Predicting head injury metrics during low- to moderate-speed frontal collisions using computational simulations.

OBJECTIVES: Subjective reports of mild traumatic brain injury (mTBI) are common following low-energy motor-vehicle collisions. Biomechanical analyses are useful in providing a quantitative means for determining the likelihood of sustaining mTBI. While occupant dynamics in low-speed rear impacts have been extensively investigated, peer-reviewed studies on occupant dynamics during low-speed frontal collisions are sparse. The objective of this study is to present a validated computational method to quantify the biomechanical response of the head in low- and moderate-speed frontal collisions. DATA AND METHODS: This study used data from a previously-published series of four instrumented in-line front-to-rear staged collisions using 2014 Honda Accord sedans at closing velocities of approximately 7.4 kph (test L1), 12.7 kph (test L2), 21.7 kph (test L3), and 33.6 kph (test L4) kph. A model of the test vehicle occupant compartment was created using the MAthematical DYnamic MOdeling (MADYMO) software using methods previously described. Crash pulse data from L4 were applied to the MADYMO model. Seat belt parameters were optimized to achieve reasonable agreement between simulation results and test data for relevant head injury metrics (linear head acceleration [LHA], angular head acceleration [AHA], and HIC15). Crash pulses from the other tests in the series (L1, L2, and L3) were then applied to the model and peak values for LHA, AHA, and HIC15 were compared to the physical test data to demonstrate validation. RESULTS: The optimization of seat belt and seat parameters within the MADYMO model resulted in accurate prediction of ATD dynamics demonstrated in Test L4. The simulation-predicted peak LHA was within 0.2 g of the test value, peak AHA was within 64 rad/s2, and HIC15 was within 0.46. When applied to the remainder of the tests (L1, L2, and L3), the optimized model showed excellent accuracy in predicting peak LHA and HIC15. When compared to the physical test data, the simulation-predicted values for LHA were within 0.4 g or less and the HIC15 values were within 0.4 or less across all tests. The model generally over-predicted AHA, particularly for the lower-severity collisions (L1 and L2). CONCLUSIONS: We have demonstrated a reliable methodology for developing a biomechanical computational model to predict head injury metrics in low- to moderate-speed frontal collisions. This approach can be particularly valuable in forensic investigations of real-world crashes. Pre-existing crash test data can be used in conjunction with exemplar vehicle information to validate a MADYMO model. Appropriate crash pulse data from classical accident reconstruction techniques, event data recorders, or simulations can then be applied to the model to accurately predict head dynamics for real-world vehicle occupants without the need for full-scale staged crash tests.

Transtibial prosthetic alignment has small effects on whole-body angular momentum during functional tasks.

Due to the loss of ankle function, many people with a transtibial amputation (TTA) have difficulty maintaining balance during functional tasks. Prosthetic alignment may affect how people with TTA maintain balance as it affects ground reaction forces (GRFs) and centers of pressure. We quantified the effect of prosthetic alignment on dynamic balance during several functional tasks. Ten people with TTA and 10 controls without TTA completed tasks including walking and transitioning from a chair. Participants with TTA completed all tasks with their prescribed alignment and six shifted alignments, including ±10 mm anterior/posterior, medial/lateral, and ±20 mm in the vertical direction. For each task, we quantified dynamic balance as the range of whole-body angular momentum (H→WB) and quantified trunk range of motion (ROM) and peak GRFs. Compared to controls, participants with TTA using their prescribed alignment had a greater range of H→WB in the sagittal plane during walking, in all planes during sit-to-stand, and in the transverse plane during stand-to-sit. These results were associated with GRF and trunk ROM differences between participant groups. Alignment only affected the range of H→WB in the frontal plane during walking. The larger range for the tall alignment coincided with a greater difference in vertical GRF between intact and amputated legs compared to other alignments. Our findings suggest that people with TTA can adapt to small, translational, alignment changes to maintain similar levels of dynamic balance during chair transitions. Future work should investigate alignment changes during other tasks and in lower functioning individuals.

The ins and outs of dynamic balance during 90-degree turns in people with a unilateral transtibial amputation.

The ability to maintain balance when turning is essential to functional and independent living. Due to the lack of neuromuscular ankle control on the prosthetic side in people with a transtibial amputation (TTA), turning is likely more challenging. The purpose of this study was to quantify how people with TTA maintain dynamic balance during 90-degree turns made with the prosthesis on the inside and outside of the turn compared to people without amputation. Eight participants with TTA and eight age-, height-, and sex- matched non-amputee controls performed left and right 90-degree step turns at a self-selected speed. The primary outcomes were range of whole-body angular momentum and positive and negative contributions of six segment groups (head/trunk, pelvis, arms, and legs) to whole-body angular momentum during the continuation stride. Participants with TTA had greater range of frontal- and sagittal-plane whole-body angular momentum when turning with the prosthesis on the inside compared controls. They also had a greater range of whole-body angular momentum in all planes of motion when turning with the prosthesis on the inside compared to outside of the turn. The contributions for the head/trunk and inside and outside legs differed between groups and turns, suggesting altered interactions between segment momenta to compensate for the reduced contribution of the amputated leg. This study provides insight into possible training paradigms to reduce the high incidence of turn related falls in people with TTA and, potentially, ways to alter prosthetic function to promote balance control.

Effects of anterior-posterior shifts in prosthetic alignment on the sit-to-stand movement in people with a unilateral transtibial amputation.

The sit-to-stand movement can be challenging for people with a transtibial amputation (TTA). The alignment of the prosthesis may influence the movement strategies people with TTA use to transfer from sit-to-stand by affecting foot placement. The purpose of this study was to determine how shifting the prosthetic foot anterior and posterior relative to the socket affects movement strategies used to transfer from sit-to-stand. To aid in interpretation, we compared movement strategies between people with and without TTA. Nine people with TTA and nine sex-, and age-matched non-amputee controls completed five self-paced sit-to-stand trials. With the posterior alignment, participants with TTA had 1) smaller braking GRF impulse on the prosthetic side and greater impulse on the intact side compared to the anterior alignment, 2) no significant differences between sides, which suggests greater braking impulse symmetry compared to anterior and prescribed alignments, and 3) smaller axial trunk range of motion compared to the prescribed alignment. There were also differences between participants with TTA and controls in braking GRF impulse, knee extension moment, anterior/posterior center of pressure position, and lateral and axial trunk range of motion. Based on these results, shifting the prosthetic foot posterior to the socket may be a useful tool to reduce braking impulse asymmetry and trunk motion in people with TTA during sit-to-stand. Thus, prosthetic alignment can have important implications for the comfort and ability of people with TTA to transfer from sit-to-stand as well as for development of secondary health conditions like low back pain, which is associated with compensatory movements.

The effect of lower-limb prosthetic alignment on muscle activity during sit-to-stand.

People with a transtibial amputation (TTA) have altered motion during daily tasks, which may be influenced by prosthetic alignment. This study aimed to determine the effect of medial/lateral prosthetic alignment shifts on muscle activity, measured by integrated electromyography (iEMG), and to compare muscle activity between people with and without TTA during sit-to-stand. We quantified ground reaction forces and three-dimensional center-of-mass position to interpret muscle activity results. Compared to the prescribed alignment, the bilateral knee extensors had greater activity in the medial alignment (p < 0.001) and the amputated side gluteus medius and less activity in the lateral alignment (p = 0.035), which may be a result of altered muscular requirements for postural control. In people with TTA, smaller intact side gluteus medius activity was associated with frontal plane motion of the center-of-mass, which was not observed in non-amputees. Compared to non-amputees, people with TTA had greater iEMG in the intact side tibialis anterior (p = 0.031) and amputated side rectus femoris (p < 0.001), which may be required to brake the body center-of-mass in the absence of amputated side tibialis anterior. These results suggest that lateral alignment shifts may reduce muscle activity during sit-to-stand for people with TTA and emphasize the importance of analyzing sit-to-stand in three dimensions.

Whole-body and segment angular momentum during 90-degree turns.

BACKGROUND: Turning is a frequently performed, asymmetric task of daily living. The asymmetric nature makes turning challenging to perform while maintaining balance. RESEARCH QUESTION: How do healthy individuals maintain dynamic balance, quantified as whole-body angular momentum, during a 90-degree turn compared to straight-line walking? METHODS: The kinematics of sixteen healthy individuals were tracked during walking in a straight-line and during left and right 90-degree turns at a comfortable pace. Whole-body and segment angular momenta were calculated and the relative contributions of the legs, arms, pelvis and head/trunk to whole-body angular momentum were evaluated. RESULTS: Average whole-body angular momentum was different during turning compared to straight-line walking in all planes of motion. The initiation of a turn required generation of whole body angular momentum in all three planes of motion, which was counteracted at the end of the turn by a generation of angular momentum in the opposite direction in the frontal and sagittal planes. Transverse plane momentum was always directed in the turn direction. All segment groups, except for the inside leg, had a greater magnitude of angular momentum during turning compared to straight-line walking. The outside leg and head/trunk segments were the largest contributors to frontal and transverse plane whole-body angular momentum. SIGNIFICANCE: Understanding how body segments contribute to maintaining balance during a 90-degree turn can be useful for designing rehabilitation paradigms for people who have difficulty turning or impaired balance.

Lumbar loads and trunk kinematics in people with a transtibial amputation during sit-to-stand.

People with a transtibial amputation have numerous secondary health conditions, including an increased prevalence of low back pain. This increased prevalence may be partially explained by altered low back biomechanics during movement. The purpose of this study was to compare trunk kinematics and L4-L5 lumbar loads in people with and without a transtibial amputation during sit-to-stand. Motion capture, ground reaction force and electromyographic data were collected from eight people with a unilateral transtibial amputation and eight people without an amputation during five self-paced sit-to-stand motions. A musculoskeletal model of the torso, lumbar spine, pelvis, lower limbs, and 294 muscles was used in a static optimization framework to quantify L4-L5 loads, low back muscle forces, and trunk kinematics. Participants with an amputation had greater peak and average L4-L5 loading in compression compared to control participants, with peak loading occurring shortly after liftoff from the chair. At the instant of peak loading, participants with an amputation had significantly greater segmental trunk lateral bending and trunk-pelvis axial rotation toward the intact side, and significantly greater segmental trunk axial rotation toward the prosthetic side compared to control participants. Participants with an amputation also had greater peak frontal plane and transverse plane segmental trunk angular velocity. The postural differences observed in people with a transtibial amputation were consistent with their ground reaction force asymmetry. The cumulative effects of the altered movement strategy used by people with an amputation may result in an increased risk for low back pain development over time.

Validation of lumbar spine loading from a musculoskeletal model including the lower limbs and lumbar spine.

Low back mechanics are important to quantify to study injury, pain and disability. As in vivo forces are difficult to measure directly, modeling approaches are commonly used to estimate these forces. Validation of model estimates is critical to gain confidence in modeling results across populations of interest, such as people with lower-limb amputation. Motion capture, ground reaction force and electromyographic data were collected from ten participants without an amputation (five male/five female) and five participants with a unilateral transtibial amputation (four male/one female) during trunk-pelvis range of motion trials in flexion/extension, lateral bending and axial rotation. A musculoskeletal model with a detailed lumbar spine and the legs including 294 muscles was used to predict L4-L5 loading and muscle activations using static optimization. Model estimates of L4-L5 intervertebral joint loading were compared to measured intradiscal pressures from the literature and muscle activations were compared to electromyographic signals. Model loading estimates were only significantly different from experimental measurements during trunk extension for males without an amputation and for people with an amputation, which may suggest a greater portion of L4-L5 axial load transfer through the facet joints, as facet loads are not captured by intradiscal pressure transducers. Pressure estimates between the model and previous work were not significantly different for flexion, lateral bending or axial rotation. Timing of model-estimated muscle activations compared well with electromyographic activity of the lumbar paraspinals and upper erector spinae. Validated estimates of low back loading can increase the applicability of musculoskeletal models to clinical diagnosis and treatment.

Kinematic differences during a jump cut maneuver between individuals with and without a concussion history.

Recent evidence suggests that athletes are at a higher risk of lower-body injuries in the months and years following a concussion. However, little is known about how people modify their movements post-concussion. This study examined kinematics during a jump cut motion in young adults with a concussion history (n=9; 4 males, 5 females; 3.1years' post-injury) and 10 controls (6 males, 4 females). Peak center of mass and peak knee angles during the landing phase of a jump-cut maneuver were evaluated. Participants with a concussion history demonstrated decreased knee varus (left: Mconc=-0.5±1.0°, Mctrl=3.6±1.0°; right: Mconc=5.1±1.2°, Mctrl=7.8±1.12°) and external rotation (left: Mconc=2.5±1.6°, Mctrl=13.0±1.5°; right: Mconc=7.7±1.6°, Mctrl=12.8±1.5°) regardless of whether the cut was oriented towards to the left or right. The kinematic patterns demonstrated in individuals with a concussion history may be suggestive of increased knee injury risk. This study adds to the growing body of literature linking orthopedic injury in those no longer displaying the acute signs and symptoms of concussion.