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Papers by David Hawkins

Anterior cruciate ligament (ACL) injuries are common, costly and have long-term health consequenc... more Anterior cruciate ligament (ACL) injuries are common, costly and have long-term health consequences. Despite decades of ACL injury research, ACL injury incidence remains high, highlighting the need for new approaches. We hypothesize that many non-contact ACL injuries result from an overuse, fatigue failure mechanism and that advances in wearable technology provide an exciting opportunity to obtain data needed to test this hypothesis. In this study, we created and evaluated an inertial measurement unit (IMU) based wearable device to collect a clinically relevant metric of ACL loading events from athletes in the field. The wearable device measures relative anterior tibial acceleration (RATA), a surrogate measure of ACL loading defined as the relative acceleration between a point on the proximal tibia and a point on the distal femur. The device was evaluated using a mechanical test system designed to simulate human leg movement, and then used to quantify peak RATA profiles for two part...

JMIR research protocols, Jan 4, 2015

Computer tailored, Web-based interventions have emerged as an effective approach to promote physi... more Computer tailored, Web-based interventions have emerged as an effective approach to promote physical activity. Existing programs, however, do not adjust activities according to the participant's compliance or physiologic adaptations, which may increase risk of injury and program attrition in sedentary adults. To address this limitation, objective activity monitor (AM) and heart rate data could be used to guide personalization of physical activity, but improved Web-based frameworks are needed to test such interventions. The objective of this study is to (1) develop a personalized physical activity prescription (PPAP) app that combines dynamic Web-based guidance with multi-sensor AM data to promote physical activity and (2) to assess the feasibility of using this system in the field. The PPAP app was constructed using an open-source software platform and a custom, multi-sensor AM capable of accurately measuring heart rate and physical activity. A novel algorithm was written to use...

Journal of Biomechanics, 2003

Bone mass and size clearly affect the safety and survival of wild animals as well as human beings... more Bone mass and size clearly affect the safety and survival of wild animals as well as human beings, however, little is known about the interactions between bone size and movement dynamics. A modeling approach was used to investigate the hypothesis that increased bone cortical area causes increased limb moments of inertia, decreased lower-limb movement maximum velocities, and increased energy requirements to sustain submaximum lower-limb locomotion movements. Custom software and digital data of a human leg were used to simulate femur, tibia, and fibula cortical bone area increases of 0%, 22%, 50%, and 80%. Limb segment masses, center of mass locations, and moments of inertia in the sagittal plane were calculated for each bone condition. Movement simulations of unloaded running and cycling motions were performed. Linear regression analyses were used to determine the magnitude of the effect cortical area has on limb moment of inertia, velocity, and the internal work required to move the limbs at a given velocity. The thigh and shank moment of inertia increased linearly up to 1.5% and 6.9%, respectively for an 80% increase in cortical area resulting in 1.3% and 2.0% decreases in maximum unloaded cycling and running velocities, respectively, and in 3.0% and 2.9% increases in internal work for the cycling and running motions, respectively. These results support the hypothesis and though small changes in movement speed and energy demands were observed, such changes may have played an important role in animal survival as bones evolved and became less robust.

American Journal of Veterinary Research, 2005

Objective—To investigate the biomechanics of cervical vertebral motion units (VMUs) before and af... more Objective—To investigate the biomechanics of cervical vertebral motion units (VMUs) before and after a ventral slot procedure and after subsequent pin-polymethylmethacrylate (pin-PMMA) fixation and to assess the use of smooth and positive-profile threaded (PPT) pins in pin-PMMA fixation and intravertebral pin placement. Sample Population—Cervical portions (C3 through C6 vertebrae) of 14 cadaveric canine vertebral columns. Procedure—Flexion and extension bending moments were applied to specimens before and after creation of a ventral slot across the C4-C5 intervertebral space and after subsequent smooth or PPT pin-PMMA fixation at that site. Data for the C3-C4, C4-C5, and C5-C6 VMUs were compared among treatments and between pin types, and pin protrusion was compared between pin types. Results—Compared with values in intact specimens, ventral slot treatment increased neutral zone range of motion (NZ-ROM) by 98% at the treated VMUs and appeared to decrease overall ROM at adjacent VMUs...

Sports Injuries & Medicine, 2017

Background: Increased Anterior Knee Laxity (AKL) is a risk factor for Anterior Cruciate Ligament ... more Background: Increased Anterior Knee Laxity (AKL) is a risk factor for Anterior Cruciate Ligament (ACL) injury, and has been reported to fluctuate throughout a woman's menstrual cycle in proportion to fluctuations in hormone concentrations. Women have a greater rate of ACL injuries compared to men, which might be explained by fluctuations in hormone concentrations and AKL in women. Though studies have reported a linear relationship between hormone fluctuations and AKL within a menstrual cycle, it is not clear if this relationship is consistent between cycles. To gain a better understanding of hormone fluctuation effects on ACL injury risk, we need a better understanding of the relationship between hormone concentrations and AKL, and the repeatability of this relationship. Hypothesis/Purpose: We hypothesized that AKL would vary linearly with estrogen concentration and this linear relationship would not vary significantly between months in healthy women with regular menstrual cycles. Study Design: This is a cross-sectional study. Methods: Thirteen females aged 21±1 years with regular menstrual cycles completed the study. Their AKL and estradiol concentrations were quantified at menses and ovulation for three months. Linear regression analysis was used to determine relationships between AKL and estradiol concentrations for each month and compared for each subject. These relationships were compared using Fisher-Z-transformation and an equality test of correlations. Results: AKL at menses and ovulation, and the difference in AKL between menses and ovulation, were highly variable between months for 63%, 52%, and 97% of the comparisons respectively. Estradiol concentration at menses and ovulation, and the difference in estradiol concentration between menses and ovulation, were highly variable between months for 69%, 33%, and 81% of the comparisons, respectively. The relationship between estradiol and AKL was significantly different for at least one month for four subjects. Conclusion: There can be high variability between menstrual cycles for AKL at menses, ovulation, and the change between menses and ovulation, and for estradiol concentration at menses and the change between menses and ovulation. The relationship between estradiol and AKL can vary significantly between months for some women. Salivary estradiol concentration changes alone are not sufficient to predict AKL changes.

Advances in Orthopedics and Sports Medicine

Force generated within a muscle-tendon unit (either through muscle activation or stretch) is comm... more Force generated within a muscle-tendon unit (either through muscle activation or stretch) is commonly thought to be transmitted serially from one structure to the next, referred to as myotendinous force transmission. However, there exist alternate pathways that employ intramuscular and intermuscular connective tissues to transmit force laterally, referred to as lateral force transmission (LFT). The extensiveness of LFT and the mechanisms responsible for it are poorly understood. Lateral force transmission was investigated in normal and partially compromised passive skeletal muscle systems to determine the fraction of total system force that can be transferred laterally and to investigate possible mechanisms contributing to LFT. Chicken peroneus longus (PL)/middle gastrocnemius (MG)/fascia complexes were isolated taking care to maintain their normal connection to each other. They were attached to a testing fixture that allowed removal and reattachment of the distal end of the muscles. Tensile tests were conducted under three levels of tenotomy (i.e. both muscles attached, only PL attached, only MG attached) and three levels of fasciotomy (100%, 66% and 33% intact). LFT between muscles was sizeable and showed directionality (32% of the force applied distally to the MG muscle was transferred laterally to the PL muscle while only 16% of the force applied distally to the PL was transferred laterally to the MG). There was poor correlation between LFT and the ratio of muscle elastic moduli. The contact area between muscles and the ratio of fascia to muscle elastic moduli were the greatest contributors to LFT.

Journal of biomechanics, Jun 4, 2016

Race surface mechanics contribute to musculoskeletal injury in racehorses. These mechanics affect... more Race surface mechanics contribute to musculoskeletal injury in racehorses. These mechanics affect ground reaction forces applied to the hoof, and thus limb motions during stance that can contribute to musculoskeletal pathologies. Race surface design has been largely empirical within the industry, with little uniform consensus for injury prevention and performance. Furthermore, race surface installations are too expensive to install experimentally. The objective of this research was to develop and evaluate an integrated racehorse limb and race surface computational model. Combined forward/inverse dynamic simulations of distal leading forelimb motions of a galloping horse during stance were compared to 2D distal leading forelimb kinematics of actual galloping racehorses on race surfaces with measured mechanics. Model predicted angular and translational kinematic profiles had similar qualitative shapes as experimental data, with comparable peak magnitudes. Model predictions of peak met...

PloS one, 2014

Measures of cardiorespiratory fitness (CRF) and heart rate recovery (HRR) can improve risk strati... more Measures of cardiorespiratory fitness (CRF) and heart rate recovery (HRR) can improve risk stratification for cardiovascular disease, but these measurements are rarely made in asymptomatic individuals due to cost. An exercise field test (EFT) to assess CRF and HRR would be an inexpensive method for cardiovascular disease risk assessment in large populations. This study assessed 1) the predictive accuracy of a 12-minute run/walk EFT for estimating CRF ([Formula: see text]) and 2) the accuracy of HRR measured after an EFT using a heart rate monitor (HRM) in an asymptomatic population. Fifty subjects (48% women) ages 18-45 years completed a symptom-limited exercise tolerance test (ETT) (Bruce protocol) and an EFT on separate days. During the ETT, [Formula: see text] was measured by a metabolic cart, and heart rate was measured continuously by a HRM and a metabolic cart. EFT distance and sex independently predicted[Formula: see text]. The average absolute difference between observed and...

Journal of Biomechanics, 2015

Race surfaces have been associated with the incidence of racehorse musculoskeletal injury, the le... more Race surfaces have been associated with the incidence of racehorse musculoskeletal injury, the leading cause of racehorse attrition. Optimal race surface mechanical behaviors that minimize injury risk are unknown. Computational models are an economical method to determine optimal mechanical behaviors. Previously developed equine musculoskeletal models utilized ground reaction floor models designed to simulate a stiff, smooth floor appropriate for a human gait laboratory. Our objective was to develop a computational race surface model (two force-displacement functions, one linear and one nonlinear) that reproduced experimental race surface mechanical behaviors for incorporation in equine musculoskeletal models. Soil impact tests were simulated in a musculoskeletal modeling environment and compared to experimental force and displacement data collected during initial and repeat impacts at two racetracks with differing race surfaces - (i) dirt and (ii) synthetic. Best-fit model coefficients (7 total) were compared between surface types and initial and repeat impacts using a mixed model ANCOVA. Model simulation results closely matched empirical force, displacement and velocity data (Mean R(2)=0.930-0.997). Many model coefficients were statistically different between surface types and impacts. Principal component analysis of model coefficients showed systematic differences based on surface type and impact. In the future, the race surface model may be used in conjunction with previously developed the equine musculoskeletal models to understand the effects of race surface mechanical behaviors on limb dynamics, and determine race surface mechanical behaviors that reduce the incidence of racehorse musculoskeletal injury through modulation of limb dynamics.

PLoS ONE, 2014

Simple methods to quantify ground reaction forces (GRFs) outside a laboratory setting are needed ... more Simple methods to quantify ground reaction forces (GRFs) outside a laboratory setting are needed to understand daily loading sustained by the body. Here, we present methods to estimate peak vertical GRF (pGRFvert) and peak braking GRF (pGRFbrake) in adults using raw hip activity monitor (AM) acceleration data. The purpose of this study was to develop a statistically based model to estimate pGRFvert and pGRFbrake during walking and running from ActiGraph GT3X+ AM acceleration data. 19 males and 20 females (age 21.261.3 years, height 1.7360.12 m, mass 67.6611.5 kg) wore an ActiGraph GT3X+ AM over their right hip. Six walking and six running trials (0.95-2.19 and 2.20-4.10 m/s, respectively) were completed. Average of the peak vertical and anterior/posterior AM acceleration (ACCvert and ACCbrake, respectively) and pGRFvert and pGRFbrake during the stance phase of gait were determined. Thirty randomly selected subjects served as the training dataset to develop generalized equations to predict pGRFvert and pGRFbrake. Using a holdout approach, the remaining 9 subjects were used to test the accuracy of the models. Generalized equations to predict pGRFvert and pGRFbrake included ACCvert and ACCbrake, respectively, mass, type of locomotion (walk or run), and type of locomotion acceleration interaction. The average absolute percent differences between actual and predicted pGRFvert and pGRFbrake were 8.3% and 17.8%, respectively, when the models were applied to the test dataset. Repeated measures generalized regression equations were developed to predict pGRFvert and pGRFbrake from ActiGraph GT3X+ AM acceleration for young adults walking and running. These equations provide a means to estimate GRFs without a force plate.

To address a variety of questions pertaining to the interactions between physical activity, muscu... more To address a variety of questions pertaining to the interactions between physical activity, musculoskeletal loading and musculoskeletal health/injury/adaptation, simple methods are needed to quantify, outside a laboratory setting, the forces acting on the human body during daily activities. The purpose of this study was to develop a statistically based model to estimate peak vertical ground reaction force (pVGRF) during youth gait. 20 girls (10.960.9 years) and 15 boys (12.560.6 years) wore a Biotrainer AM over their right hip. Six walking and six running trials were completed after a standard warm-up. Average AM intensity (g) and pVGRF (N) during stance were determined. Repeated measures mixed effects regression models to estimate pVGRF from Biotrainer activity monitor acceleration in youth (girls 10-12, boys 12-14 years) while walking and running were developed. Log transformed pVGRF had a statistically significant relationship with activity monitor acceleration, centered mass, sex (girl), type of locomotion (run), and locomotion type-acceleration interaction controlling for subject as a random effect. A generalized regression model without subject specific random effects was also developed. The average absolute differences between the actual and predicted pVGRF were 5.2% (1.6% standard deviation) and 9% (4.2% standard deviation) using the mixed and generalized models, respectively. The results of this study support the use of estimating pVGRF from hip acceleration using a mixed model regression equation.

Journal of Applied Biomechanics, 2014

Wearable accelerometer-based activity monitors (AMs) are used to estimate energy expenditure and ... more Wearable accelerometer-based activity monitors (AMs) are used to estimate energy expenditure and ground reaction forces in free-living environments, but a lack of standardized calibration and data reporting methods limits their utility. The objectives of this study were to (1) design an inexpensive and easily reproducible AM testing system, (2) develop a standardized calibration method for accelerometer-based AMs, and (3) evaluate the utility of the system and accuracy of the calibration method. A centrifuge-type device was constructed to apply known accelerations (0-8g) to each sensitive axis of 30 custom and two commercial AMs. Accelerometer data were recorded and matrix algebra and a least squares solution were then used to determine a calibration matrix for the custom AMs to convert raw accelerometer output to units of g’s. Accuracy was tested by comparing applied and calculated accelerations for custom and commercial AMs. AMs were accurate to within 4% of applied accelerations....

Veterinary Surgery, 2003

Introduction and Hypothesis: Local intramuscular injections of botulinum toxin in humans and seve... more Introduction and Hypothesis: Local intramuscular injections of botulinum toxin in humans and several animal species inhibit the release of acetylcholine in pre-synaptic motor end-plates resulting in prolonged reversible muscle relaxation. The hypothesis of this study was that intramuscular injections of Myoblock® (botulinum toxin type-B) in the external anal sphincter of horses would cause reduction of anal sphincter tone without causing systemic side effects. Materials and Methods: Peak and resting anal sphincter pressures were measured with a custom made rectal probe connected to a pressure transducer. Pressures were measured before treatment and after injections with Myoblock® or saline and until any changes in sphincter pressure returned to normal. The horses' physical changes, behavior, and anal pressure were recorded. Results: Injections of botulinum toxin type-B produced a reduction in anal sphincter tone and peak anal sphincter pressure a few days after injections. Return to baseline peak anal pressure was observed within 168 days. When compared to controls, there was a significant decrease in peak anal pressure after botulinum toxin injection. Injection in the anal sphincter with 2500 units of type-B toxin in one horse produced transient signs of depression, generalized weakness, and dysphagia. Clinical side effects were not observed in horses after injections with 500, 1000, or 1500 units of toxin. Discussion: The effect of intramuscular injection of botulinum toxin type-B in horses is similar to other species. However, horses, compared to other species, are very sensitive to the toxin and clinical signs of botulism may develop with doses that don't cause a systemic effect in other species. The results of this study suggest that a single treatment with botulinum toxin type-B in the external anal sphincter of mares has the potential to reduce incisional dehiscence after repair of perineal lacerations.

Sports Engineering, 2003

Rowing requires an orderly, coordinated , and powerful sequence of actions from every major muscl... more Rowing requires an orderly, coordinated , and powerful sequence of actions from every major muscle group in the body; thus, a successful rower must have good technique, timing, and power. Currently, coaches alter a rower's technique based on fundamental physical principles and, more commonly, the coach's experience. Rowers can also benefit from objective visual feedback during training (Henry et al., 1995; Macfarlane et al., 1997). Coaches and athletes could more accurately and scientifically make technique changes if they had real-time, quantitative kinematic and kinetic data available. While a biomechanical feedback system incorporated into a rowing shell would provide optimal benefit (because changes in technique could be related directly to measurement of changes in boat speed), a system of this nature is somewhat impractical due to the complexity and expense of telemetry, and the physical distance between the rower's shell and the coaching launch (Mahler et al., 1984; Kramer et al., 1994; Macfarlane et al., 1997). A land-based biomechanical feedback system provides a good alternative to an onwater system, and an efficient approach to develop technology that could be transferred to an on-water

Medicine & Science in Sports & Exercise, 2006

... PURPOSe: the aim of this study was to evaluate the effect of physical training on the reactiv... more ... PURPOSe: the aim of this study was to evaluate the effect of physical training on the reactivity of pulmonary artery rings from rats submitted to a pulmonary IR. MeTHOdS:Wistar male rats were divided into five groups named: control sedentary (SD); Trained (TR); sham ...

Medicine & Science in Sports & Exercise, 2007

Journal of Biomechanics, 2012

The purposes of this study were (1) determine if youth peak Achilles tendon (AT) strain, peak AT ... more The purposes of this study were (1) determine if youth peak Achilles tendon (AT) strain, peak AT stress, and AT stiffness, measured during an isometric plantar flexion, differed after six months (mos) of growth, and (2) determine if sex, physical activity level (Physical Activity Questionnaire (PAQ-C)), and/or growth rate (GR) were related to these properties. AT stress, strain, and stiffness were quantified in 20 boys (13.47±0.81 years) and 22 girls (11.18±0.82 years) at 2 times (0 and 6 mos). GR (change in height in 6 mos) was not significantly different between boys and girls (3.5±1.4 and 3.4±1.1 cm/6 mos, respectively). Peak AT strain and stiffness (mean 3.8±0.4% and 128.9±153.6 N/mm, respectively) did not differ between testing sessions or sex. Peak AT stress (22.1±2.4 and 24.0±2.1 MPa at 0 and 6 mos, respectively) did not differ between sex and increased significantly at 6 mos due to a significant decrease in AT cross-sectional area (40.6±1.3 and 38.1±1.6 mm 2 at 0 and 6 mos, respectively) with no significant difference in peak AT force (882.3±93.9 and 900.3± 65.5 N at 0 and 6 mos, respectively). Peak AT stress was significantly greater in subjects with greater PAQ-C scores (9.1% increase with 1 unit increase in PAQ-C score) and smaller in subjects with faster GRs (13.8% decrease with 1 cm/6 mos increase in GR). These results indicate that of the AT mechanical properties quantified, none differed between sex, and only peak AT stress significantly differed after 6 months and was related to GR and physical activity.

Journal of Biomechanics, 2005

The superficial (SDF) and deep digital flexor (DDF) muscles are critical for equine forelimb loco... more The superficial (SDF) and deep digital flexor (DDF) muscles are critical for equine forelimb locomotion. Knowledge of their mechanical properties will enhance our understanding of limb biomechanics. Muscle contractile properties derived from architectural-based algorithms may overestimate real forces and underestimate shortening capacity because of simplistic assumptions regarding muscle architecture. Therefore, passive and active (=total - passive) force-length properties of the SDF and DDF muscles were measured directly in vivo. Muscles from the right forelimbs of four Thoroughbred horses were evaluated during general anesthesia. Limbs were fixed to an external frame with the muscle attached to a linear actuator and load cell. Each muscle was stretched from an unloaded state to a range of prefixed lengths, then stimulated while held at that length. The total force did not exceed 4000 N, the limit for the clamping device. The SDF and DDF muscles produced 716+/-192 and 1577+/-203 N maximum active isometric force (F(max)), had ascending force-length ranges (R(asc)) of 5.1+/-0.2 and 9.1+/-0.4 cm, and had passive stiffnesses of 1186+/-104 and 1132+/-51 N/cm, respectively. The values measured for F(max) were much smaller than predicted based on conservative estimates of muscle specific tension and muscle physiological cross-sectional area. R(asc) were much larger than predicted based on muscle fiber length estimates. These data suggest that accurate prediction of the active mechanical behavior of architecturally complex muscles such as the equine DDF and SDF requires more sophisticated algorithms.