Dieter Heinrich

Relationship Between Jump Landing Kinematics and Peak ACL Force During a Jump in Downhill Skiing: A Simulation Study

Recent data highlight that competitive skiers face a high risk of injuries especially during off‐balance jump landing maneuvers in downhill skiing. The purpose of the present study was to develop a musculo‐skeletal modeling and simulation approach to investigate the cause‐and‐effect relationship between a perturbed landing position, i.e., joint angles and trunk orientation, and the peak force in the anterior cruciate ligament (ACL) during jump landing. A two‐dimensional musculo‐skeletal model was developed and a baseline simulation was obtained reproducing measurement data of a reference landing movement. Based on the baseline simulation, a series of perturbed landing simulations (n = 1000) was generated. Multiple linear regression was performed to determine a relationship between peak ACL force and the perturbed landing posture. Increased backward lean, hip flexion, knee extension, and ankle dorsiflexion as well as an asymmetric position were related to higher peak ACL forces during jump landing. The orientation of the trunk of the skier was identified as the most important predictor accounting for 60% of the variance of the peak ACL force in the simulations. Teaching of tactical decisions and the inclusion of exercise regimens in ACL injury prevention programs to improve trunk control during landing motions in downhill skiing was concluded.

Calculation of the contact pressure between ski and snow during a carved turn in Alpine skiing

The macroscopic contact area between ski and snow and the contact pressure are crucial influencing factors for carved turns in Alpine skiing. In the present paper, a simulation model is developed to quantify these factors. The ski is modelled as an Euler–Bernoulli beam with variable cross section, camber, bending and torsional stiffness using measured data from skis. The reaction forces of the snow are decomposed in penetration and shear forces. For the penetration forces a hypoplastic constitutive law is applied incorporating elastic and plastic deformation of the snow at the contact area. For the shear forces metal cutting theory is used. Ski deformation, contact area and contact pressure are computed based on quasi‐static equilibrium between forces exerted by the skier and snow reaction forces. Parameter studies are performed to investigate the influence of edging and distributing the load between the inner and outer ski. Higher edging angles as well as loading both skis affected the contact pressure positively by increasing the resistance against shearing. The results of our study agree well with measurement data taken from literature. Based on the results, the importance of actions of the skier during carved turns is concluded.

Modeling of the Ski-Snow Contact for a Carved Turn

Carved turns with alpine skis are investigated. During the movement of a ski, snow is loaded and unloaded. Compacted snow is not elastic, i.e. deformations remain. Such effects are modeled by a hypoplastic constitutive equation. During a turn the shovel digs into the snow and the tail maintains nearly the same penetration depth as the part under maximum load. This results in a higher resistance against shearing for the afterbody of the ski. In the present work we investigated the benefits of the hypoplastic against the elastic forcepenetration relationship. Simulation results for a sledge on two skis are compared to experimental track data.

Influence of Ski Bending Stiffness on the Turning Radius of Alpine Skis at Different Edging Angles and Velocities

Carved turns with Alpine Skis were investigated using a computer simulation model. Varied input data to the model were the bending stiffness of the skis, the edging angle, and the velocity. Results include the turn radius and the force distribution along the running surface of the skis.

Performance limitation and the role of core temperature when wearing light-weight workwear under moderate thermal conditions.

The objective of this investigation was to achieve an understanding about the relationship between heat stress and performance limitation when wearing a two-layerfire-resistant light-weight workwear (full-clothed ensemble) compared to an one-layer short sports gear (semi-clothed ensemble) in an exhaustive, stressful situation under moderate thermal condition (25°C). Ten well trained male subjects performed a strenuous walking protocol with both clothing ensembles until exhaustion occurred in a climatic chamber. Wearing workwear reduced the endurance performance by 10% (p=0.007) and the evaporation by 21% (p=0.003), caused a more pronounced rise in core temperature during submaximal walking (0.7±0.3 vs. 1.2±0.4°C; p≤0.001) and from start till exhaustion (1.4±0.3 vs. 1.8±0.5°C; p=0.008), accelerated sweat loss (13±2 vs. 15±3gmin(-1); p=0.007), and led to a significant higher heart rate at the end of cool down (103±6 vs. 111±7bpm; p=0.004). Correlation analysis revealed that core temperature development during submaximal walking and evaporation may play important roles for endurance performance. However, a critical core temperature of 40°C, which is stated to be a crucial factor for central fatigue and performance limitation, was not reached either with the semi-clothed or the full-clothed ensemble (38.3±0.4 vs. 38.4±0.5°C). Additionally, perceived exertion did not increase to a higher extent parallel with the rising core temperature with workwear which would substantiate the critical core temperature theory. In conclusion, increased heat stress led to cardiovascular exercise limitation rather than central fatigue.

Modeling the ski–snow contact in skiing turns using a hypoplastic vs an elastic force–penetration relation

A ski–snow interaction model is presented. The force between ski and snow is decomposed into a penetration force normal to the snow surface, a shear force parallel to it, and friction. The purpose of this study was to investigate the benefits of a hypoplastic vs an elastic contact for penetration in the simulation of skiing turns. To reduce the number of influencing factors, a sledge equipped with skis was considered. A forward dynamic simulation model for the sledge was implemented. For the evaluation of both contact models, the deviation between simulated trajectories and experimental track data was computed for turns of 67 and 42 m. Maximum deviations for these turns were 0.44 and 0.14 m for the hypoplastic contact, and 0.6 and 7.5 m for the elastic contact, respectively. In the hypoplastic contact, the penetration depth of the skis afterbody maintained nearly the same value as the part under maximum load, whereas it decreased in the elastic contact. Because the shear force is proportional to the penetration depth, the hypoplastic contact resulted in a higher shearing resistance. By replacing the sledge with a skier model, one may investigate more complex skier actions, skiing performance, or accident‐prone skiing maneuvers.

Viscose as an alternative to aramid in workwear: Influence on endurance performance, cooling, and comfort

Workwears are required to maintain optimum performance during dangerous, exhausting activities (e.g. those involving the fire brigade or police). The purpose of the present study was to compare two workwears (A and B) composed of underwear and outerwear with different fiber blends (A: 100% aramid; B: combination of fire resistant (FR) viscose/merino wool underwear and FR viscose/aramid outerwear) during strenuous physical activity. In a climatic chamber (25℃, 50% RH) participants had to walk on a treadmill until exhaustion occurred. Weight measurements were made for calculating evaporation, sweat residue, and sweat distribution. Endurance performance was assessed by time to exhaustion. Core temperature, heart rate, lactate, thermal comfort, microclimate between skin and underwear, surface temperature of the outerwear, and perceived exertion were also measured. The tested workwears caused no significant differences in time to exhaustion, core temperature, and thermal comfort. Sweat distribution differed significantly in the workwears. The underwear of workwear B caused less moisture accumulation in the outerwear and this may be a beneficial safety feature for the prevention of hazardous burns of the skin. Moisture accumulation in the outerwear may reduce thermal insulation and increase the possibility of evaporation whereby hot steam may move to the skin. The potential protective feature of the FR viscose/merino wool blended underwear and the economical price of viscose support the use of fire resistant (FR) viscose blended fabrics in workwears.

Effect of ski boot rear stiffness (SBRS) on maximal ACL force during injury prone landing movements in alpine ski racing: A study with a musculoskeletal simulation model

ABSTRACT A common anterior cruciate ligament (ACL) injury situation in alpine ski racing is landing back-weighted after a jump. Simulated back-weighted landing situations showed higher ACL-injury risk for increasing ski boot rear stiffness (SBRS) without considering muscles. It is well known that muscle forces affect ACL tensile forces during landing. The purpose of this study is to investigate the effect of different SBRS on the maximal ACL tensile forces during injury prone landings considering muscle forces by a two-dimensional musculoskeletal simulation model. Injury prone situations for ACL-injuries were generated by the musculoskeletal simulation model using measured kinematics of a non-injury situation and the method of Monte Carlo simulation. Subsequently, the SBRS was varied for injury prone landings. The maximal ACL tensile forces and contributing factors to the ACL forces were compared for the different SBRS. In the injury prone landings the maximal ACL tensile forces increased with increasing SBRS. It was found that the higher maximal ACL force was caused by higher forces acting on the tibia by the boot and by higher quadriceps muscle forces both due to the higher SBRS. Practical experience suggested that the reduction of SBRS is not accepted by ski racers due to performance reasons. Thus, preventive measures may concentrate on the reduction of the quadriceps muscle force during impact.

The Effect of Uphill and Downhill Walking on Joint-Position Sense: A Study on Healthy Knees

CONTEXT During sport activity, knee proprioception might worsen. This decrease in proprioceptive acuity negatively influences motor control and therefore may increase injury risk. Hiking is a common activity characterized by a higher-intensity-exercise phase during uphill walking and a lower-intensity-exercise phase during downhill walking. Pain and injuries are reported in hiking, especially during the downhill phase. OBJECTIVE To examine the effect of a hiking-fatigue protocol on joint-position sense. DESIGN Repeated measures. SETTING University research laboratory. PARTICIPANTS 24 nonprofessional sportswomen without knee injuries. MAIN OUTCOME MEASURES Joint-position sense was tested at the beginning, after 30 min uphill walking, and after 30 min downhill walking on a treadmill (continuous protocol). RESULTS After downhill walking, joint-position sense was significantly worse than in the test at the beginning (P = .035, α = .05). After uphill walking, no differences were observed in comparison with the test at the beginning (P = .172, α = .05) or the test after downhill walking (P = .165, α = .05). CONCLUSION Downhill walking causes impairment in knee-joint-position sense. Considering these results, injury-prevention protocols for hiking should focus on maintaining and improving knee proprioception during the descending phase.

Effects of two different battings (sheep wool versus polyester microfiber) in an outdoor jacket on the heat and moisture management and comfort sensation in the cold

Wool in outdoor clothing systems is stated to be a good temperature and humidity regulator giving the wearer a warm and dry feeling during physical activities in the cold. The objective was to compare two different battings (sheep wool “tirolwool” (TW) versus polyester microfiber (PMF)) in an outdoor jacket of a two-layer outdoor clothing system consisting of a shirt and jacket on the heat and moisture management and comfort sensation during a moderate mountain walking protocol which was followed by a cool-down phase in the cold (−5℃, 43% relative humidity). Due to its beneficial moisture management properties it was hypothesized that TW in the outermost layer would cause a higher skin and core temperature, reduce the after chill effect, increase moisture transfer, decrease moisture accumulation within the clothing system, and lead to more pleasant comfort sensations. Twelve sport students performed the simulated walking protocol (60 min, 7.7% inclination, 5 km/h) and cool-down phase (20 min) with TW and PMF in a climatic chamber. The use of TW compared to PMF was stronger moisture absorbent led to a dryer shirt (p = 0.043) and lower humidity underneath the shirt and jacket (chest: p ≤ 0.05). Core temperature (p = 0.017) and skin temperature drop (chest: p = 0.003) were attenuated with TW after the physical activity. Therefore, the after chill effect was reduced with TW but moisture accumulated to a higher extent in the jacket (p = 0.001), which might lead to a longer drying rate. Correlation analysis revealed that tested underwear blended with merino wool, elastane, and polyamide might feel less scratchy in the slightly damp state than in the dry state.