Angular Velocity, Moment, and Power Analysis of the Ankle, Knee, and Hip Joints in the Goalkeeper's Diving Save in Football.

The aim of this study was to identify biomechanical characteristics of goalkeeper's diving performance in football. Lower extremity joints powers, moments, and angular velocities, were investigated in seven elite goalkeepers diving to save balls, shot from a ball canon to unanticipated heights (high and low) and sides (right and left). Our result showed that there was a proximal-to-distal sequence for each leg in timing of peak joints powers (p < 0.05). Hip extensors produced the largest (p < 0.05) peak moment, the contralateral (relative to dive side) peak was significantly larger than the ipsilateral one for high (4.56 ± 1.02 N·m·kg-1, and 3.52 ± 0.79 N·m·kg-1) and low dives (3.52 ± 0.79 N·m·kg-1, and 2.52 ± 0.56 N·m·kg-1). The ankle plantar flexors produced the second largest peak moment (p < 0.05), and the peak ipsilateral ankle power and angular velocity were the largest (p < 0.05) of all joints, during high (1,502 ± 338 W, and 14.73 ± 1.36 rad·s-1) and low dives (868 ± 263 W, and 14.14 ± 3.09 rad·s-1). Strength and conditioning coaches need to focus on hip extensors and ankle plantar flexors, and for specificity in power training that should elicit triple extension of the lower limbs' joints in a proximal-to-distal sequence.

Kinematic and kinetic analysis of the goalkeeper's diving save in football.

Kinetics and full body kinematics were measured in ten elite goalkeepers diving to save high and low balls at both sides of the goal, aiming to investigate their starting position, linear and angular momentum, and legs' contribution to end-performance. Our results showed that goalkeepers adopted a starting position with a stance width of 33 ± 1% of leg length, knee flexion angle of 62 ± 18° and hip flexion angle of 63 ± 18°. The contralateral leg contributed more than the ipsilateral leg to COM velocity (p < 0.01), both for the horizontal (2.7 ± 0.1 m·s-1 versus 1.2 ± 0.1 m·s-1) and for the vertical component (3.1 ± 0.3 m·s-1 versus 0.4 ± 0.2 m·s-1). Peak horizontal and peak angular momenta were significantly larger (p < 0.01) for low dives than for high dives with a mean difference of 55 kg·m·s-1 and 9 kg·m2·s-1, respectively. In addition, peak vertical momentum was significantly larger (p < 0.01) for high dives with a mean difference between dive heights of 113 kg·m·s-1. Coaches need to highlight horizontal lateral skills and exercises (e.g. sideward push-off, sideward jumps), with emphasis on pushing-off with the contralateral leg, when training and assessing goalkeeper's physical performance.

Effects of a passive exoskeleton on the mechanical loading of the low back in static holding tasks.

With mechanical loading as the main risk factor for LBP in mind, exoskeletons are designed to reduce the load on the back by taking over a part of the required moment. The present study assessed the effect of a passive exoskeleton on back and abdominal muscle activation, hip and lumbar flexion and on the contribution of both the human and the exoskeleton to the L5/S1 net moment, during static bending at five different hand heights. Two configurations of the exoskeleton (LOW & HIGH) differing in angle-torque characteristics were tested. L5/S1 moments generated by the subjects were significantly reduced (15-20% for the most effective type) at all hand heights. LOW generated 4-11 Nm more support than HIGH at 50%, 25% and 0% upright stance hand height and HIGH generated 4-5 Nm more support than LOW at 100% and 75%. Significant reductions (11-57%) in back muscle activity were found compared to WITHOUT for both exoskeletons for some conditions. However, EMG reductions compared to WITHOUT were highly variable across subjects and not always significant. The device allowed for substantial lumbar bending (up to 70°) so that a number of participants showed the flexion-relaxation phenomenon, which prevented further reduction of back EMG by the device and even an increase from 2% to 6% MVC in abdominal activity at 25% hand height. These results indicate that flexion relaxation and its interindividual variation should be considered in future exoskeleton developments.

The Effect of Preparatory Posture on Goalkeeper's Diving Save Performance in Football.

Identifying the optimal preparatory posture of football goalkeepers can be very relevant for improving goalkeepers' diving save performance, and coaching practices of technical and strength and conditioning coaches. This study aimed to analyse the effect of different starting stance widths and knee flexion angles on movement time, center of mass (CoM) trajectory and velocity in goalkeepers' diving saves. Ten elite goalkeepers performed dives from preferred (PT) and imposed postures, by altering knee angle (45, 75, and 90°) and stance width (50, 75, and 100% of leg length) independently, at the starting position. Repeated measures ANOVA showed a main effect of preparatory posture on dive time (p < 0.01). Pairwise comparisons showed that the fastest dive movement time was observed when goalkeepers started from a stance width of 75% (SW75). CoM traveled a larger distance between contralateral and ipsilateral peak ground reaction forces in SW75 than PT (p < 0.05). The goalkeepers were also more efficient in SW75, as a smaller countermovement and vertical velocity range were observed during high and low dives, respectively, from SW75 than PT (p < 0.05). Thus, diving from a position with wider stance width than the preferred one leads to shorter movement time, and a faster and more direct CoM trajectory toward the ball.

An accurate estimation of the horizontal acceleration of a rower's centre of mass using inertial sensors: a validation.

For a valid determination of a rower's mechanical power output, the anterior-posterior (AP) acceleration of a rower's centre of mass (CoM) is required. The current study was designed to evaluate the accuracy of the determination of this acceleration using a full-body inertial measurement units (IMUs) suit in combination with a mass distribution model. Three methods were evaluated. In the first two methods, IMU data were combined with either a subject-specific mass distribution or a standard mass distribution model for athletes. In the third method, a rower's AP CoM acceleration was estimated using a single IMU placed at the pelvis. Experienced rowers rowed on an ergometer that was placed on two force plates, while wearing a full-body IMUs suit. Correspondence values between AP CoM acceleration based on IMU data (the three methods) and AP CoM acceleration obtained from force plate data (reference) were calculated. Good correspondence was found between the reference AP CoM acceleration and the AP CoM accelerations determined using IMU data in combination with the subject-specific mass model and the standard mass model (intraclass correlation coefficients [ICC] > 0.988 and normalized root mean square errors [nRMSE] 3.81%). Correspondence was lower for the AP CoM accelerations determined using a single pelvis IMU (0.877 < ICC < 0.960 and 6.11% < nRMSE < 13.61%). Based on these results, we recommend determining a rower's AP CoM acceleration using IMUs in combination with the standard mass model. Finally, we conclude that accurate determination of a rower's AP CoM acceleration is not possible on the basis of the pelvis acceleration only.

Estimating the L5S1 flexion/extension moment in symmetrical lifting using a simplified ambulatory measurement system.

Mechanical loading of the spine has been shown to be an important risk factor for the development of low-back pain. Inertial motion capture (IMC) systems might allow measuring lumbar moments in realistic working conditions, and thus support evaluation of measures to reduce mechanical loading. As the number of sensors limits applicability, the objective of this study was to investigate the effect of the number of sensors on estimates of L5S1 moments. Hand forces, ground reaction forces (GRF) and full-body kinematics were measured using a gold standard (GS) laboratory setup. In the ambulatory setup, hand forces were estimated based on the force plates measured GRF and body kinematics that were measured using (subsets of) an IMC system. Using top-down inverse dynamics, L5S1 flexion/extension moments were calculated. RMSerrors (Nm) were lowest (16.6) with the full set of 17 sensors and increased to 20.5, 22 and 30.6, for 8, 6 and 4 sensors. Absolute errors in peak moments (Nm) ranged from 17.7 to 16.4, 16.9 and 49.3 Nm, for IMC setup's with 17, 8, 6 and 4 sensors, respectively. When horizontal GRF were neglected for 6 sensors, RMSerrors and peak moment errors decreased from 22 to 17.3 and from 16.9 to 13 Nm, respectively. In conclusion, while reasonable moment estimates can be obtained with 6 sensors, omitting the forearm sensors led to unacceptable errors. Furthermore, vertical GRF information is sufficient to estimate L5S1 moments in lifting.

Changes in gait characteristics of women with early and established medial knee osteoarthritis: Results from a 2-years longitudinal study.

BACKGROUND: Despite the large number of cross-sectional studies on gait in subjects with knee osteoarthritis, there are scarcely any longitudinal studies on gait changes in knee osteoarthritis. METHODS: Gait analysis was performed on 25 women with early and 18 with established medial knee osteoarthritis, as well as a group of 23 healthy controls. Subjects were asked to walk at their comfortable speed. Kinematic and kinetic data were measured at baseline and after 2years follow-up. FINDINGS: Results indicated that the early osteoarthritis group, similar to established osteoarthritis group, showed significantly higher maximum knee adduction angles compared to the controls during the early stance phase of gait. None of the kinematic or kinetic measures, changed over two years in the early osteoarthritis group. In the established osteoarthritis group, at the time of entry, an increased first and second peak knee adduction moment, as well as higher mid-stance knee adduction moment and knee adduction moment impulse, were present compared to the control and the early osteoarthritis groups. Mid-stance knee adduction moment and knee adduction moment impulse, further increased over two years only in the established osteoarthritis group. For all three groups, the peak knee flexion angle during the stance phase decreased significantly over time. INTERPRETATION: Increased maximum knee adduction angle during stance phase was the only alteration in the gait pattern of subjects with early knee osteoarthritis compared to the controls. This suggests that, unlike in the later stages of the disease, gait is rather stable over two years in early osteoarthritis.

Validation of a Step Detection Algorithm during Straight Walking and Turning in Patients with Parkinson's Disease and Older Adults Using an Inertial Measurement Unit at the Lower Back.

INTRODUCTION: Inertial measurement units (IMUs) positioned on various body locations allow detailed gait analysis even under unconstrained conditions. From a medical perspective, the assessment of vulnerable populations is of particular relevance, especially in the daily-life environment. Gait analysis algorithms need thorough validation, as many chronic diseases show specific and even unique gait patterns. The aim of this study was therefore to validate an acceleration-based step detection algorithm for patients with Parkinson's disease (PD) and older adults in both a lab-based and home-like environment. METHODS: In this prospective observational study, data were captured from a single 6-degrees of freedom IMU (APDM) (3DOF accelerometer and 3DOF gyroscope) worn on the lower back. Detection of heel strike (HS) and toe off (TO) on a treadmill was validated against an optoelectronic system (Vicon) (11 PD patients and 12 older adults). A second independent validation study in the home-like environment was performed against video observation (20 PD patients and 12 older adults) and included step counting during turning and non-turning, defined with a previously published algorithm. RESULTS: A continuous wavelet transform (cwt)-based algorithm was developed for step detection with very high agreement with the optoelectronic system. HS detection in PD patients/older adults, respectively, reached 99/99% accuracy. Similar results were obtained for TO (99/100%). In HS detection, Bland-Altman plots showed a mean difference of 0.002 s [95% confidence interval (CI) -0.09 to 0.10] between the algorithm and the optoelectronic system. The Bland-Altman plot for TO detection showed mean differences of 0.00 s (95% CI -0.12 to 0.12). In the home-like assessment, the algorithm for detection of occurrence of steps during turning reached 90% (PD patients)/90% (older adults) sensitivity, 83/88% specificity, and 88/89% accuracy. The detection of steps during non-turning phases reached 91/91% sensitivity, 90/90% specificity, and 91/91% accuracy. CONCLUSION: This cwt-based algorithm for step detection measured at the lower back is in high agreement with the optoelectronic system in both PD patients and older adults. This approach and algorithm thus could provide a valuable tool for future research on home-based gait analysis in these vulnerable cohorts.

Dynamic and static knee alignment at baseline predict structural abnormalities on MRI associated with medial compartment knee osteoarthritis after 2 years.

BACKGROUND: Dynamic and static varus alignment, both, have been reported as risk factors associated with structural progression of knee osteoarthritis. However the association of none of the static and dynamic alignment with structural, clinical, and functional progression associated with knee osteoarthritis has not been assessed yet in a longitudinal study. METHODS: Forty-seven women with early and established medial knee osteoarthritis were evaluated. Static and dynamic alignment as well as MRI detected structural features, clinical, and functional characteristics of patients were assessed at baseline and at 2 years follow-up. Associations between baseline static and dynamic alignment with structural, functional, and clinical characteristics at the time of entry, as well as the changes over 2 years were evaluated. FINDINGS: Both static and dynamic varus alignment at baseline were significantly associated with osteoarthritis related tibio-femoral joint structural abnormalities detected on MRI, at the time of entry. Only the magnitude of varus thrust at baseline was predictive of the changes in the presence of meniscal maceration over two years. None of the static or dynamic measures of knee joint alignment were associated with clinical characteristics associated with medial knee osteoarthritis. INTERPRETATION: The key finding of this study is that both frontal plane dynamic and static alignment, are associated with structural abnormalities in patients with medial knee osteoarthritis.

Algorithm for Turning Detection and Analysis Validated under Home-Like Conditions in Patients with Parkinson's Disease and Older Adults using a 6 Degree-of-Freedom Inertial Measurement Unit at the Lower Back.

INTRODUCTION: Aging and age-associated disorders such as Parkinson's disease (PD) are often associated with turning difficulties, which can lead to falls and fractures. Valid assessment of turning and turning deficits specifically in non-standardized environments may foster specific treatment and prevention of consequences. METHODS: Relative orientation, obtained from 3D-accelerometer and 3D-gyroscope data of a sensor worn at the lower back, was used to develop an algorithm for turning detection and qualitative analysis in PD patients and controls in non-standardized environments. The algorithm was validated with a total of 2,304 turns ≥90° extracted from an independent dataset of 20 PD patients during medication ON- and OFF-conditions and 13 older adults. Video observation by two independent clinical observers served as gold standard. RESULTS: In PD patients under medication OFF, the algorithm detected turns with a sensitivity of 0.92, a specificity of 0.89, and an accuracy of 0.92. During medication ON, values were 0.92, 0.78, and 0.83. In older adults, the algorithm reached validation values of 0.94, 0.89, and 0.92. Turning magnitude (difference, 0.06°; SEM, 0.14°) and duration (difference, 0.004 s; SEM, 0.005 s) yielded high correlation values with gold standard. Overall accuracy for direction of turning was 0.995. Intra class correlation of the clinical observers was 0.92. CONCLUSION: This wearable sensor- and relative orientation-based algorithm yields very high agreement with clinical observation for the detection and evaluation of ≥90° turns under non-standardized conditions in PD patients and older adults. It can be suggested for the assessment of turning in daily life.

Kinematic analysis of the drag flick in field hockey.

Attaining high speed of the stick head and consequently of the ball is essential for successful performance of the drag flick in field hockey, but the coordination pattern used to maximise stick head speed is unknown. The kinematics of the drag flick was studied in ten elite hockey players who performed twenty shots each towards a target located 1.5 m high. A 150 Hz active marker motion analysis system was used, alongside two force plates to detect foot touchdown. Angular velocity and contribution to stick endpoint speed of upper body joints were analysed. Repeated measures ANOVA was used to compare timing of onset and peak angular velocities between joints. Participants used a kinematic pattern that was close to a proximal-to-distal sequence. Trunk axial rotation and lateral rotation towards the target, right wrist flexion and left wrist extension were the main contributors to stick endpoint speed. Coaches should emphasise trunk rotations and wrist flexion and extension movements for maximising stick head speed. Given the high level of the participants in this study, the coordination of joints motions, as reported here, can serve as a guideline for drag flick training.

Regulation of step frequency in transtibial amputee endurance athletes using a running-specific prosthesis.

Running specific prostheses (RSP) are designed to replicate the spring-like behaviour of the human leg during running, by incorporating a real physical spring in the prosthesis. Leg stiffness is an important parameter in running as it is strongly related to step frequency and running economy. To be able to select a prosthesis that contributes to the required leg stiffness of the athlete, it needs to be known to what extent the behaviour of the prosthetic leg during running is dominated by the stiffness of the prosthesis or whether it can be regulated by adaptations of the residual joints. The aim of this study was to investigate whether and how athletes with an RSP could regulate leg stiffness during distance running at different step frequencies. Seven endurance runners with an unilateral transtibial amputation performed five running trials on a treadmill at a fixed speed, while different step frequencies were imposed (preferred step frequency (PSF) and -15%, -7.5%, +7.5% and +15% of PSF). Among others, step time, ground contact time, flight time, leg stiffness and joint kinetics were measured for both legs. In the intact leg, increasing step frequency was accompanied by a decrease in both contact and flight time, while in the prosthetic leg contact time remained constant and only flight time decreased. In accordance, leg stiffness increased in the intact leg, but not in the prosthetic leg. Although a substantial contribution of the residual leg to total leg stiffness was observed, this contribution did not change considerably with changing step frequency. Amputee athletes do not seem to be able to alter prosthetic leg stiffness to regulate step frequency during running. This invariant behaviour indicates that RSP stiffness has a large effect on total leg stiffness and therefore can have an important influence on running performance. Nevertheless, since prosthetic leg stiffness was considerably lower than stiffness of the RSP, compliance of the residual leg should not be ignored when selecting RSP stiffness.

Varus thrust in women with early medial knee osteoarthritis and its relation with the external knee adduction moment.

BACKGROUND: Varus thrust, defined as an abrupt increase of the knee varus angle during weight-bearing in gait, has been shown to be present in patients with moderate to severe knee osteoarthritis and is considered to be one of the risk factors for progression of symptomatic medial knee osteoarthritis. We evaluated the presence and magnitude of varus thrust and its relation with the Knee Adduction Moment in women with early medial knee osteoarthritis, and compared it to that in a group of controls and in a group of subjects with established medial knee osteoarthritis. METHODS: Twenty-seven women with early medial knee osteoarthritis, 20 women with established medial knee osteoarthritis and 24 asymptomatic controls were evaluated. Varus thrust was estimated as an increase of the knee varus angle during the weight-bearing phase of gait at self-selected speed, assessed by 3D motion analysis. FINDINGS: Varus thrust was significantly higher in both early and established osteoarthritis groups compared to the control group (P<0.001), but not different between osteoarthritis groups. While the knee adduction moments were higher than controls only in the established osteoarthritis group, the magnitude of varus thrust was significantly correlated with the second peak knee adduction moment. INTERPRETATION: Higher varus thrust was found both in early and established stages of knee osteoarthritis, suggesting that problems with dynamic stabilization of the knee are present early in the development of knee osteoarthritis. This highlights the necessity of considering dynamic alignment in rehabilitation already in the early stages of the disease.

Hip abductor neuromuscular capacity: A limiting factor in mediolateral balance control in older adults?

BACKGROUND: Mediolateral balance impairment is an important cause of falling in older adults. We aimed to investigate whether hip abductor muscular strength and/or position sense are limiting factors in mediolateral balance control. METHODS: Sixteen community-dwelling older adults performed three different mediolateral weight-shifting tasks, by tracking (1) a sinusoidally moving visual target, "visual-MELBA"; (2) a sinusoidally translating platform without explicit visual feedback, "mechanical-MELBA"; and (3) an unpredictable platform translation, "sudden-platform-translation." Balance performance was quantified for each task and correlated with hip abductor position sense, isometric strength, and peak hip abduction/adduction moments and moment rates. FINDINGS: Participants with better balance performance showed higher and faster hip abduction/adduction moment production during the tasks. Isometric hip abductor strength was significantly correlated with accuracy of tracking the visual target, while hip abductor position sense was associated with the bandwidth over which the mechanical target could be tracked and with a smaller delay between CoM movement and the sudden-platform movement. INTERPRETATION: Hip abductor muscles play an important role in mediolateral balance control. Accurate balance performance appears limited by lower hip abductor strength when explicit visual information on balance reduces the need for hip abductor proprioception, while proprioceptive acuity may limit balance performance when no explicit enhanced feedback is presented and required weight shifts have to be inferred from "normal" sensory information.

Changes in proprioceptive weighting during quiet standing in women with early and established knee osteoarthritis compared to healthy controls.

OBJECTIVES: Knee osteoarthritis (OA) is highly prevalent in people above the age of 60, and is typically associated with pain, stiffness, muscle weakness and proprioceptive deficits. Muscle-tendon vibration has been used to assess the spatial reweighting of proprioceptive input during standing. The current study aimed to investigate whether weighting of proprioceptive input is altered in patients with early and established knee OA compared to asymptomatic controls. METHODS: The upright posture of 27 participants with early OA, 26 with established OA, and 27 asymptomatic controls was perturbed by vibrating (frequency: 70Hz and amplitude: approximately 0.5mm) ankle muscles (i.e. tibialis anterior and triceps surae) and knee muscles (vastus medialis). Center of pressure displacements of the participants were recorded using a force plate. RESULTS: Both patients with early and established OA were more sensitive to triceps surae vibration compared to their healthy peers (P<0.01 for both). No such difference was found for the vibration of tibialis anterior or vastus medialis muscles between patients with knee OA and healthy controls. CONCLUSIONS: These results suggest that the early stages of knee OA may already lead to reweighting of proprioceptive information, suggesting more reliance on ankle proprioceptive input for postural control.

The validity of assessing temporal events, sub-phases and trunk kinematics of the sit-to-walk movement in older adults using a single inertial sensor.

The aim of this study was to develop a method to identify temporal events, sub-phases and trunk kinematics of the sit-to-walk (STW) using a single inertial sensor (IS) worn at the lower back and to determine the validity of this method. Twenty-seven healthy older adults performed a STW movement, which started from sitting in a chair and included walking 3m. Participants׳ movements were recorded with the IS, a reference measurement system consisting of an optical motion capture system (3 markers on the IS and one on each foot) and on-off switches located in the seat of the chair. Using the data from the IS and the reference measurement systems, the following signals and variables were calculated and compared: 3D IS motion (accelerations, velocities, displacements and angles), temporal events (start of trunk movement, seat-off, end of trunk flexion phase, end of trunk rising phase and gait initiation) and trunk kinematics (flexion range, maximum flexion velocity, maximum forward velocity and forward velocity during seat-off and at first heel-strike and maximum vertical velocity and vertical velocity at first heel-strike). For most variables acceptable differences (RMSE<10%) were found between IS and reference measurement systems, except for sideways displacements and non-sagittal plane rotations. Furthermore, good results were found for temporal event detection, with ICC values for all variables being 0.988 or higher. With exception of the vertical velocity at heel-strike agreement for trunk kinematics was high, with ICC values being 0.867 or higher.

Supporting the upper body with the hand on the thigh reduces back loading during lifting.

When picking objects from the floor, low back pain patients often tend to support the upper body by leaning with one hand on a thigh. While this strategy may reduce back load, this has not yet been assessed, probably due to the difficulty of measuring the forces between hand and thigh. Ten healthy male subjects lifted a pencil and a crate from the floor, with four lifting techniques (free, squat, stoop and a Weight Lifters Technique (WLT)), each of which was performed with and without supporting with one hand on the thigh. A six Degrees of Freedom force transducer, with a comfortable surface to support the hand on, was mounted just above the subject׳s left knee. Hand forces, ground reaction forces, full body kinematics, and trunk EMG were measured. Using inverse dynamics and taking the forces between hand and thigh into account, we calculated 3D L5S1 joint moments, and subsequently estimated spine forces using an EMG-assisted trunk model. For lifting a pencil, hand support reduced average peak total moments by 17-25%, dependent on lifting technique. For crate lifting, hand support reduced total moments by 13-19% compared with one-handed lifting and by 14-26% compared to two-handed lifting. Hand support slightly increased asymmetric motions and caused a substantial increase in asymmetric moments in crate lifting. For compression forces, reductions (up to 28%) were seen in all techniques except in stoop lifts. It is concluded that leaning with a hand on the thigh can lead to substantial reductions of low back loading during lifting.

Learning to balance on one leg: motor strategy and sensory weighting.

We investigated motor and sensory changes underlying learning of a balance task. Fourteen participants practiced balancing on one leg on a board that could freely rotate in the frontal plane. They performed six, 16-s trials standing on one leg on a stable surface (2 trials without manipulation, 2 with vestibular, and 2 with visual stimulation) and six trials on the balance board before and after a 30-min training. Center of mass (COM) movement, segment, and total angular momenta and board angles were determined. Trials on stable surface were compared with trials after training to assess effects of surface conditions. Trials pretraining and posttraining were compared to assess rapid (between trials pretraining) and slower (before and after training) learning, and sensory manipulation trials were compared with unperturbed trials to assess sensory weighting. COM excursions were larger on the unstable surface but decreased with practice, with the largest improvement over the pretraining trials. Changes in angular momentum contributed more to COM acceleration on the balance board, but with practice this decreased. Visual stimulation increased sway similarly in both surface conditions, while vestibular stimulation increased sway less on the balance board. With practice, the effects of visual and vestibular stimulation increased rapidly. Initially, oscillations of the balance board occurred at 3.5 Hz, which decreased with practice. The initial decrease in sway with practice was associated with upweighting of visual information, while later changes were associated with suppression of oscillations that we suggest are due to too high proprioceptive feedback gains.

Lumbar compression forces while lifting and carrying with two and four workers.

Team lifting and carrying is advised when loads exceed 25 kg and mechanical lifting is not feasible. The aim of this study was to assess mean, maximum and variability of peak lumbar compression forces which occur daily at construction sites. Therefore, 12 ironworkers performed 50-kg two-worker and 100-kg four-worker lifting and carrying tasks in a laboratory experiment. The 50-kg two-worker lifts resulted in significantly higher mean (Δ 537 N) and maximum (Δ 586 N) peak lumbar compression forces compared with the 100-kg four-worker lifts. The lowest mean and maximum peak lumbar compression forces were found while carrying on level ground and increased significantly when stepping over obstacles and up platforms. Lifting 100 kg with four workers in a rectangular line up resulted in lower compression forces compared with lifting 50 kg with two workers standing next to each other. When loads are carried manually routes should be free of any obstacles to be overcome.

Upper extremity kinematic and kinetic adaptations during a fatiguing repetitive task.

Repetitive low-force contractions are common in the workplace and yet can lead to muscle fatigue and work-related musculoskeletal disorders. The current study aimed to investigate potential motion adaptations during a simulated repetitive light assembly work task designed to fatigue the shoulder region, focusing on changes over time and age-related group differences. Ten younger and ten older participants performed four 20-min task sessions separated by short breaks. Mean and variability of joint angles and scapular elevation, joint net moments for the shoulder, elbow, and wrist were calculated from upper extremity kinematics recorded by a motion tracking system. Results showed that joint angle and joint torque decreased across sessions and across multiple joints and segments. Increased kinematic variability over time was observed in the shoulder joint; however, decreased kinematic variability over time was seen in the more distal part of the upper limb. The changes of motion adaptations were sensitive to the task-break schedule. The results suggested that kinematic and kinetic adaptations occurred to reduce the biomechanical loading on the fatigued shoulder region. In addition, the kinematic and kinetic responses at the elbow and wrist joints also changed, possibly to compensate for the increased variability caused by the shoulder joint while still maintaining task requirements. These motion strategies in responses to muscle fatigue were similar between two age groups although the older group showed more effort in adaptation than the younger in terms of magnitude and affected body parts.