results revealed that the CBS increased the hip joint moment and GM activation, while it decreased the knee joint moment and quadriceps activation as compared with the RS. Therefore, the CBS produces a more hip-dominant and less knee-dominant squat movement pattern and could be used in exercise programs aimed at producing more hip-dominant movement patterns.     

Influence of lifting technique on perceptual and cardiovascular responses to submaximal repetitive lifting

Oxygen consumption ( O₂), heart rate, ventilation and central rating of perceived exertion (RPE) in repetitive lifting while executing squat and stoop techniques were investigated in ten male forestry workers. In all five mass/frequency combinations studied, O₂ was significantly higher for the squat than for the stoop technique. No differences were found in RPE between the techniques. The O₂ and RPE recordings were also related to those obtained during maximal repetitive lifting (same lifting technique) and maximal treadmill running. The O₂ expressed as a percentage of that obtained during maximal repetitive lifting with the same lifting technique was defined as relative aerobic intensity (% O_{2max, lifting}). The % O_{2max, lifting} was not significantly different between the techniques except for the lowest mass lifted (1 kg). This study therefore would support the hypothesis that RPE is more closely related to % O_{2max, lifting} than to absolute aerobic intensity. Related to maximal treadmill running, it was demonstrated for both lifting techniques that relative RPE (percentage of the RPE during maximal running) was more accurate than relative O₂ (percentage of maximal O₂ during maximal running) for determining the % O_{2max, lifting} in repetitive lifting. The study showed that the higher O₂ during squat. lifting compared to stoop lifting was caused by the O₂ expended in lifting and lowering the body rather than the O₂ expended lifting and lowering the external mass. It was concluded that the stoop technique was not superior to the squat technique in terms of central RPE. Based on % O_{2max, lifting}, there may be a rationale for choosing the stoop technique during repetitive lifting with light masses, but not with heavy masses.     

Determination of patient-specific multi-joint kinematic models through two-level optimization

Dynamic patient-specific musculoskeletal models have great potential for addressing clinical problems in orthopedics and rehabilitation. However, their predictive capability is limited by how well the underlying kinematic model matches the patient's structure. This study presents a general two-level optimization procedure for tuning any multi-joint kinematic model to a patient's experimental movement data. An outer level optimization modifies the model's parameters (joint position and orientations) while repeated inner level optimizations modify the model's degrees of freedom given the current parameters, with the goal of minimizing errors between model and experimental marker trajectories. The approach is demonstrated by fitting a 27 parameter, three-dimensional, 12 degree-of-freedom lower-extremity kinematic model to synthetic and experimental movement data for isolated joint (hip, knee, and ankle) and gait (full leg) motions. For noiseless synthetic data, the approach successfully recovered the known joint parameters to within an arbitrarily tight tolerance. When noise was added to the synthetic data, root-mean-square (RMS) errors between known and recovered joint parameters were within 10.4 degrees and 10 mm. For experimental data, RMS marker distance errors were reduced by up to 62% compared to methods that estimate joint parameters from anatomical landmarks. Optimized joint parameters found using a loaded full-leg gait motion differed significantly from those found using unloaded individual joint motions. In the future, this approach may facilitate the creation of dynamic patient-specific musculoskeletal models for predictive clinical applications.     

Validation of magneto-inertial measuring units for measuring hip joint angles

Camera-based motion capture systems are the current gold standard for motion analysis. However, the use of wireless inertial sensor-based systems is increasing in popularity, largely due to convenient portability. The purpose of this study was to validate the use of wireless inertial sensors for measuring hip joint motion with a functional calibration requiring only one motion (walking) and neutral standing. Data were concurrently collected using a 10-camera motion capture system and a wireless inertial sensor-based system. Hip joint angles were measured for 10 participants during walking, jumping jack, and bilateral squat tasks and for a subset (n = 5) a jump turn task. Camera-based system hip joint angles were calculated from retro-reflective marker positions and sensor-based system angles were calculated in MATLAB using the sensor output quaternions. Most hip joint angles measured with the sensor-based system were within 6° of angles measured with the camera motion capture system. Accurate measurement of motion outside of a laboratory setting has broad implications for diagnosing movement abnormalities, monitoring sports performance, and assessing rehabilitation progress.     

Improving joint torque calculations: optimization-based inverse dynamics to reduce the effect of motion errors

The accuracy of joint torques calculated from inverse dynamics methods is strongly dependent upon errors in body segment motion profiles, which arise from two sources of noise: the motion capture system and movement artifacts of skin-mounted markers. The current study presents a method to increase the accuracy of estimated joint torques through the optimization of the angular position data used to describe these segment motions. To compute these angular data, we formulated a constrained nonlinear optimization problem with a cost function that minimizes the difference between the known ground reaction forces (GRFs) and the GRF calculated via a top-down inverse dynamics solution. To evaluate this approach, we constructed idealized error-free reference movements (of squatting and lifting) that produced a set of known “true” motions and associated true joint torques and GRF. To simulate real-world inaccuracies in motion data, these true motions were perturbed by artificial noise. We then applied our approach to these noise-induced data to determine optimized motions and related joint torques. To evaluate the efficacy of the optimization approach compared to traditional (bottom-up or top-down) inverse dynamics approaches, we computed the root mean square error (RMSE) values of joint torques derived from each approach relative to the expected true joint torques. Compared to traditional approaches, the optimization approach reduced the RMSE by 54% to 79%. Average reduction due to our method was 65%; previous methods only achieved an overall reduction of 30%. These results suggest that significant improvement in the accuracy of joint torque calculations can be achieved using this approach.     

Postural Sway and Gaze Can Track the Complex Motion of a Visual Target

Variability is an inherent and important feature of human movement. This variability has form exhibiting a chaotic structure. Visual feedback training using regular predictive visual target motions does not take into account this essential characteristic of the human movement, and may result in task specific learning and loss of visuo-motor adaptability. In this study, we asked how well healthy young adults can track visual target cues of varying degree of complexity during whole-body swaying in the Anterior-Posterior (AP) and Medio-Lateral (ML) direction. Participants were asked to track three visual target motions: a complex (Lorenz attractor), a noise (brown) and a periodic (sine) moving target while receiving online visual feedback about their performance. Postural sway, gaze and target motion were synchronously recorded and the degree of force-target and gaze-target coupling was quantified using spectral coherence and Cross-Approximate entropy. Analysis revealed that both force-target and gaze-target coupling was sensitive to the complexity of the visual stimuli motions. Postural sway showed a higher degree of coherence with the Lorenz attractor than the brown noise or sinusoidal stimulus motion. Similarly, gaze was more synchronous with the Lorenz attractor than the brown noise and sinusoidal stimulus motion. These results were similar regardless of whether tracking was performed in the AP or ML direction. Based on the theoretical model of optimal movement variability tracking of a complex signal may provide a better stimulus to improve visuo-motor adaptation and learning in postural control.     

Computational modeling of a dynamic knee simulator for reproduction of knee loading

As a first step towards reproducing desired three-dimensional joint loading and motion on a dynamic knee simulator, the goal of this study was to develop and verify a three-dimensional computational model that generated control profiles for the simulator using desired knee loading and motion as model inputs. The developed model was verified by predicting tibio-femoral loading on an instrumented analog knee for given actuator forces and the ability to generate simulator control profiles was demonstrated using a three-dimensional walking profile. The model predicted axial tibia loading for a sagittal-plane dual-limb squat within 1% of measured peak loading. Adding out-of-sagittal-plane forces decreased the accuracy of load prediction. The model generated control profiles to the simulator that produced axial tibia loading within 16% of desired for walking. Discrepancies in predicted and measured quadriceps forces influenced the accuracy of the generated control profiles. Future work will replace the analog knee in both the model and machine with a prosthetic knee.