The VJH was significantly lower for both groups of young competitors compared with 1A and 2A (Figure 5).
We found more statistically significant differences between the young and adult competitors in SJ athletes compared with NC athletes. Body mass index decreased significantly for both age groups from the first to the second period (Figure 3). The BMI values of both young groups were significantly lower than those of both adult groups. Body fat was significantly higher in 2Y than in 1A and 2A.
Statistically significant differences in MRIF in both lower limbs, with lower values in groups of younger competitors, were found in 1Y vs. 1A and 2A and in 2Y vs. 2A (Figures 2 and 4). The VJH was significantly lower in 1Y vs. 1A, 2A and in 2Y vs. 1A, 2A (Figure 5).
The BMI was lower in the SJ athletes compared with the NC competitors with a significant difference between groups 1A in the NC athletes and 2A in the SJ athletes (Figure 3).
Statistically significant differences were found in the MRIF in both lower limbs between the NC group 1A and SJ group 2A (Figures 2 and 4). The VJH of 1A among the NC competitors was significantly lower than in SJ groups 1A and 2A (Figure 5).
For the anthropometric parameters, a significant relationship was found only in group 2A between the VJH and the amount of subcutaneous fat (r = −0.42; p < 0.025).
There was a strong positive correlation in the MRIF between the left and right lower limbs for all measured groups (1Y, r = 0.83, p < 0.001; 2Y, r = 0.86, p < 0.001; 1A, r = 0.75, p < 0.001; and 2A, r = 0.84, p < 0.001). In addition, we found a positive significant relationship between the VJH and MRIF for both groups of young competitors and for group 2A of adult competitors (Figure 6).
We found several statistically significant relationships between anthropometric and force parameters (Table 3). The MRIF in the lower limb was negatively correlated with the amount of subcutaneous fat. A negative relationship was also found between the subcutaneous fat and VJH as well as between the BMI and MRIF in the lower limb. A positive correlation existed in the groups of youth competitors between BMI and the VJH.
There was a strong positive correlation for MRIF between the left and right lower limbs for all measured groups (1Y, r = 0.72, p < 0.001; 2Y, r = 0.83, p < 0.001; 1A, r = 0.82, p < 001; 2A, r = 0.84, p < 0.001). Similar to the NC groups, we found a significant positive correlation between the MRIF on both limbs and the VJH (except for group 1A on the left limb) (Figure 7).
The most important findings of this study were that (a) the BMI and the MRIF in both lower limbs of young competitors in both disciplines were lower than in the adult groups; (b) the adult SJ competitors exhibited greater values of the bilateral MRIF and VJH than NC competitors; and (c) there was a strong positive correlation for MRIF between the left and right lower limbs as well as between the MRIF and the VJH in both disciplines.
These findings support our research hypotheses with the exception that the magnitude of the observed parameters significantly differed between the 2 periods, 1992–2000 and 2001–10.
Changes in the SJ rules, which determined the relationships between ski jumpers' body masses and their ski surfaces, caused a halt in the sharp decline of BMI (31). Still, we found several competitors among the young ski jumpers with a BMI below 18.5 kg·m−2 and thus below the underweight level.
The basic test for evaluating the explosive force of the lower limbs is a vertical jump with the VJH as a measured parameter. From a variety of takeoffs, the in-run takeoff is preferred for testing (50,53), which we used in our study. The VJH in the NC athletes in our study was lower than that in the SJ athletes during the whole observed period; the differences for adult competitors were statistically significant. These differences may be caused by the variety of training methods that are used in the SJ and NC athletes, for example, strength training, which improves the takeoff force of NC competitors, does not influence endurance development; however, muscular explosive power can be reduced during endurance training (25,40). When strength and endurance training are performed simultaneously, a potential interference in strength development may occur (32).
For the correct interpretation of the results, it is important to determine the relationship between the magnitude of muscle force during isometric contraction and explosive power (represented by the VJH). We found a strong relationship between the MRIF on the right and left lower limbs, for which the coefficients of determination R2 for individual groups of competitors ranged from 0.52 to 0.74. The correlation between the MRIF and the VJH was also significant, but the relationship was weaker.
At the present time, during the explosive force training in both disciplines, the maximum resistance exercises are partially discontinued and exercises with a greater speed and power along with coordination exercises (e.g., the use of a balance board) are used more often. Although the training of special explosive forces in the lower limbs is similar for both disciplines, the training of the trunk and upper-limb muscles and muscle endurance is fundamentally different. Ski jumpers train trunk and arm strength solely with compensatory exercises.
Because rules change frequently in SJ, it is necessary to take these changes into account for the interpretation of the results of the tests. This is also true for applying the results of the laboratory tests to practice, when it is necessary to take into account the changes (takeoff without plantar flexion and the effects of external forces) that occur during a takeoff on a SJ hill. We recommend that future researchers not only use explosive strength (VJH) as the final outcome but also focus on the symmetry of the lower limbs at takeoff.
Supported by the research grant MSM 6198959221 of the Ministry of Education, Youth and Sport, Czech Republic, “Physical activity and inactivity of inhabitants of the Czech Republic in the context of behavioural changes.”
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