ABSTRACT:Pinto, BL, Beach, TAC, Howarth, SJ, and Callaghan, JP. Immediate responses to trunk muscle coordination cueing on maximal isometric lifting force, low back postural displacement, lumbar spine rotational stiffness, and trunk muscle co-contraction. J Strength Cond Res XX(X): 000-000, 2026-Cueing trunk muscle bracing has the potential to reduce the risk of injury by stiffening the spine to prevent motion. However, the effects on force production in high-demand tasks such as deadlifting are unpredictable. In addition, various approaches have been used in research and practice settings to cue trunk muscle coordination, ranging from simple verbal directives to detailed coaching. However, the efficacy of these approaches to immediately modify spine stiffness has not been compared. The objective of this investigation was to compare the immediate effects of a simple verbal directive to "activate your core" and detailed trunk muscle bracing coaching on maximal isometric lifting force production, low back postural displacement, muscular contributions to spine rotational stiffness, and trunk muscle agonist-antagonist co-contraction. Forty subjects (50% women) performed maximal isometric lifting exertions across 3 conditions (baseline, directive, and coached). Data were assessed with linear mixed-effects models ( α = 0.05). Postural displacement decreased and rotational stiffness increased in the coached, but not the directive condition. However, both cueing approaches decreased maximal lifting force to a similar extent, which was not solely explained by agonist-antagonist co-contraction, as co-contraction only increased in the coached condition. Decreases in lifting force may instead be attributable to the cognitive effort and attentional focus imposed by the cues. Thus, cueing trunk muscle coordination for the first time can decrease force production, and simple verbal directives are insufficient at modifying trunk muscle coordination. Instead, detailed coaching with practice may ensure the acquisition and implementation of new muscle coordination patterns.

To uncover a link between prolonged sitting and low back disorders, this study assessed how lumbar spine passive stiffness varied with activities and seated spine kinematics. Data were collected from twenty participants performing seated office work throughout a work week. The time spent sitting, standing/stepping, and moderate-vigorous physical activity (MPVA) were derived from a thigh-worn activity monitor. Thorax inclination and lumbar spine flexion and movement were calculated from low back tri-axial accelerometers. Lumbar spine flexion passive stiffness was measured on Monday morning and evening, Tuesday morning, Friday evening, and the following Monday morning. Forward-backward subset regression models were constructed between spine stiffness and activities and seated kinematics, then models were subjected to bootstrapping. The median adjusted R-2 ranged from 0.10 to 0.65 for stiffness measures in low to moderate flexion ranges. Sitting time, particularly at work, was associated with earlier tissue engagement and reductions in the low stiffness zone slope, while MVPA demonstrated the opposite effects. More frequent and smaller spine micromovements while seated were associated with smaller changes in passive stiffness. Modifying workplace sitting behaviours and incorporating movement, through MVPA and in sitting, may help mitigate the mechanical changes in the spine that could contribute to low back disorders.

Musculoskeletal injury risk is often assessed using models that assume damage accumulates linearly with loading. However, biological tissues may exhibit history-dependent changes in tolerance, particularly under repeated or variable loading. In this study, we propose a nonlinear cumulative damage model grounded in a mechanistic description of collagen fibril engagement and failure. The model predicts the evolving tissue tolerance over time, with damage rates governed by a Tobolsky-Eyring-type law modulated by prior damage history.The model was calibrated using experimental fatigue data from functional spinal units and evaluated through a series of simulations designed to reflect common ergonomic exposures. These included constant-load cycling, variable-load sequences, and heavy-tailed loading distributions. Notably, the model predicts that tissue already compromised by prior loading is more susceptible to additional damage, even under identical external conditions—a form of path dependence not captured by the classical Miner-Palmgren rule.Perturbation analysis reveals that commonly used fatigue models can be recovered as successive approximations of the proposed framework, offering a formal connection between linear cumulative load theory, including ergonomics tools like LiFFT, and our nonlinear formulation. This unifying perspective helps reconcile chronic and acute injury risk models and highlights the importance of accounting for load history and variability in injury risk assessments.These findings suggest that ergonomic models should be sensitive not only to cumulative load, but also to its temporal structure and variability. Incorporating such nonlinearities could improve predictions of tissue failure and inform guidelines for safer task design.

End-stage osteoarthritis (OA) alters bone density in the humeral head, complicating implant fixation during stemless shoulder arthroplasty. Current preoperative assessments fail to consider the mechanical properties of bone directly supporting the humeral component. While in-scan phantom calibration is used to determine volumetric bone mineral density (vBMD), phantoms are rarely used clinically. An internal density calibration method has been developed, but not yet applied in the proximal humerus. This study examined correlations between phantom and internal density calibration, and between vBMD and estimated stiffness in the proximal humerus. Nonpathologic cadaveric CT images containing a K2HPO4 phantom were used to analyze a 10 mm region of interest below the anatomic neck. Phantom calibration was performed. Internal calibration used air (A), adipose (Ad), skeletal muscle (M), and cortical bone (C) in three combinations (AAdCM, ACM, AAdC). Finite element models (FEMs) were generated from each. Strong correlations were observed between phantom- and internally calibrated vBMD (AAdC R² = 0.80; AAdCM R² = 0.88; ACM R² = 0.90), with ACM showing the lowest error (9.98%). Estimated stiffness and vBMD were strongly correlated across calibrations (R² = 0.61-0.66), with ACM showing the lowest error (5.46%). Findings support internal calibration for determining vBMD and FEMs for estimating stiffness in the proximal humerus.

There is considerable variability in low back pain reporting during seated exposures, where pain ratings vary day-to-day, and previous pain does not consistently translate to future pain. The current study assessed within-subject differences in seated activities and spine kinematics between days with minimum and maximum pain ratings, with the goal of understanding the relationship between spine kinematic strategies and sitting-related low back pain. Over one workweek, twenty young and back-healthy participants completed their usual seated office work at their own workstation. Sitting and standing/stepping time were measured from a thigh-worn activity monitor, and spine kinematics were measured from two accelerometers on the low back. Low back pain ratings were collected on a 100 mm visual analog scale before and after each workday. Nine participants exhibited mild differences in pain between two days (≥5 mm) and time-varying activities and kinematics on their Minimum and Maximum pain days were assessed. Additionally, data from the eleven participants with stable pain responses were evaluated in an extended Bland-Altman to characterize normal daily variability. On their higher pain days, participants exhibited larger but less frequent spine movements, with no differences in activities or seated spine postures. Although the mean differences in movement amplitude (0.4°-1.7°) and timing (4-32 seconds) were small, the differences consistently exceeded the mean difference between days for the participants with stable pain responses. Thorax reclination also increased throughout the day on both pain days. Collectively, responses to low back pain in sitting appear to be linked to strategies in lumbar spine micromovements and reclining on the seatback.

Obesity is associated with increased risk of work-related musculoskeletal disorders; however, specific biomechanical effects during manual material handing (MMH) tasks remain unclear. This study examined the influence of body mass index (BMI) on shoulder and lumbar joint moments and postures during a lift/lower task and load transfer task. Sixty-three participants (29 male, 34 female), spanning normal weight to class III obesity, completed MMH tasks under varying load and distance conditions. Higher BMI was associated with greater lumbar and shoulder moments across tasks, with condition-dependent effects indicating loading did not increase uniformly with body size. During lifting, BMI-related increases in lumbar loading were most pronounced during floor-level lifts and under lighter load conditions, with effects amplified in males. During the load transfer, BMI increased shoulder moments, with stronger effects at lighter loads. Trunk and shoulder angles changed minimally, highlighting body composition as an important differential modifier of mechanical exposure during MMH.

Epidemiological studies have reported a prevalence of neck pain among rotary-wing aircrew (RWA), attributed in part to head-supported mass (HSM) including helmet and night vision goggles (NVG). Combined HSM and movement of the head away from the neutral position results in increased muscle activation and increased loads in the neck; however, the effect of HSM has not been quantified at the tissue level. In the present study, the tissue-level response to HSM was investigated using a detailed human finite element head-neck (HN) model. Non-neutral HN positions were achieved by activating the neck muscles, and three conditions were simulated for 25° flexion: (1) baseline HN model (HNF), (2) HN with helmet (HNF-H), and (3) HN with helmet, NVG and counterweight (HNF-N). In addition, a combined HN position was investigated with the helmet, NVG and counterweight, comprising 25° flexion, 8° lateral bending, and 23° axial rotation (HNC-N). Endplate stresses and annulus fibrosus (AF) fiber strains increased by 17% and 4%, respectively, with the added helmet (HNF to HNF-H). Endplate stresses and AF fiber strains increased by 24% and 12%, respectively, with the inclusion of NVG and counterweight to the helmet (HNF-H to HNF-N). In HNC-N, endplate stresses and AF fiber strains further increased by 10% and 9%, respectively, relative to HNF-N. The addition of NVG and counterweight to the helmet had a stronger influence than the helmet alone. The results can potentially quantify the high incidence of neck pain in RWA and can be applied to assess the consequences of mass additions and distribution in future systems.

Objective Observe how instruction to avoid rounding the low back while lifting a relatively light mass impacts isometric lifting strength. Background As opposed to manual materials handling training directives recommending whole-body techniques such as a squat lift, targeting specific body regions such as low back curvature, theoretically affords workers greater flexibility to organize the rest of the body to reduce musculoskeletal loading without reducing physical performance. However, providing these directives during sub-maximal tasks may not prompt prioritization of physical performance as individuals self-organize, eventually making the intervention ineffective. Methods Forty participants (50% female) lifted a crate with and without the instruction to avoid rounding the low back. Postures at the initiation of crate lifting were replicated to test isometric strength. Results At the group-level, instruction decreased low back flexion (p < 0.0001) but did not change strength (p = 0.862). However, high heterogeneity motivated examining individual responses. Thirty-seven participants (92.5% of the sample) exhibited greater than 40% of their flexion range-of-motion during baseline lifting, a threshold below which passive tissue strain is typically minimized. Yet, 22 participants (55%) were unsuccessful in reducing low back flexion below this threshold with instruction. Independent from these postural response groups, 23 maintained (57.5%), 8 increased (20%) and 9 decreased (22.5%) isometric strength. Conclusion On average, physical performance potential was maintained in response to a low back postural directive. However, personalized movement coaching is needed to ensure the desired response for all. Application Manual materials handling training should include personalized movement coaching that considers both musculoskeletal loading and performance.

Movement assessment and coaching within health and human performance settings has targeted trunk inclination and low back curvature to reduce low back injury risk. However, it remains unclear how modifying these postural characteristics may also impact the ability to exert force in multijoint tasks to influence physical performance. This study investigated the independent and interacting effects of trunk inclination and low back curvature on maximal isometric lifting strength. Forty participants (50% female) exerted maximal isometric lifting force in 4 experimentally controlled postural conditions consisting of 2 trunk inclinations (vertical and horizontal), each performed with 2 low back curvatures (neutral and flexed). A linear mixed-effects model revealed a significant group level interaction between trunk inclination and low back curvature ( β = 0.13, P = .002), but heterogeneity was high and indicated that some individual responses opposed the estimated group level response (random effect SD = 0.12). Individual responses substantially varied in magnitude (up to 620 N) and direction (increase/decrease). Modifying trunk inclination and low back curvature each have a similar potential to influence strength in multijoint tasks, but the response varies across individuals. Strength in multijoint tasks cannot be inferred solely from posture, and a single postural profile cannot be generalized as the strongest for all individuals.