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Evaluation of Explosive Power Performance in Ski Jumpers and Nordic Combined Competitive Athletes: A 19-Year Study

Janura, Miroslav1; Cabell, Lee2; Svoboda, Zdenek1; Elfmark, Milan1

Journal of Strength and Conditioning Research: January 2016 - Volume 30 - Issue 1 - p 71–80
doi: 10.1519/JSC.0000000000001046
Original Research
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Janura, M, Cabell, L, Svoboda, Z, and Elfmark, M. Evaluation of explosive power performance in ski jumpers and Nordic combined competitive athletes: A 19-year study. J Strength Cond Res 30(1): 71–80, 2016—Between 1992 and 2010, a total of 334 males participated in this study that assessed the differences and relationships between anthropometric variables and lower limb muscle strength in young and adult ski jumpers (n = 207) and Nordic combined (NC, n = 127) athletes. All athletes completed a maximal vertical jump from an in-run position and a maximal relative isometric force (MRIF) of the knee extensor measurement in a laboratory setting. The body mass index (BMI) in young competitors was lower than in adult groups (NC: p < 0.001; ski jumping [SJ]: p < 0.001). Similarly, the MRIF in both limbs was lower for both disciplines in the groups of young competitors. The vertical jump height (VJH) was lower for young competitors than for adults (NC: p ≤ 0.05; SJ: p < 0.001). When comparing SJ and NC athletes, BMI was lower in SJ athletes. In addition, the adult SJ competitors exhibited greater values of bilateral MRIF (p ≤ 0.05) and VJH (p < 0.01). There was a strong positive correlation in MRIF between the left and right lower limbs (p < 0.001) for all groups of SJ and NC athletes; therefore, it was determined to be sufficient to measure the MRIF on a single limb. Application of the new training methods (e.g., less emphasis on maximum resistance exercises) resulted in improved explosive power in ski jumpers even at lower-body weights. These changes are in accordance with the change in ski jump techniques.

1Department of Natural Sciences in Kinanthropology, Faculty of Physical Culture, Palacký University Olomouc, Olomouc, Czech Republic; and

2Department of Interprofessional Health Sciences and Health Administration, School of Health and Medical Sciences, Seton Hall University, New Jersey

Address correspondence to Lee Cabell, Lee.Cabell@shu.edu.

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Introduction

Ski jumpers must develop muscular strength and explosive power for peak performance in their sport, similar to other athletic disciplines, such as long jump, high jump, and weightlifting. There are a number of sports in which a combination of strength and endurance training is required (32). A similar demand on muscle strength and endurance can be found in Nordic Combined (NC) athletes. Whether an athlete participates in NC or solely in ski jumping (SJ), the predominant requirement for a successful ski jump is the development of power. Regular testing of SJ and NC athletes' anthropometry, knee extensor strength, and vertical jump height (VJH) in a laboratory setting can help determine the effectiveness of athletes' training regimens as they prepare for both SJ and NC events.

Ski jumpers must focus on strength training with fewer repetitions to increase their maximal strength with minor impact on muscle hypertrophy and with a significant effect on the rate of force development and power production (17,18,40). This practice may be described as a specific explosive power training of the lower limb muscles (34). Nordic Combined athletes must further adapt the content and form of their training to accommodate both the strength demands of the ski jump and the endurance demands of the cross-country race. Nordic Combined athletes must focus on explosive force-generating capacity (32); however, the effect of this strength training on endurance development (V[Combining Dot Above]O2max), crucial to cross-country skiing, is minimal.

Despite the difference in overall training focus, both disciplines include a ski jump. The takeoff—the most important phase of a ski jump—establishes the basic conditions for the flight to achieve maximum jump length (44). A successful takeoff, executed within 0.25–0.35 seconds, requires optimal values in vertical velocity (takeoff force) along with maintenance of the release velocity (51,54).

Effective execution of the takeoff phase of a ski jump demands a large takeoff force, which is achieved through a combination of the muscular strength, power, and coordination capabilities of the athlete. Long-term systematic training will help the athlete develop the muscular power required at takeoff (2,7,11,12,22,24,42,45,55). Above all, this training must be specific, individualized to each athlete's needs, and conducted with optimal loading (9,13,20,28,43).

A ski jumper's muscle strength, body coordination, and balance can be measured with several biomechanical and motor tests to determine the current status of the athlete's performance and to adjust his or her training regimen and subsequent training phases (15,38). Specifically, VJH, which displays particularly predictive results for SJ success, should be examined. Vertical jumps are among the most thoroughly studied motor tasks in various sport populations (23,33,36,47).

The results from an assessment of the strength in a single-joint isometric knee extension compared with a countermovement jump (CMJ) height suggest that isometric strength is a poor predictor of performance in multijoint explosive movements such as a CMJ (1). However, the increase in maximal strength relative to body mass was found to be more strongly correlated with explosive movements (33).

Previous studies in SJ and NC athletes have found that increased leg extensor strength results in an increase in vertical jump ability (40). Maintaining the strength level during the competitive season would also increase jumping potential (39). The most suitable motor tests to examine these variables and the results of various training regimens for individual athletes are those that are implemented in conditions similar to training or competition.

Significant changes have been observed in SJ technique over the past 30 years. We wondered whether these changes were apparent in the physical preparedness of the athletes and their somatometric parameters (44).

The objective of this study was to compare the anthropometric variables, strength, and power of the lower limbs in SJ and NC competitors in 2 observed periods (1992–2000, 2001–10) and to assess differences between young (adolescent) and adult athletes. Another objective was to identify the relationship between the maximal relative isometric force (MRIF) of knee extensors and VJH.

For the purposes of this study, we tested the following hypotheses: (a) the magnitude of the anthropometric and force parameters observed in young athletes was significantly different from that observed in adult athletes; (b) the magnitude of observed parameters significantly differed between 1992–2000 and 2001–10; and (c) strong positive relationships existed between MRIF and VJH for all observed groups of athletes.

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Methods

Experimental Approach to the Problem

In this cross-sectional study, we monitored young (adolescent) and adult SJ and NC athletes in a laboratory setting for 19 years, split into 2 periods. The subjects were tested during 1-day visits to the University Laboratory of Human Movement Studies. During the tests, the subjects were barefoot and all wore shorts and T-shirts. Each subject first performed 2 bilateral maximal isometric force (MIF) trials with 1-minute rests between trials, followed by a 5-minute rest before 3 VJH (in-run position, a position similar to that of ski jumper's on SJ hill) trials were completed with a 1-minute rest between trials. The value from the best trial was used for subsequent statistical analyses.

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Subjects

The subjects in this study were Czech male SJ (n = 207) and NC (n = 127) athletes trained at various competitive levels. All recruited subjects were healthy volunteers who participated in regular laboratory biomechanical testing from 1992 to 2010. The division of the study duration into 2 comparable time periods was performed in accordance with the changes in SJ rules, which were aimed at preventing the extreme decline in competitors' body weights. The subjects were further divided into 2 age groups (18 years or younger and older than 18 years) (Table 1). Because there were changes in the classification of competitors in the age categories at that time, we chose 18 years of age as a cutoff as the change to the junior category. The group of adult athletes (older than 18 years) consisted of ski jumpers at junior and senior level rankings. If a competitor was measured within the group several times during a time period, the mean value was calculated from these measurements; therefore, each competitor was represented in the group by not more than 1 value for each parameter. The study protocol was approved by the University's Institutional Review Board. The parental consent was obtained for all participants younger than 18 years, all participants signed informed consent documents.

Table 1

Table 1

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Procedures

Anthropometry

On arrival at the laboratory, the participants' heights and body masses were measured and the body mass indices (BMIs) were determined from these variables using standard calculations. The subcutaneous fat thickness was measured with the use of 10 skinfolds by a skinfold caliper (35). The value of subcutaneous body fat (%BF) was calculated from the equations:

where x is the sum of the 10 skinfold thickness (in millimeters) measurements.

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Maximal Knee Extensor Isometric Force Measurements

Figure 1A shows the subject sitting on a chair holding on to 2 horizontal bars while his thigh was horizontal, and the segmental angle between his thigh and shank was 120°. A piezoelectric load cell (Model 9301A; Kistler Instrumentation, Corp., Winterthur, Switzerland) was attached to the ankle joint such that its axis was directed at a right angle to the shank's axis. The other side of the load cell was attached to a tube that was part of the apparatus. The piezoelectric force transducer (undamped resonant frequency, 90 kHz; mass, 11.93 g) was connected to a charge amplifier (Model 5006; Kistler Instrumentation, Corp.).

Figure 1

Figure 1

A white flash LED (type LEMWH51; LG Innotek, Seoul, South Korea) was positioned 3 m from the subject at his eye level. It was connected to a PC and was controlled by a keyboard. When the investigator pressed a key on the keyboard, the LED flashed and the subject exerted maximal knee extensor force against resistance. After an automatically set 3-second interval, the LED flashed again and the subject moved his leg back to the original resting position. The MIF was recorded and the MRIF was normalized (in N·kg−1): MRIF = MIF per body mass.

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Height of a Vertical Jump Measurement

The ground reaction force (GRF) was measured with a piezoelectric force platform (Model 9261/A; Kistler Instrumentation, Corp.) while the subject was squatting in an in-run position and instructed to take off and land at the same place to achieve a maximal jump height (Figure 1B). Each subject performed a minimum of 3 practice jumping trials to ensure he felt comfortable with the setting.

The VJH was calculated from the GRF impulses using the following equation: VJH = I2/2gm2, where I is impulse (in Newton second), m is body mass (in kilograms), and g = 9.81 m·s−2 is gravity acceleration.

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Data Processing

Following amplification, the analog force data were converted to digital signals using a 12-bit A/D converter (WSH 572A; Research Institute for Mathematical Instruments, Prague, Czech Republic) and sampled at a nominal rate of 1 kHz per channel. The signals were analyzed using Pekol software (Palacky University, Olomouc, Czech Republic), and all data were stored on a TNS microcomputer and later on IBM-compatible PCs.

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Statistical Analyses

The data were checked, and no model assumptions of statistical tests were violated. Data were analyzed using a 2 × 4 (sport discipline × group) analysis of variance followed by Fisher's least significant difference test as a post hoc test when the F-ratio was significant to determine the pairwise differences between mean values. The effect size was computed in accordance with Cohen's d statistic. An effect size smaller than 0.20 was considered a negligible difference, between ≥0.20 and <0.50, a small difference; between ≥0.50 and <0.80, a moderate difference; and ≥0.80, a large difference. Pearson correlations (r) were used to determine if any bivariate correlations existed among selected variables. Descriptive statistics for the outcome measures are presented as the mean ± SD. Statistical significance was set a priori at p ≤ 0.05. The statistical analyses were performed using the STATISTICA v10.0 (StatSoft, Inc., Tulsa, OK, USA) statistical package program.

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Results

The average age of young (18 years or younger) and adult (older than 18 years) athletes did not differ significantly between the SJ and NC groups.

The mean ± SD values of the measured parameters of all monitored groups are presented in Table 2.

Table 2

Table 2

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Differences Between the Two Observed Periods Within a Given Discipline

We did not find any significant differences either in the young (1Y vs. 2Y) or the adult (1A vs. 2A) NC athletes. The MRIF in both lower limbs increased in both age groups among the SJ competitors with a statistically significant difference in the right lower limb between 1Y and 2Y (Figure 2). The body height increase resulted in a reduction of BMI for all observed groups, but the differences were not statistically significant.

Figure 2

Figure 2

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The Comparison Between Young and Adult Groups

Nordic Combined

The BMI for groups 1Y and 2Y was significantly lower than in groups 1A and 2A (Figure 3). The MRIF in the right lower limb was significantly lower in 2Y than in 1A and 2A (Figure 2), and, similarly, the MRIF in the left lower limb was significantly lower in 2Y than in 2A (Figure 4).

Figure 3

Figure 3

Figure 4

Figure 4

The VJH was significantly lower for both groups of young competitors compared with 1A and 2A (Figure 5).

Figure 5

Figure 5

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Ski Jumping

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).

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Comparison of Ski Jumpers and Nordic Combined Athletes

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).

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Correlations Between the Measured Parameters

Nordic Combined

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).

Figure 6

Figure 6

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Ski Jumping

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.

Table 3

Table 3

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).

Figure 7

Figure 7

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Discussion

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.

The body mass in both age groups of the SJ competitors was smaller than in the NC competitors throughout the entire study time period. The most significant reduction of mass occurred in adult ski jumpers. The BMI reached lower values in the SJ competitors compared with the NC competitors. All groups had slightly decreased BMIs during the monitored time period, with the largest decrease noted for adult ski jumpers. These trends are in line with the changes in SJ techniques, for which smaller BMI values allow for the better use of aerodynamics during the flight phase of a ski jump. There has been a reduction of BMI up to 4 units since 1970 (31). Ski jumping performance is not only determined by the level of motor skills (46) because the best performances can be achieved even by young ski jumpers with optimal somatic parameters. A body mass reduction of 5 kg could cause an increase in jump length of 6–7 m on jump hills with a large critical point (30); however, Müller (29) did not find any significant correlations between BMI values and competitors' placements during the 1999/2000 World Cup SJ season.

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.

We used MIF normalized to body mass because relative strength has been shown to be more relevant than absolute strength measurements (33). The MRIF in both lower limbs increased during the monitored time period in all measured groups with the exception of young NC competitors. These changes were not statistically significant (except for young SJ competitors in the right lower limb). The number of significant differences between young and adult groups was larger for the SJ competitors than the NC competitors. This fact might be caused by neuromuscular adaptations because of training (37), which seems to play a greater role than muscle hypertrophy (16).

Improvement of strength-related performance is accomplished through neuromuscular learning, increased fiber-recruitment synchronicity, and muscle cell hypertrophy (26,41). There is a possible lack of efficient utilization of resources that are necessary for maximal strength production in pubescent athletes (including our young groups) (21). Long-term periodized strength and power training also influence qualitative muscle tissue adaptations (3). Improved strength-related performance is accomplished through neuromuscular learning and increased fiber-recruitment synchronicity and possibly hyperplasia without changes in V[Combining Dot Above]O2max or in the capacity to generate adenosine triphosphate through oxidative metabolism (27,41). Seasonal changes in leg strength and vertical jump ability in ski jumpers were examined by Rønnestad (39). The main finding in their study was that isometric squat strength was reduced between the beginning and the end of the competition season. However, neuromuscular efficiency increased during the competitive season and no changes occurred in the squat jump performance during the season. The comparison of lower limb isometric strength between the SJ and NC athletes was also made by Bösl et al. (6), and the SJ athletes showed significantly greater absolute and relative isometric strength than NC athletes. Additionally, in our study, the MRIF in SJ athletes was greater in both limbs than in the NC groups (with the exception of group 1Y), but the measured differences were not significant.

Explosive lower limb power is the key for successful execution of the takeoff (49). It has been shown that the force perpendicular to the takeoff table exerted by ski jumpers at takeoff influences the length of the ski jump (52). Different exercises for training explosive power in the lower limbs are used in SJ in which various forms of the takeoff are practiced. Lower limb extensor muscle force in a closed kinematic chain is more relevant to jumping performance than knee extensor force in an open kinematic chain (5).

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.

The results from previous studies vary. Baker et al. (4) determined that the relationship between the mechanisms that contribute to enhanced isometric and dynamic strength was poor. Anderson et al. (1) and Haff et al. (14) did not find significant correlations between isometric knee extension force and CMJ height. When relative strength was compared with CMJ performance, it was demonstrated that squat relative 1 repetition maximum was significantly correlated with the CMJ height (10). Similar findings were also found by Nuzzo et al. (33) who reported that relative measures of strength, rather than absolute measures of strength, are more strongly correlated with CMJ height. Bruhn et al. (8) also found a significant relationship between the MRIF of the knee extensor and the lower limb explosive force with interindividual differences among competitors. The influence of anthropometric, body composition, and force variables on jumping performance in elite junior basketball players was examined by Ugarkovic et al. (48). Correlation coefficients between the selected variables and jump height were statistically insignificant, but multiple correlation coefficients between monitored parameters suggested a moderate predictability of jumping performance.

If we want to assess the changes in measured parameters during long-term monitoring, we also need to focus on the changes that have occurred in the sport discipline (techniques, equipment materials, and SJ hills). In SJ, a fundamental change was in the flight phase when execution styles evolved from the classic ski-holding to a so-called “V” style. This change increased the requirements of a jumper's aerodynamics during the flight phase, which resulted in a significant reduction of body weight (27). The different nature of NC (i.e., SJ and cross-country skiing) requires a combination of several training methods. For the basic movement abilities (strength and endurance), a major change (either an increase or decrease) can be beneficial in one discipline but can have a negative influence in the other discipline. Nordic combined competitors have poorer results in SJ tests than SJ competitors (19). This is also true when the values of the basic somatometric parameters of NC and SJ competitors are compared (6).

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Practical Applications

The results of this study are important for strength and conditioning coaches who work in the area of SJ and NC. These results allow the coaches to obtain information about the anthropometric (important for the flight phase of a ski jump) and the explosive power parameters in the lower limbs and their changes over a long period of time. A decrease in BMI in all groups was found between the 2 observed time periods. It was more influenced by an increase of body height, and thus there was an excessive reduction of the mass of competitors. The decrease in BMI was not reflected in a reduction of the power parameters in the lower limbs (with the exception of young NC athletes), but the MRIF of the knee extensor in the knee joint and explosive power increased.

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.

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Acknowledgments

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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Keywords:

biomechanics; age; anthropometry; strength; jump height; monitoring

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