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Tsai L.-C.,University of Southern California | Tsai L.-C.,Northwestern University | Scher I.S.,Guidance Engineering and Applied Research | Powers C.M.,University of Southern California
Journal of Applied Biomechanics | Year: 2013

The purpose of this study is to describe an MRI-based EMG-driven knee model to quantify tibiofemoral compressive and shear forces. Twelve healthy females participated. Subjects underwent 2 phases of data collection: (1) MRI assessment of the lower extremity to quantify muscle volumes and patella tendon orientation and (2) biomechanical evaluation of a drop-jump task. A subject-specific EMG-driven knee model that incorporated lower extremity kinematics, EMG, and muscle volumes and patella tendon orientation estimated from MRI was developed to quantify tibiofemoral shear and compressive forces. A resultant anterior tibial shear force generated from the ground reaction force (GRF) and muscle forces was observed during the first 30% of the stance phase of the drop-jump task. All of the muscle forces and GRF resulted in tibiofemoral compression, with the quadriceps force being the primary contributor. Acquiring subject-specific muscle volumes and patella tendon orientation for use in an EMG-driven knee model may be useful to quantify tibiofemoral forces in persons with altered patella position or muscle atrophy following knee injury or pathology. © 2013 Human Kinetics, Inc.

Shealy J.,Guidance Engineering and Applied Research | Shealy J.,Rochester Institute of Technology | Scher I.,Guidance Engineering and Applied Research | Scher I.,University of Southern California | And 2 more authors.
Journal of ASTM International | Year: 2010

The performance of individuals jumping tabletop features in terrain parks has noteen widely studied. A field study was conducted to measure the takeoff speed and horizontal distance achieved by jumpers at two tabletop style jump features: A smaller jump at Snow Summit, CA and a larger jump at Mammoth Mountain, CA, USA. Analyses were also conducted to determine the effects of equipment type (skis versus snowboard) on jumper kinematics. Before each data collection session, the physical dimensions of the jump were measured and recorded. For consecutive jumpers, the speed parallel to the ramp was measured at the end of the takeoff by using a laser speed trap accurate to 0.11 m/s. The landing zone was marked at 3.0 m intervals with colored dye, beginning just past the flat deck portion of the jump. A high-definition video camera was used to record the landing of each jumper, and the landing point of the projected center of mass was determined by using photogrammetry, accurate to within 5 cm. The actual landing distances measured in the field were compared to predicted landing distances by using ordinary ballistic equations. A total of 280 jumps was observed on the two features: 105 on skis and 175 on snowboards. The correlation coefficients between the square of the takeoff speed and the landing distance for the smaller and larger jumps were 0.75 (R2=0.56) and 0.41 (R2=0.16), respectively. The measured landing distances differed significantly from those predicted by models using the ballistic equations of motion. The average landing distance beyond the knuckle was 2.4 m on the smaller jump and 2.1 m on the larger jump. For both features, skier and snowboarder jumpers did not differ in average landing distance. Copyright © 2010 by ASTM International.

Harley E.M.,Failure Analysis Associates | Scher I.S.,Guidance Engineering and Applied Research | Stepan L.,Failure Analysis Associates | Young D.E.,Failure Analysis Associates | And 3 more authors.
Journal of ASTM International | Year: 2010

Collisions with obstacles, such as trees, rocks, and other people, are a common occurrence in the sports of skiing and snowboarding. Once an obstacle becomes visible, whether or not the skier has time to avoid it is largely determined by that skier's reaction time (RT--the time it takes to detect and identify the obstacle, make a decision about how to respond, and initiate that response. Stopping and turning RTs were measured in ten expert skiers and four expert snowboarders at Mammoth Mountain, California. Participants were told to search for a sign along a closed intermediate course and to execute the instruction on the sign as quickly as possible. The sign was positioned such that it was not visible until participants crested a berm. Two high-speed video cameras captured the movements of each participant. RT was defined as the time between when the sign first came into view and when the skier or snowboarder initiated a response (the time of initial ski, snowboard, or body movement away from the original path or arc of the participant-. The average RT for skiers and snowboarders was 856 and 1056 ms, respectively. No difference in RT was observed between stopping and turning responses. These data can be used to estimate the limits of performance for an attentive, experienced skier or snowboarder under good environmental conditions. Copyright © 2010 by ASTM International.

Shealy J.E.,Rochester Institute of Technology | Johnson R.J.,University of Vermont | Ettlinger C.F.,Underhill Center | Scher I.S.,Guidance Engineering and Applied Research
ASTM Special Technical Publication | Year: 2015

Helmets have been proposed as a means of injury mitigation. Head injuries are of particular interest due to the potential for death or permanent cognitive impairment. The objective of this paper was to determine the degree that a recreational ski sports helmet can mitigate head injuries. The authors conducted a prospective epidemiological study of all medically significant skiing injuries at the Sugarbush Resort. All injuries were diagnosed and initially treated at a clinic at the base of the resort by orthopedic physicians. Various control group strategies were used to assess the characteristics of the population at risk. The numbers of resort visits by various sub-groups of the population were carefully audited. Controls consist of random assessments of the population at risk as well as equipment examinations and evaluations. During the time period of this study (17 seasons from the 1995/1996 season through the 2011/2012 season), within the population at risk, helmet usage increased from 8 to 84%. Our analysis began at the time that helmet usage became popular. We specifically focused on all injuries to the region of the head. For the 17 seasons of interest, the prevalence of all injuries to the head decreased from 8.4 to 6.8%; the prevalence of potentially serious head injuries (PSHI) declined from 4.2 to 3.0 %. The incidence of PSHI declined from 1 in 4200 days of activity to 1 in 11 000 days of activity; the incidence of any head injury declined from 1 in 8600 days of activity to 1 in 26 000 days of activity. Results of the study also stated that over this same time, the incidence of helmet usage increased from 8 to 84%. The average helmet use for the period of interest was 45 %. Of the 10 observed skull fractures, only 1 was to a person wearing a helmet. Of 47 scalp lacerations, only 1 was to a person wearing a helmet. Helmets are mechanical devices that attenuate a finite amount of energy during a head impact. We observed that helmets offer very effective mitigation for head injuries such as skull fractures and scalp lacerations. Increased use of helmets was also associated with a significant reduction in potentially serious head injuries, as well as all head injuries. Copyright © 2015 by ASTM International, West Conshohocken, PA. All rights reserved.

Scher I.,Guidance Engineering and Applied Research | Scher I.,University of Washington | Shealy J.,Guidance Engineering and Applied Research | Stepan L.,Guidance Engineering and Applied Research | And 2 more authors.
ASTM Special Technical Publication | Year: 2015

It has been suggested that contouring the landing area of a terrain park jump, by increasing the landing slope with increasing horizontal distance from the takeoff ramp of a jump, would reduce the likelihood of injury. In theory, this limits the component of center-of-mass velocity that is normal to the snow surface at contact. In published works that recommend this jump design, velocity normal to the snow surface at contact is converted into an equivalent height above the ground, referred to as equivalent fall height (EFH). The purpose of the current research is to evaluate the injury mitigation potential of a landing surface that limits EFH. An instrumented 50th-percentile male Hybrid III anthropomorphic test device (ATD) fitted with snowboarding equipment was used to determine the head accelerations, cervical spine loads, and lumbar spine loads associated with landing on a snow surface in backward rotated configurations. For these tests, the ATD was suspended above a hard-packed, snow-filled box, rotated backwards, and allowed to fall onto the snow. The ATD fall distance and backward rotation were varied in order to adjust the EFH (range: 0.23 to 1.52 m) and torso to snow angle at impact (range: 0 to 92°). The peak resultant linear and angular head accelerations, peak cervical spine load, and peak lumbar spine load were determined for each trial and compared to the loads associated with severe injuries from the biomechanical engineering literature. Full sets of data were recorded for thirteen test trials. The peak resultant linear and angular head accelerations were well below the levels associated with severe brain injury. For eight of the tests, the cervical spine compression exceeded the average compression known to create severe injuries [Nightingale, R. W., McElhaney, J. H., Richardson, W. J. and Myers, B. S., "Dynamic Responses of the Head and Cervical Spine to Axial Impact Loading," J. Biomech., Vol. 29,1996, pp. 307-318; Maiman, D. J., Sances, A. Jr., Myklebust, J. B., Larson, S. J., Houterman, C., Chilbert, M., and El-Ghatit, A. Z., "Compression Injuries of the Cervical Spine: A Biomechanical Analysis," Neurosurgery, Vol. 13, 1983, pp. 254-260]. All of the tests produced cervical spine flexion moments above those associated with cervical spine failure found in the literature. There was no correlation between cervical spine compression and EFH (R2 = 0.03), but there was a significant correlation with torso to snow surface angle at landing (R2 = 0.90). Results of the present study indicate that the likelihood of severe brain injury was low for all impacts within the EFHs examined. Despite this, even low EFHs can produce cervical spine loads well above the levels associated with severe cervical spine injury; these results support the findings of Dressier et al. [Dressler, D., Richards, D., Bates, E., Van Toen, C. and Cripton, P., "Head and Neck Injury Potential With and Without Helmets During Head-First Impacts on Snow," Skiing Trauma Safety, 19th Volume, STP 1553, R. Johnson, J. Shealy, R. Greenwald and I. Scher, Eds., ASTM International, West Conshohocken, PA, 2012, pp. 235-249], who used a partial ATD without rotational kinematics. Furthermore, the lack of relationship between EFH and the metrics related to severe neck injury in the testing suggest that landing configuration is more important than EFH in determining injury likelihood of cervical spine from a backward rotated, unsuccessful jump landing. Copyright © 2015 by ASTM International, West Conshohocken, PA. All rights reserved.

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