One-handed Exertions Research Articles

One versus two-handed lifting and lowering: lumbar spine loads and recommended one-handed limits protecting the lower back

The objectives of this study were to quantify loads imposed upon the lumbar spine while lifting/lowering with one versus two hands and to create guidelines for one-handed lifting/lowering that are protective of the lower back. Thirty subjects (15 male, 15 female) performed one- and two-handed exertions in a laboratory, lifting from/lowering to 18 lift origins/destinations using medicine balls of varying masses. An electromyography-assisted model predicted peak spinal loads, which were related to tissue tolerance limits to create recommended weight limits. Compared to two-handed exertions, one-handed exertions resulted in decreased spinal compression and A/P shear loading (p < 0.001) but increased lateral shear (p < 0.001). Effects were likely driven by altered moment exposures attributable to altered torso kinematics. Differences between spinal loads for one- versus two-handed exertions were influenced by asymmetry (p < 0.001) and amplified at lower lift origin/destination heights, lower object masses and larger horizontal distances between the body and the load (p < 0.001). Practitioner summary: A biomechanical model was utilised to compare spinal loading for one versus two-handed lifting/lowering. Spinal loads in compression and A/P shear were reduced for one-handed relative to two-handed exertions. As current lifting guidelines cannot appropriately be applied to one-handed scenarios, one-handed weight limits protecting the lower back are presented herein. Abbreviations: LBD: low back disorder, EMG: electromyography, A/P: anterior/posterior, MVC: maximum voluntary contraction

Spinal Loading During One and Two-Handed Lifting

Background: Lifting at work remains one of the most highly studied potential risk factors for low back disorders, but most of the studies that have been conducted on this topic focus on lifting scenarios in which two hands are used to lift the weight. Few investigations have examined one-handed lifting scenarios, as might be observed when lifting objects from industrial storage bins or stocking products onto shelves. Moreover, there remains a need to re-examine one versus two-handed lifting scenarios utilizing a more advanced and accurate biomechanical model than was used previously (Ferguson et al., 2002; Kingma and van Dieen, 2004; Marras and Davis, 1998). The objective of this study was to quantify biomechanical loads imposed upon the lumbar spine during one-handed lifting versus two-handed lifting, specifically in relation to any interaction effects present between the number of hands used to perform the lift and external lifting conditions like the lift origin or object weight. Methods: Thirty subjects (15 male, 15 female) were recruited for this laboratory study. In each experimental task, subjects lifted one of three medicine balls (of the same size/shape but differing in weight) with either one or two hands from the lift origin predefined by the study design to a common lift destination directly in front of the body. All one-handed exertions were performed with the dominant hand, and asymmetric conditions were tested on the dominant side of the body. Independent variables included lift origin height (ankle, knee, waist), lift origin asymmetry (0 degrees, 45 degrees, 90 degrees), load weight (2.7 kg, 7.3 kg, 11.4 kg), horizontal distance of the lift origin from the body (40 cm, 70 cm), and the number of hands used to perform the lift (one, two). An EMG-driven biomechanical spine model was implemented to evaluate lumbar spinal loads in compression, anterior/posterior (A/P) shear, and lateral shear (Dufour et al., 2013; Hwang et al., 2016a, 2016b). Results and Discussion: One-handed lifting resulted in 6% lower spinal compression and 16% lower A/P shear loads than two-handed lifting on average, but lateral shear was increased by 23% for one-handed lifting relative to two-handed lifting (p&lt;0.001). Consistent with Marras and Davis (1998), spinal compression increased with increased lift origin asymmetry in two-handed lifting but decreased with increased lift origin asymmetry in one-handed lifting (p&lt;0.001). The effects of using one versus two hands to perform the lift were generally amplified at lower lift origin heights, lower weights, and for the far reach distance. Effects were likely driven by differing moment exposures on the spine attributable to the weight of the torso. Conclusion: One-handed lifting resulted in lower peak spinal compression and peak A/P shear loads on the lumbar spine, so it may be preferred to two-handed lifting if the load to be lifted falls within the strength capabilities of the worker population. However, while this study shows benefits of one-handed lifting for the low back, these results should be placed in context with future studies aimed at assessing the impacts of one-handed lifting on the upper extremity. Acknowledgement: This study was funded, in part, by the Ohio Bureau of Workers’ Compensation.

Postural Behaviors during One-Hand Force Exertions

Posture and external loads such as hand forces have a dominant effect on ergonomic analysis outcomes. Yet, current digital human modeling tools used for proactive ergonomics analysis lack validated models for predicting postures for standing hand-force exertions. To address this need, the effects of hand magnitude and direction on whole-body posture for standing static hand-force exertion tasks were quantified in a motion-capture study of 19 men and women with widely varying body size. The objective of this work was to identify postural behaviors that might be incorporated into a postureprediction algorithm for standing hand-force tasks. Analysis of one-handed exertions indicates that, when possible, people tend to align their bodies with the direction of force application, converting potential crossbody exertions into sagittal plane exertions. With respect to the hand-force plane, pelvis position is consistent with a postural objective of reducing rotational trunk torques. One-handed task postures are characterized by axial rotation of the torso towards or away from the point of force application.

The influence of task resistance on the characteristics of maximal one- and two-handed lifting exertions in men and women.

Dynamic lifting strength was assessed at lift velocities ranging from 0 to 1 m.s-1 using a hydrodynamometer in which the viscous resistance to motion could be preset. Nine men and nine women performed one- and two-handed dynamic exertions against low, medium, and high resistances. The subjects were required to lift as forcefully and as fast as possible from a height of 400 mm to just above head height. The instantaneous forces recorded at knee, knuckle, hip, shoulder, elbow, and head heights were compared with those obtained during maximal one- and two-handed isometric lifts performed at the same heights. The position of peak force relative to stature was lower for one-handed lifts (35.9% of stature) than for two-handed lifts (38.4% of stature) (P < 0.005), but was unaffected by lift resistance (P > 0.05) and subject sex (P > 0.05). Lifting forces during two-handed exertions were on average 1.5 times greater than for one-handed exertions (P < 0.0001). However, the difference in the strength of one- and two-handed lifts was highly dependent on hand height (P < 0.0001) and task resistance (P < 0.005), and was greater for the men than for the women (P < 0.001). When static lifting strength was measured at heights relative to stature and normalized by dividing by body weight, there was no significant difference in performance between the men and the women [mean normalized female:male: (f:m) strength ratio = 0.76, P > 0.05]. Under dynamic conditions the sex difference in lifting performance was pronounced (mean normalized f:m strength ratio = 0.68, P < 0.01), especially for power output (mean f:m power.unit body weight-1 ratio = 0.52, P < 0.005). The proportion of generality (r2 x 100) between dynamic and static lifting strength was found to range between 63% and 87%. These data suggest that the dynamic and static tests of lifting strength measured a common, intrinsic ability to produce maximal lifting forces. The finding that sex differences in strength are dependent on whether the exertion is static or dynamic has important implications for biomechanical models of human strength.

The measurement and prediction of isometric lifting strength in symmetrical and asymmetrical postures.

Static lifting strengths of nine men and nine women were measured at six heights from just above the floor to just above the head, at two horizontal reaches from the mid-ankles (equal to the elbow to grip and acromium to grip distances), in the sagittal plane and also at 45 degrees and 90 degrees to the right for two-handed exertions and at 45 degrees and 90 degrees to each side for one-handed exertions, making a total of 96 postures. A second and different group of 18 subjects (nine men and nine women) were studied in 20 two-handed and 40 one-handed postures intermediate to those of the first group. A third group of 16 subjects (eight men and eight women), with six drawn from the other groups, were used to determine maximum possible reach (at which lifting strength is zero) at the same heights and planes as those for the first group. When strength was expressed as a fraction of body weight and height and reach were expressed as fractions of stature, predictive equations of static lifting strength were obtained which were gender free. The predictive equations may be used to generate isodyne contours for an individual in any chosen planes. Individuals exist whose strengths are consistently greater or less than the prediction. The possibility of identifying such persons in a process of worker selection is discussed.

Human strength capabilities during one-handed maximum voluntary exertions in the fore and aft plane

Maximal static strengths were determined for one-handed exertions in all directions in the fore and aft plane. Data from 12 males and 10 females (mean age 30.7 yrs, standard deviation (SD) = 8.9 yrs, n = 22) were obtained with handle heights of 1.0 and 1.75 m. Twelve of the subjects also performed two-handed exertions at the same handle heights. The ratio of mean strengths of females to that of males ranged from 0.50 to 0.83 (for absolute forces) and from 0.63 to 1.00 for forces normalized to body weight. The ratios of one-handed to two-handed strengths ranged from 0.64 to 1.04. Two-handed strengths commonly exceeded one-handed strengths at the lower handle height, but showed fewer significant strength differences (p less than 0.05) according to direction at 1.75 m. Both female/male and one-handed/two-handed strength ratios were found to be dependent on direction of exertion and handle height. The observed strength dependencies upon number of hands (one or two-handed), direction of exertion, handle height and sex are discussed. The strength data have implications for use in biomechanical models and task analysis.