Sex-Dependent Estimation of Spinal Loads During Static Manual Material Handling Activities—Combined in vivo and in silico Analyses

Manual material handling (MMH) is considered as one of the main contributors to low back pain. While males traditionally perform MMH tasks, recently the number of females who undertake these physically-demanding activities is also increasing. To evaluate the risk of mechanical injuries, the majority of previous studies have estimated spinal forces using different modeling approaches that mostly focus on male individuals. Notable sex-dependent differences have, however, been reported in torso muscle strength and anatomy, segmental mass distribution, as well as lifting strategy during MMH. Therefore, this study aimed to use sex-specific models to estimate lumbar spinal and muscle forces during static MHH tasks in 10 healthy males and 10 females. Motion-capture, surface electromyographic from select trunk muscles, and ground reaction force data were simultaneously collected while subjects performed twelve symmetric and asymmetric static lifting (10 kg) tasks. AnyBody Modeling System was used to develop base-models (subject-specific segmental length, muscle architecture, and kinematics data) for both sexes. For females, female-specific models were also developed by taking into account for the female’s muscle physiological cross-sectional areas, segmental mass distributions, and body fat percentage. Males showed higher absolute L5-S1 compressive and shear loads as compared to both female base-models (25.3% compressive and 14% shear) and female-specific models (41% compressive and 23.6% shear). When the predicted spine loads were normalized to subjects’ body weight, however, female base-models showed larger loads (9% compressive and 16.2% shear on average), and female-specific models showed 2.4% smaller and 9.4% larger loads than males. Females showed larger forces in oblique abdominal muscles during both symmetric and asymmetric lifting tasks, while males had larger back extensor muscle forces during symmetric lifting tasks. A stronger correlation between measured and predicted muscle activities was found in females than males. Results indicate that female-specific characteristics affect the predicted spinal loads and must be considered in musculoskeletal models. Neglecting sex-specific parameters in these models could lead to the overestimation of spinal loads in females.

A Comparision Of Spinal Loads While Lifting In Confined Vertical Space

Background: Lifting in confined vertical space is observed across several industries, including airline baggage handling, mining, construction, maintenance, and shipbuilding. However, only a few studies have investigated confined space lifting scenarios with biomechanical methods (Gallagher et al., 1988; Stalhammar et al., 1986), and fewer have quantified biomechanical loads on the intervertebral discs of the lumbar spine while lifting in confined vertical spaces using a biomechanical model (Gallagher et al., 1994; Middelton et al., 2016; Splittstoesser et al., 2007). The objective of this study was to quantify spinal loads for kneeling and sitting lifting styles in confined vertical space. This was accomplished via the replication of the baggage compartment of a narrow-bodied aircraft in a laboratory. Methods: Ten males performed airline baggage handling tasks for this study. A fully balanced design was implemented, and independent variables included lifting style (3), exertion type (4), bag weight (2), and their interactions. Lifting styles investigated included stooping, kneeling, and cross-legged sitting. In kneeling and sitting exertions, subjects were confined within 1.22 m of vertical space (i.e., the vertical constraint within a narrow-bodied airplane). However, stooping conditions were performed as a control condition in unconfined vertical space. Exertion type included loading bags from the floor to a low and high vertical heights or unloading bags from low and high vertical heights to the floor. Subjects lifted bags weighing either 14.5 kg (industry average) or 22.7 kg (95th percentile in U.S.) (Lu et al., 2018). Dependent measures included the peak torso flexion for each lift and peak spinal loads in compression, anterior/posterior (A/P) shear, and lateral shear derived from an electromyography (EMG)-driven spine model (Dufour et al., 2013; Hwang et al., 2016a, b). These spinal loads were compared to documented tolerance limits for spinal loading (Gallagher and Marras, 2012; Waters et al., 1993). Results and Discussion: Stooping, kneeling, and sitting all posed significant risk of injury to the lumbar spine via excessive compressive and A/P shear loading. Statistically significant differences attributable to lift style (stooping, kneeling, sitting) were not observed for peak compressive or lateral shear, but kneeling decreased anterior/posterior (A/P) shear spinal loads relative to stooping (p=0.02). Collectively, kneeling presented the least risk for injury to the low back when lifting in confined spaces because the torso remained more upright, subsequently reducing shear forces on the lumbar spine. However, future studies should also aim to assess the biomechanical risks associated with confined lifting scenarios to other regions of the body in which musculoskeletal disorders might be of concern (i.e., shoulders, knees). Though experimental conditions specific to baggage handling were examined, it is expected that the results of this investigation are applicable across all industries in which lifting in confined vertical spaces is observed. Conclusion: Baggage handling tasks performed in confined vertical space pose significant biomechanical risk to the lumbar spine in compression and A/P shear. From a low back loading perspective, kneeling lifting styles should be preferred to sitting when lifting in a confined vertical because of the ability to keep a more upright torso.