Effect of Hand Grip and Center of Mass Location on Muscle Activity While Manipulating a Surgical Table Segment.

OCCUPATIONAL APPLICATIONSHand grip location relative to the center of mass of an object can impact the activity of trunk and upper limb muscles. Aligning the hand grip location with the center of mass in the anterior/posterior direction minimizes muscle activity. Whether a proximal or distal grip requires more effort appears to be muscle dependent. Our work illustrates how design features influencing hand grip and center of mass location, such as handles and hand-operated mechanisms, can impact the user. Reducing physical effort via design is important to improve usability and help mitigate the high incidence of musculoskeletal injury resulting from manual materials handling tasks.

Background Manual materials handling tasks are associated with a high risk of injury. The physical effort required to lift and manipulate objects can be influenced by design.Purpose Examine the effect of hand grip location and center of mass on physical effort during a surgical table section attachment task.Methods Twelve participants lifted, carried, and placed a table section onto a surgical table. Hand grip and center of mass location of the table section were both modified in three anteroposterior axis directions (proximal, aligned, and distal), as was the mass (6.8, 9.1, and 11.4 kg). Physical effort was quantified as the normalized peak activity from six unilateral trunk and upper limb muscles recorded via surface electromyography.Results As hypothesized, when an effect was present, aligning the hand grip with the center of mass resulted in the lowest level of muscle activity for all muscles. Whether a proximal or distal relationship between hand grip and the center of mass was more arduous differed by muscle: the deltoid, biceps, and extensor digitorum had greater activity with a center of mass located distal to the hand grip, while erector spinae and trapezius muscles had greater activity with a hand grip distal to the center of mass. Flexor digitorum activity was high in both misaligned conditions of hand grip. Mass, as has been previously documented, had a significant and direct effect on all muscle groups.Conclusions This work has implications for design features such as handles, buttons, or release mechanisms that can dictate where the user grips. By quantifying the impact of anteroposterior axis hand grip and center of mass location on the physical demands of manipulating an object, ergonomists and designers can consider the consequences of incorporating features that could misalign the hand grip location and center of mass.

An OpenSim thoracolumbar spine model applying a bottom-up modelling approach is similar to a top-down approach.

The kinetic demands of the spine can be assessed using a top-down (TD) or bottom-up (BU) approach, which start calculations from the either the hands or from the feet, respectively. Biomechanists have traditionally favored a BU approach, though existing modeling approaches encourage a TD approach. Regardless of the approach the demands should be similar, provided the external forces and linked segment parameters are equivalently measured and modeled. Demonstrating a level of agreement between the two approaches can help evaluate a model. Further, having both approaches can be advantageous when data is inaccurate or unavailable for one. The purpose of this study was to compare the internal moments and forces at multiple lumbar and thoracic intervertebral joint (IVJ) levels during lifting tasks from an established OpenSim thoracolumbar spine model that applies a TD approach and a similar model modified to adopt a BU approach. Kinematics and external forces were recorded from twelve participants during sagittal and lateral lifts of different lifting speeds and crate masses. For both approaches IVJ kinetics were estimated using a standard OpenSim modeling pipeline. The BU and TD approach IVJ joint moments generally agreed both temporally (R2 = .94 ± .17) and in magnitude (RMSE=6.2 ± 3.5 Nm) of the primary planes of movement. There were however some temporal fit exceptions for off axes moments with low magnitudes (i.e., < 10 Nm). Bland-Altman plots also indicated acceptable agreement for IVJ peak forces (BU-TD difference of 12 ± 111 and 8 ± 31 N in compression and resultant shear, respectfully). These results support the application of the BU approach and the assigned linked segment parameters of the model. The new BU model is available on the SimTK site (https://simtk.org/projects/spine_ribcage).

Factors associated with safe patient handling practice in acute care and its relationship with patient mobilization: A cross-sectional study.

BACKGROUND: Mobilizing hospital patients is associated with improved outcomes and shorter length of stay. Safe patient handling and mobility programs that include mechanical lift use facilitate mobilizing patients and reduce the likelihood of musculoskeletal disorders in staff. However, there is little information on the prevalence of lift use or why some patients are more likely to have a lift used than others. Such information is needed to inform public policy, benchmark lift use over time, and contextualize barriers for lift use. OBJECTIVE: To determine the percentage of patients that had a lift used during care in US acute care facilities, identify attributes related to the patient and their hospital stay that affect the lift use, examine whether state legislation increased lift use, and determine whether lift use was correlated with more frequent mobilization out of bed. DESIGN: Retrospective analysis of the 2018 International Pressure Ulcer Prevalence ™ data. PARTICIPANTS: 40,856 patients in 642 US acute care hospitals over the age of 18 with complete data. METHODS: Lift use prevalence was calculated as the percentage of patients that met inclusion criteria that had a lift used for care. Prevalence was then analyzed by patient mobility level. A logistic regression examined the influence of patient and facility related attributes. For patients with limited mobility (that could not stand or turn themselves), a t-test of proportions evaluated whether lift use during a patient's stay was correlated with an increased likelihood of being out of bed at the time of the survey. RESULTS: 3.7% of patients had a lift used during their care. 11.1% of limited mobility patients had a lift used. Lift use was associated with higher body mass, longer length of stay, lower Braden score, pressure injury prevention methods in place, being in an intensive care unit, being in a smaller hospital, and being in a state with safe patient handling and mobility legislation. Limited mobility patients moved with lifts during their stay were more likely to be observed in a bedside chair and less likely to be observed in bed, as compared to patients that never had a lift used. CONCLUSIONS: Despite the benefits to patients and caregivers, US acute care facilities are largely not using lifts to safely mobilize patients. Results suggested that safe patient handling and mobility legislation has increased the rate of lift use. Finally, lift use was correlated with patients being mobilized out of bed.

The effect of prolonged standing on touch sensitivity threshold of the foot: a pilot study.

OBJECTIVE: To determine the effect of prolonged standing on touch sensitivity of the foot. DESIGN: An observational study with replications. SETTING: University laboratory. PARTICIPANTS: Ten healthy college students (5 men and 5 women), with a mean ± SD age of 23.5 ± 4.1 years and body mass of 67.4 ± 12.6 kg. METHODS: Semmes-Weinstein monofilament tests were administered to 12 locations on the dorsal and plantar surfaces of the foot before and after 4 hours of standing. These locations were formed into several groupings (toes, metatarsal heads, midfoot, heel, all plantar sites, all dorsal sites), and paired t-tests were used to test for significant changes in sensitivity threshold after standing. MAIN OUTCOME MEASUREMENT: The difference between sensitivity thresholds measured before and after standing for different locations on the foot. RESULTS: The average of all sensitivity thresholds on the plantar surface of the foot decreased (indicating increased sensitivity) from 0.56 to 0.36 g (P < .01) after 4 hours of prolonged standing. This change in threshold equated to a difference of 1 Semmes-Weinstein monofilament level. Changes in the sensitivity threshold of the dorsal aspect of the foot were not significant. CONCLUSIONS: Analysis of the results suggests that the plantar foot has greater sensitivity to touch after prolonged standing. These findings may be useful for identifying potential unintended bias in clinical touch sensitivity testing. Future research is necessary to understand the underlying mechanisms for this sensitivity change and to determine the onset and recovery times for sensitivity changes.