Publications by authors named "Jackie D Zehr"

10 Publications

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Reconstructing an accelerometer-based pelvis segment for three-dimensional kinematic analyses during laboratory simulated tasks with obstructed line-of-sight.

J Biomech 2021 May 8;123:110512. Epub 2021 May 8.

Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada. Electronic address:

Close interface between humans and inanimate objects (furniture, assistive devices, and external loads) can obstruct line-of-sight in biomechanics studies that utilize optoelectronic motion capture systems. This specific problem is frequently encountered with the pelvis segment. This study sought to compare joint and pelvis angles computed from a pelvis-fixed local coordinate system (LCS) that was constructed from optically tracked pelvis landmarks (gold standard) and landmarks derived from angular deviations calculated from triaxial accelerometer data. One participant performed seven tasks: sitting, forward bend, sit-to-stand-to-sit, forward lunge, symmetrical squat, asymmetrical squat, and gait. The root mean square error (RMSE) and coefficient of determination (R) were examined for the pelvis, lumbar spine, and hip joint angles calculated using the standard and accelerometer-based methods for creating a LCS. The RMSE values for global pelvis angles ranged from 2.2° (gait; R = 0.47) to 4.9° (sit-to-stand-to-sit; R = 0.98), 0.6° (sitting; R = 0.88) to 7.4° (gait; R = 0.39), and 1.5° (forward bend; R = 0.99) to 2.9° (sit-to-stand-to-sit; R = 0.99) for motion about the X, Y, and Z axes, respectively. The magnitude of error observed for adjacent joint motion was lowest about the Z axis for all tasks. In conclusion, the accelerometer-based LCS offers an alternative method for computing pelvis and adjacent joint angles without the reliance on a visual line-of-sight. For motion about the X and Y axes, time-series data derived with the accelerometer-based method may be less representative of discrete events, particularly for gait and lunging tasks.
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http://dx.doi.org/10.1016/j.jbiomech.2021.110512DOI Listing
May 2021

Exploring the influence of impact severity and posture on vertebral joint mechanics in an in-vitro porcine model.

J Biomech 2021 Jun 25;122:110479. Epub 2021 Apr 25.

Department of Kinesiology, University of Waterloo, Waterloo, ON, Canada. Electronic address:

To date, no in vitro studies have been conducted to explore lumbar soft tissue injury potential and altered mechanical properties from exposure to impact forces. After a motor vehicle collision (MVC), the cause of reported acute onset low back pain is difficult to associate with potential soft tissue strain injury sites including the facet joint and innervated facet joint capsule ligament (FJC). Thus, the purpose of this investigation was to quantify intervertebral anterior-posterior (AP) translation and facet joint capsule strain under varying postures and impact severities. Seventy-two porcine spinal units were exposed to three levels of impact severity (4 g, 8 g, 11 g), and posture (Neutral, Flexion, Extension). Impacts were applied using a custom-built impact track that replicated parameters experienced in low to moderate speed rear-end MVCs. Flexion-extension and anterior-posterior shear neutral zone testing were completed prior to impact. AP intervertebral translation and the strain tensor of the facet capsule ligament were measured during impacts. A significant main effect of collision severity was observed for peak AP intervertebral translation (4 g-2.8 ±0.53 mm; 8 g-6.4 ±2.9 mm; 11 g-8.3 ±0.45 mm) and peak FJC shear strain (2.37% strain change from 4 g to 11 g impact severity). Despite observed main effects of impact severity, no influence of posture was observed. This lack of influence of posture and small FJC strain magnitudes suggest that the FJC does not appear to undergo injurious or permanent mechanical changes in response to low-to-moderate MVC impact scenarios.
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http://dx.doi.org/10.1016/j.jbiomech.2021.110479DOI Listing
June 2021

Joint fatigue-failure: A demonstration of viscoelastic responses to rate and frequency loading parameters using the porcine cervical spine.

J Biomech 2020 12 24;113:110081. Epub 2020 Oct 24.

Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada. Electronic address:

Fatigue-failure in low back tissues is influenced by parameters of cyclic loading. Therefore, this study quantified the effect of loading rate and frequency on the number of tolerated compression cycles. Energy storage and vertical deformation were secondarily examined. Thirty-two porcine spinal units were randomly assigned to experimental groups that differed by loading rate (4.2 kN/s, 8.3 kN/s) and loading frequency (0.5 Hz, 1 Hz). Following preload and range-of-motion tests, specimens were cyclically loaded in a neutral posture until fatigue-failure occurred or 10800 cycles were tolerated. Macroscopic dissection was performed to identify the fracture morphology, and measurements of energy storage and vertical displacement were calculated throughout the specimen lifespan (1%, 10%, 50%, 90%, 99%). Given the differences in compression dose-force-time integral-between experimental conditions, the number of sustained cycles were assessed following linear and nonlinear dose-normalization via correction factors calculated from existing risk-exposure approximations. Without dose-normalization, an 8.3 kN/s loading rate and 0.5 Hz loading frequency reduced the fatigue lifetime by 3541 and 5977 cycles, respectively (p < 0.001). Linear and nonlinear dose-normalization resulted in a significant rate × frequency interaction (p < 0.001). For a 1 Hz loading frequency, the number of sustained loading cycles did not differ between loading rates (p ≥ 0.988), but at 0.5 Hz, spinal units compressed at 8.3 kN/s sustained 99% (linear) and 97% (nonlinear) fewer cycles (p < 0.001). These findings demonstrate that the interacting effects of loading frequency and loading rate on spinal fatigue-failure depend on the normalization of dose discrepancies between experimental groups.
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http://dx.doi.org/10.1016/j.jbiomech.2020.110081DOI Listing
December 2020

Strain of the facet joint capsule during rotation and translation range-of-motion tests: an in vitro porcine model as a human surrogate.

Spine J 2020 03 26;20(3):475-487. Epub 2019 Sep 26.

Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada. Electronic address:

Background Context: Prior data about the modulating effects of lumbar spine posture on facet capsule strains are limited to small joint deviations. Knowledge of facet capsule strain during rotational and translational intervertebral joint motion (ie, large joint deviations) under physiological loading could be useful as it may help explain why visually normal lumbar spinal joints become painful.

Purpose: This study quantified the strain tensor of the facet capsule during rotation and translation range-of-motion tests.

Study Design/setting: Strain was calculated in isolated porcine functional spinal units. Following a preload, each specimen underwent a flexion/extension rotation (F/E) followed by an anterior/posterior translation (A/P) range-of-motion test while under a 300 N compression load.

Methods: Twenty porcine spinal units (10 C3-C4, 10 C5-C6) were tested. Joint flexion/extension was imposed by applying a ±8 Nm moment at a rate of 0.5°/s, and translation was facilitated by loading the caudal vertebra with a ±400 N shear force at a rate of 0.2 mm/s. Points were drawn on the exposed capsule surface and their coordinates were optically tracked throughout each test. Strain was calculated as the displacement of the point configuration with respect to the configuration in a neutral joint position.

Results: Compared to a neutral posture, superior-inferior strain increased and decreased systematically during flexion and extension, respectively. Posterior displacement of the caudal vertebra by more than 1.3 mm was associated with negative strains, which was significantly lower than the +4.6% strain observed during anterior displacement (p≥.199). The shear strain associated with anterior translation was, on average, -1.1% compared to a neutral joint posture.

Conclusions: These results demonstrate that there is a combination of strain types within the facet capsule when spinal units are rotated and translated. The strains documented in this study did not reach the thresholds associated with nociception.

Clinical Relevance: The magnitude of flexion-extension rotation and anterior-translation may glean insight into the facet capsule deformation response under low compression (300 N) loading scenarios. Further, intervertebral joint motion alone, even under low compression loading, does not appear to initiate a clinically relevant pain response in the lumbar facet capsule of a nondegenerated spinal joint.
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http://dx.doi.org/10.1016/j.spinee.2019.09.022DOI Listing
March 2020

Spine loading during laboratory-simulated fireground operations - inter-individual variation and method of load quantification.

Ergonomics 2019 Nov 9;62(11):1426-1438. Epub 2019 Sep 9.

Department of Kinesiology, University of Waterloo , Waterloo , ON , Canada.

Spine loading data are needed to design low-back health-preserving ergonomic interventions for firefighters. Study objectives were to quantify spine loads during simulated fireground operations using simple (polynomial) and advanced (EMG-assisted musculoskeletal model) methods and to describe the variation in spine loads between performers ( = 20). Spine compression forces differed by as much as 5.5 times bodyweight between individuals performing identical tasks. Anteroposterior and mediolateral shear forces varied by as much 3.2 and 2.1 times bodyweight between individuals performing the same tasks, respectively. Large variations in spine load magnitudes were documented regardless of whether simple or advanced quantification methods were used. Results suggest that low-back loading demands on the fireground would vary widely depending on the physical characteristics of individual firefighters, movement strategies employed, and tasks performed. Thus, personalised ergonomic interventions are warranted to regulate spine loading and load tolerance in firefighters. Even when performing the same work, the associated spine loading demands will vary widely across people due to differences in their body sizes, shapes, and movement strategies. Therefore, personalised interventions are needed to regulate spine loading and load tolerance in workers (e.g. obesity prevention, physical capacity-building exercise, and movement [re]training).
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http://dx.doi.org/10.1080/00140139.2019.1657183DOI Listing
November 2019

Examining endplate fatigue failure during cyclic compression loading with variable and consistent peak magnitudes using a force weighting adjustment approach: an study.

Ergonomics 2019 Oct 7;62(10):1339-1348. Epub 2019 Aug 7.

Department of Kinesiology, University of Waterloo , Waterloo , ON , Canada.

Repetitive movement is common in many occupational contexts. Therefore, cumulative load is a widely recognised risk factor for lowback injury. This study quantified the effect of force weighting factors on cumulative load estimates and injury prediction during cyclic loading. Forty-eight porcine cervical spine motion segments were assigned to experimental groups that differed by average peak compression magnitude (30%, 50% and 70% of predicted tolerance) and amplitude variation (consistent, variable). Cyclic loading was performed at a frequency of 0.5 Hz until fatigue failure occurred. Weighting factors were determined and applied instantaneously. Inclusion of weighting factors resulted in statistically similar cumulative load estimates at injury between variable and consistent loading ( > .071). Further, survivorship was generally greater when the peak compression magnitude was consistent compared to variable. These results emphasise the importance of weighting factors as an equalisation tool for the evaluation of cumulative low back loading exposures in occupational contexts. Weighting factors can equalise the risk of injury based on compression magnitude. When weighted, the cumulative compression was similar between consistent and variable cyclic loading protocols, despite being significantly different when unweighted and having similar injury rates. Therefore, assessing representative occupational exposures without evaluating task performance variability may underestimate injury risk. FSU: functional spinal unit; UCT: ultimate compression tolerance.
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http://dx.doi.org/10.1080/00140139.2019.1648879DOI Listing
October 2019

Partitioning the total seatback reaction force amongst the lumbar spine motion segments during simulated rear-impact collisions.

Int J Occup Saf Ergon 2021 Jun 7;27(2):613-619. Epub 2019 Jul 7.

Department of Kinesiology, University of Waterloo, Canada.

. This study aimed to determine how the seatback force is distributed across lumbar spine motion segments during a simulated low-velocity rear-impact collision with and without the application of mechanical lumbar support. . A ferroresistive pressure-sensing system was used during simulated rear-impact collisions (Δ = 7.66 km/h). Total seatback reaction force was derived from pressure recordings as the product of calibrated pressure outputs and sensel areas. The three-dimensional position of the pressure mat and the lumbar spinous processes were tracked and then used to extract the seatback force that was applied to the lumbar motion segments. . On average, 77% (637 N) and 53% (430 N) of the total seatback force was applied directly to the lumbar spine with and without lumbar support, respectively (< 0.001). In addition to four of five individual motion segments bearing a greater force with lumbar support ( < 0.029), the distribution of the total lumbar force was found to be significantly different between support type conditions. . Although lumbar supports can alter the magnitude and distribution of shear force applied to the lumbar spine during low-velocity rear-impact collisions, they do not appear to elevate the injury risk.
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http://dx.doi.org/10.1080/10803548.2019.1617455DOI Listing
June 2021

Incorporating loading variability into in vitro injury analyses and its effect on cumulative compression tolerance in porcine cervical spine units.

J Biomech 2019 May 18;88:48-54. Epub 2019 Mar 18.

Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada. Electronic address:

During repetitive movement, low-back loading exposures are inherently variable in magnitude. The current study aimed to investigate how variation in successive compression exposures influences cumulative load tolerance in the spine. Forty-eight porcine cervical spine units were randomly assigned to one of six combinations of mean peak compression force (30%, 50%, 70% of the predicted tolerance) and loading variation (consistent peak amplitude, variable peak amplitude). Following preload and passive range-of-motion tests, specimens were positioned in a neutral posture and then cyclically loaded in compression until failure occurred or the maximum 12 h duration was reached. Specimens were dissected to classify macroscopic injury and measurements of cumulative load, cycles, and height loss sustained at failure were calculated. Statistical comparisons were made between loading protocols within each normalized compression group. A significant loading variation × compression interaction was demonstrated for cumulative load (p = 0.026) and cycles to failure (p = 0.021). Cumulative compression was reduced under all normalized compression loads (30% p = 0.016; 50% p = 0.030; 70% p = 0.020) when variable loading was incorporated. The largest reduction was by 33% and occurred in the 30% compression group. The number of sustained cycles was reduced by 31% (p = 0.017), 72% (p = 0.030), and 76% (p = 0.009) under normalized compression loads of 30%, 50%, and 70%, respectively. These findings suggest that variation in compression exposures interact to reduce cumulative compression tolerance of the spine and could elevate low-back injury risk during time-varying repetitive tasks.
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http://dx.doi.org/10.1016/j.jbiomech.2019.03.011DOI Listing
May 2019

Using relative phase analyses and vector coding to quantify Pelvis-Thorax coordination during lifting-A methodological investigation.

J Electromyogr Kinesiol 2018 Apr 16;39:104-113. Epub 2018 Feb 16.

Faculty of Kinesiology & Physical Education, University of Toronto, Toronto, Ontario M5S 2W6, Canada. Electronic address:

Low-back disorder risk can be modulated by pelvis-thorax coordination when lifting. To objectively discriminate between coordination patterns during lifting, the analytical methods used require evaluation. The primary study objective was to determine if continuous relative phase (CRP) and vector coding (VC) analyses can discriminate between lifting techniques that differ based on biomechanical risk criteria. The secondary objective was to determine if normalization/transformation of input segmental angular position and velocity data is required to discriminate between lifting techniques. Sixteen volunteers performed a sagittal lifting task using freestyle (FRE), flexed spine (FLX), and neutral spine (NTL) techniques. CRP and VC analyses were implemented to quantify pelvis-thorax coordination patterns based on time-normalized, phase-normalized, and Hilbert-transformed segmental angular kinematic data. Mean relative phase angles along with thorax-only and in-phase coupling patterns were significantly different between FRE-NTL and FLX-NTL techniques (p < 0.01), but not FRE-FLX (p > 0.44). This finding was consistent across all relative phase normalization/transformation methods. Therefore, CRP and VC analyses successfully discriminated between different lifting techniques, regardless of the relative phase normalization/transformation method used.
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http://dx.doi.org/10.1016/j.jelekin.2018.02.004DOI Listing
April 2018

A comparative analysis of lumbar spine mechanics during barbell- and crate-lifting: implications for occupational lifting task assessments.

Int J Occup Saf Ergon 2020 Mar 19;26(1):1-8. Epub 2018 Mar 19.

Faculty of Kinesiology and Physical Education, University of Toronto, Canada.

. To compare the effects of object handled and handgrip used on lumbar spine motion and loading during occupational lifting task simulations. . Eight male and eight female volunteers performed barbell and crate lifts with a pronated (barbell) and a neutral (crate) handgrip. The mass of barbells/crates lifted was identical across the objects and fixed at 11.6 and 9.3 kg for men and women, respectively. The initial heights of barbells/crates were individualized to mid-shank level. Body segment kinematics and foot-ground reaction kinetics were collected, and then input into an electromyography-assisted dynamic biomechanical model to quantify lumbar spine motion and loading. . Lumbar compression and net lumbosacral moment magnitudes were 416 N and 17 Nm lower when lifting a barbell than when lifting a crate ( < 0.001), respectively. There were no between-condition differences in lumbar flexion displacements ( > 0.392) or flexion/extension velocities ( > 0.085). . Crate- and barbell-lifting tasks can be used interchangeably if assessing lifting mechanics based on peak spine motion variables. If assessments are based on the spine loading responses to task demands, however, then crate- and barbell-lifting tasks cannot be used interchangeably.
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http://dx.doi.org/10.1080/10803548.2018.1439872DOI Listing
March 2020