INTRODUCTION: The value of take-off speed at the ski jumping hill amounts to approximately 72% of the size achieved in laboratory take-off with training footwear (Vaverka et al., 1993). The value increases to 93% with jumping boots (Jost, 1993). Jumping boots restrict execution of plantar flexion in the final phase of take-off (Virmavirta & Komi, 2001). This study aims to evaluate the differences between magnitudes of take-off forces during different executions of take-off in the laboratory and at a ski jumping hill. METHOD: A group of young Nordic combined competitors (n = 22, age = 16.18 ± 1.67 yrs, mass = 64.7 ± 7.28 kg) performed three different modifications of the vertical jump (IRJ [in run position Jump], CMJ, and SJ [counter movement and squat jump with exclusion of engagement of upper limbs]). Two trials in training footwear (t) were followed by two trials in jumping boots and outfit (s). All tests were conducted on two force plates Kistler 9286AA. This was followed by 10 Jumps (hill) at a jumping hill (force piates Kistler Z18402-100). Data were analyzed with programme STATISTICA, Version 6.0. RESULTS: Values of average heights of the vertical jumps are shown in Table 1. When comparing individual executions of the take-off with performance of IRJt, the remaining values range between 82.1% and 98.9%. An exception is the height of a jump under field conditions at a jumping hill when the value achieved amounts to 46.5%. Differences are statistically significant (p<0.05). This conclusion is not valid for comparing IRJ and CMJ in training (jumping) footwear. DISCUSSION: The ascertained differences between the height of jump under laboratory and field conditions differ from the values stated earlier. This might be affected by the age of ski Jumpers, through the manner of performing the take-off, and/or also by changes in the take-off techniques noted in recent years. When selected competitors performed the take-off in a laboratory while attempting to shift the centre of gravity forwards ("bouncing"), the jump height was significantly reduced. CONCLUSION: When performing take-off at a ski jumping hill, the height of the jump is reduced by over 50% compared with take-off from in-run Position in a laboratory with training footwear. When applying data resulting from laboratory take-offs for training purposes, the individual preferences to take-off execution must be taken into account.
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