| 11. |
- Hosseini, Seyed, et al.
(författare)
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Comparison of machining performance of stainless steel 316L produced by selective laser melting and electron beam melting
- 2022
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Ingår i: Procedia CIRP. - : Elsevier B.V.. - 2212-8271. ; , s. 72-77
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Konferensbidrag (refereegranskat)abstract
- Powder bed fusion processes based additively manufactured SS 316L components fall short of surface integrity requirements needed for optimal functional performance. Hence, machining is required to achieve dimensional accuracy and to enhance surface integrity characteristics. This research is focused on comparing the material removal performance of 316L produced by PBF-LB (laser) and PBF-EB (electron beam) in terms of tool wear and surface integrity. The results showed comparable surface topography and residual stress profiles. While the hardness profiles revealed work hardening at the surface where PBF-LB specimens being more susceptible to work hardening. The investigation also revealed differences in the progress of the tool wear when machining specimens produced with either PBF-LB or PBF-EB. .
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| 14. |
- Jonsson, Malin, et al.
(författare)
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Biomechanical differences in double poling (DP) for world- and national-class female elite cross-country (XC) skiers during a 10-km classical race
- 2016
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Ingår i: Proceedings ICSS in St. Christoph am Arlberg, Austria.
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Konferensbidrag (refereegranskat)abstract
- Introduction The DP technique of classical XC-skiing involves both the upper and lower body (Holmberg et al.,2005) and has become more important the last years with skiers using exclusively DP during some competitions. Our purpose was to characterize biomechanical differences in DP by world- (WC) and national-class (NC) women skiers. Methods The participants were 40 elite female XC skiers (20 WC and 20 NC) who competed in the 10-km classical race at the Norwegian National Championships, 2016. On a flat measurement section (22 m long) 0.8 km from the start, the skiers employed DP only and were video-filmed (Panasonic GH4, 96 Hz). Three DP cycles were analyzed using the Kinovea software (France, v 8.25) for joint and pole angles at pole plant (PP) and pole off (PO), as well as cycle length (CL) and rate (CR), and poling (PT) and swing times (ST). Results The total racing time for the WC-group was 10.5% faster than for the NC-skiers, with no differences in CL, CR, PT or ST. The WC-group skied faster on the flat section (6.30±0.23 vs 6.04±0.25 m/s) and exhibited a smaller ankle-shoulder angle relative to horizontal at PP (73.0±1.8 vs 75.0±1.5°) and a smaller hip angle at PO (62.7±5.2 vs 69.1±6.4°) with no difference in minimal trunk angle with respect to horizontal (19.2±3.2 vs 21.7±4.8°). 27 of the skiers (15 WC and 12 NC) used active heel raise to create force. There was a difference between the groups for when the heel raise ended, with the NC-group stopping just before PP and the WC-group after. No difference between the groups were found for when the heel raise started . There was a negative correlation between DP velocity and total racing time (r = -0.48, p<0.05) and a positive correlation between total racing time and the ankle-shoulder angle relative to horizontal at PP (r = 0.54, p<0.01), the hip angle at PO (r = 0.51, p<0.01) and minimal trunk angle relative to horizontal during the cycles (r = 0.41, p<0.01). Discussion The WC-group had 4.1% higher DP velocity which correlated with total racing time. Moreover, the finding that faster skiers have a more forward lean of the body at PP and a better timing of the ending of the heel raise, indicates that they can bring more bodyweight on their poles at PP. The WC-group had a smaller hip angle at PO which is in line with the findings of Lindinger et al.(2009). This study shows the importance of a high relative velocity during DP sections of the track and highlights the benefit of a more forward body position at PP to create higher DP velocity in female XC skiers. References Holmberg, H.C., Lindinger, S., Stöggl, T., Eitzlmair, E. & Müller, E. (2005). Biomechanical analysis of double poling in elite cross-country skiers. Med Sci Sports Exerc, 37(5), 807-818 Lindinger, S., Stöggl, T., Müller, E. & Holmberg, H.C. (2009). Control of speed during the double poling technique performed by elite cross-country skiers. Med Sci Sports Exerc, 41(1), 210-220
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| 15. |
- Karlöf, Lars, et al.
(författare)
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Snow - the performance surface for alpine skiing
- 2013
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Ingår i: Routledge Handbook of Ergonomics in Sport and Exercise. - Oxon and New York : Routledge. - 9781138657106 ; , s. 323-334
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Bokkapitel (refereegranskat)abstract
- Alpine skiing is a popular winter sport. Athletes use their equipment on different courses that are covered with snow. From this perspective, better knowledge of snow conditions and the interaction between the skis and snow is fundamental, because performance and choices between different techniques are related to the interaction beteeen the athlete, the ski eqiuipment and the snow.
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| 16. |
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| 17. |
- Sandbakk, Oyvind, et al.
(författare)
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Trening for bedre langrennsprestasjoner
- 2008
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Ingår i: Skisport. - Allan Aabech : Sportmedia AS. - 0333-3973. ; :9, s. 21-24
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- Målsetningen med denne artikkelen er å belyse ulike faktorer som påvirker utholdenhetsprestasjonen, samt presentere treningsmetoder for å utvikle prestasjonen videre. Vi mener at en bedret forståelse for hvordan treningen spesifikt påvirker de ulike faktorene, øker forutsetningene for å legge opp treningen optimalt.
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| 18. |
- Supej, Matej, et al.
(författare)
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Aerodynamic drag is not the major determinant of performance during giant slalom skiing at the elite level
- 2013
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Ingår i: Scandinavian Journal of Medicine and Science in Sports. - : Wiley-Blackwell. - 0905-7188 .- 1600-0838. ; 23:1, s. e38-e47
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Tidskriftsartikel (refereegranskat)abstract
- This investigation was designed to (a) develop an individualized mechanical model for measuring aerodynamic drag (Fd) while ski racing through multiple gates, (b) estimate energy dissipation (Ed) caused by Fd and compare this to the total energy loss (Et), and (c) investigate the relative contribution of Ed/Et to performance during giant slalom skiing (GS). Nine elite skiers were monitored in different positions and with different wind velocities in a wind tunnel, as well as during GS and straight downhill skiing employing a Global Navigation Satellite System. On the basis of the wind tunnel measurements, a linear regression model of drag coefficient multiplied by cross-sectional area as a function of shoulder height was established for each skier (r > 0.94, all P < 0.001). Skiing velocity, Fd, Et, and Ed per GS turn were 15–21 m/s, 20–60 N, −11 to −5 kJ, and −2.3 to −0.5 kJ, respectively. Ed/Et ranged from ∼5% to 28% and the relationship between Et/vin and Ed was r = −0.12 (all NS). In conclusion, (a) Fd during alpine skiing was calculated by mechanical modeling, (b) Ed made a relatively small contribution to Et, and (c) higher relative Ed was correlated to better performance in elite GS skiers, suggesting that reducing ski–snow friction can improve this performance.
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| 19. |
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| 20. |
- Örtenblad, Niels, et al.
(författare)
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Repeated sprint exercise impairs contractile force of isolated single human muscle fibers
- 2013
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Ingår i: Proceedings for the 6<sup>th</sup> International Congress on Science and Skiing. - 9783200034174 ; , s. 93-
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Konferensbidrag (refereegranskat)abstract
- INTRODUCTION: The purpose of the present study was, to examine the effects of repeated sprint skiing on the contractile apparatus of single muscle fibres obtained from a group of elite skiers. We have recently demonstrated that prolonged cycling exercise impairs the contractile apparatus of single muscle fibres, and that this can be restored following recovery. However, little is known about the effect of repeated high intensity exercise on single fibre properties, as i.e. during cross-country (cc) sprint competitions. We hypothesize that repeated high intensity exercise in highly trained subjects will impair the contractile apparatus maximum force output.METHOD: Eleven elite male sprint talented cc skiers (age 24 ± 4 years; VO2max 5.1 ± 0.5 (diagonal skiing, DIA), 4.9 ± 0.5 (double pooling, DP) L·min-1)) volunteered for the study. The skiers performed a simulated intermittent classic sprint roller skiing competition on a treadmill. The sprint exercise included 4 times1300m, with 45 min recovery between sprints. Each sprint consisted of 3 DP sections (1° uphill) and 2 DIA sections (7° uphill). Muscle biopsies were obtained in arm muscle (m. biceps brachii) before and after the sprint exercises. Muscle fibre bundles were cooled and skinned in a glycerinating solution and stored until analyzed. Single muscle fibre segments (n=232) were isolated and attached to a sensitive force recording transducer, and activated by Ca2+ buffered solutions at pH 7.1 to measure mechanically properties (maximum force Po and Po/cross sectional area (CSA)) and fibre typed by the Sr2+ sensitivity (Hvid et al. 2013).RESULTS: Average sprint time was 3min 49s ± 9s, with no difference between sprints. A total of 232 fibres were analysed (150 type I and 82 type II fibres). Type II fibres had a sign. (P<0.05) higher CSA (8103 ± 2334 µm2 (type I) and 8852 ± 2288 µm2 (type II) and Po (0.82 ± 0.43 and 1.24 ± 0.50 mN) than type I fibres. Also type II fibres had a 31% higher Po/CSA (108 ± 55 vs 142 ± 45 kN/m2). Following the intermittent sprint exercise, type II fibres exhibited a sign. (P = 0.01) 20% decrease in Po, with no difference in type I fibres. To test if the decrease in the single fibre Po were associated with oxidative stress we tested if this could be reversed with a strong reducing agent (dithiothreitol, DTT). DTT did not alter Po at pre nor the decrease in type II fibres following sprint exercise.DISCUSSION: By using a translational approach from whole body exercise to single fibre measurements, we here we demonstrate that type II fibres from highly trained cross country skiers, has a 20% decrease in Po following repeated sprint. Thus, part of the experienced fatigue following sprint competitions is due to impairments at the level of the contractile apparatus. Further, we did not find any evidence for oxidative stress as a causative component in the observed decrease in Po.CONCLUSION: Here we demonstrate for the first time, in highly trained sprint skiers, that repeated sprint impairs single fibre maximum force at the level of the contractile apparatus, which may have a significant impact on muscle function and fatigue.REFERENCES: Gejl K, Hvid LG, Ulrik Frandsen U, Jensen K, Sahlin K and Ørtenblad N. Muscle glycogen content modifies SR Ca2+ release rate in elite endurance athletes. Med. Sci. Sports Ex. (2013).
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