Introduction
Soccer is considered widely the most popular and most participated sport in the world, with approximately 265 million participants as of 2006 (8). Soccer has become a faster-paced game in recent decades (24). Wallace and Norton (24) analyzed World Cup Final broadcast footage from 1966 to 2010 to examine patterns of play and stop periods, type and duration of game stoppages, ball speed, player congestion, and passing rates. Over that time, they found that play duration decreased by 10.6% with a corresponding increase in stoppage duration, ball speed increased by 15%, player density (number of players/area) trended upward, and passing rate increased by 35% (24). In light of these game-play developments, strength and conditioning programs have shifted away from targeting aerobic adaptations to instead emphasizing sprint, repeated sprint, change of direction, strength, and power abilities (3,4,6,10,15,20–22,26). In fact, Maria Gil et al. (17) reported that velocity, change in direction, endurance, and jumping abilities were the most important factors for coaches when attempting to identify talent in the youth ranks (age, 9–10) of professional soccer clubs (17). Contemporary research has found that vertical jumping movements (countermovement jumps, squat jumps, broad jumps) correlate well with sprint and power performance (5,16). Moreover, vertical jumping performance can be a strong determinant of sport performance (2,5,16). Accordingly, coaches have begun to test jump performance to monitor the strength and power progress of their athletes (10,23,25).
In soccer, jumping can often occur with a necessary addition of an aerial rotation. For instance, a player attempting to head the ball from a corner kick may require rotational movement in the air, so the ball can be directed to the goal, cleared to a safe area, or passed to a specific player (7). Currently, the manner in which an aerial rotation affects vertical jump performance is unknown. Before takeoff, the rotational jump may incorporate medial-lateral (ML) and anterior-posterior (AP) ground reaction forces (GRFs) to generate rotation. Therefore, the purpose of this study was to compare jump height during maximum effort jumping using a vertical countermovement jump with no rotation (CMJ0) and vertical countermovement jump with 180° aerial rotation (CMJ180). A secondary purpose was to compare the kinetic profiles during the downward and upward jump phases of the CMJ0 and CMJ180. We hypothesized that the rotational demand of the CMJ180 would decrease jump height and increase AP GRFs as compared with CMJ0.
Methods
Experimental Approach to the Problem
Each participant completed a single testing session. Anthropometric and demographic data (height, mass, age, position, and dominant leg) were measured and recorded by the researchers. After anthropometric and demographic measures, participants completed a self-selected warm-up, which consisted of dynamic stretching and practice jumps (#10 minutes). After the warm-up, participants performed as many practice attempts as needed to become familiar with each jumping task (i.e., CMJ0, CMJ180). Once familiar with the tasks, participants performed 3 successful trials for each task. Trials were performed sequentially, completing 3 trials of the CMJ0 followed by 3 trials CMJ180. We did not randomize or counterbalance the condition order because participants were given ample time to practice both jump styles. We did not anticipate learning effects because participants were highly skilled individuals performing simple jumping tasks.
Subjects
Twenty-four male division-1 soccer players (height, 179.6 ± 8.4 cm; mass, 75.7 kg ± 10.1; age, 19.9 ± 1.5 years) volunteered to participate in the study. Before completing the testing protocol, participants provided written consent as approved by the local institutional review board. At the time of testing, all participants were free of any current injury to the lower extremities and were active members of the soccer team of the University of Nevada, Las Vegas.
Procedures
A dual-force platform system sampling at 1,000 Hz (Kistler Instruments, Corp., Amherst, NY, USA) was used to collect 3-dimensional kinetic data bilaterally and was interfaced to a personal computer running Bioware (version 4.0.1.2; Kistler Instruments, Corp.). Countermovement depth and arm swing were not controlled. Each attempt for the CMJ0 and CMJ180 began with participants standing still with each foot on separate force platforms. After initiation of the countermovement (initial unloading), participants performed a maximal effort vertical jump. Participants then attempted to land as normally as possible. No instructions were given for the landing phase, with the exception of completing the landing with each foot contacting separate force platforms and to return to a controlled standing position. For the CMJ180, participants were instructed to complete a 180° rotation in their preferred direction before landing. The research team detected mistrials visually. If a participant did not land with each foot contacting the necessary landing platform, was unable to return to a controlled standing position, or under or over rotated, the trial was discarded and repeated. A total of 6 trials were allowed for each task to complete 3 successful attempts. No subject required all 6 attempts to perform 3 successful trials. Despite visually confirmed trials, 4 trials were excluded from the jump height analysis because the participants did not remain motionless before the countermovement. Rest was provided between trials (≤1 minute) and tasks (≤2 minutes) to mitigate any potential fatigue effects.
Data Analyses
Data were exported to MATLAB for processing (R2014a; The Mathworks, Inc., Natick, MA, USA). During processing, data were smoothed using a fourth-order low-pass Butterworth digital filter with a cutoff frequency of 50 Hz (9). Jump height was calculated for the CMJ0 and CMJ180 using the time in the air equation (14). An Fz threshold of 20 N (summed from both force platforms) was used to distinguish takeoff and landing events. Both jumps were divided into downward and upward phases identified from the combined (left leg + right leg) vertical GRF (Fz) data. The start of the downward phase for each task was identified as the event in which body weight was reduced by 2.5% (18). The end of the downward phase for each task was identified as the event when the vertical velocity of the center of mass crossed zero after the start of the downward phase. To obtain vertical velocity, vertical acceleration was first computed from the combined Fz data using Newton's law of acceleration (force = mass·acceleration) while accounting for the acceleration because of gravity. Then, velocity was computed as the time integral of the vertical acceleration profile. Variables examined in this analysis included jump height, ML force couple, AP force couple, and vertical net impulse relative to body mass during the downward and upward phases, in addition to the peak Fz during downward and upward phases. Net impulse, which was calculated relative to body mass, was incorporated as a variable to determine jump performance (12). Medial-lateral (Fx) and AP (Fy) forces from each foot were compiled into a force coupling value for the respective plane and separated by jump phase. To compute the force couples, we used the absolute values of the AP and ML net relative impulses summed from each platform. These force couples allowed for the analysis of the ML and AP rotational forces across the participant pool. By taking the absolute value, this analysis only considered the magnitude of the rotational GRFs, effectively accounting for, and disregarding, a participant's preferred direction of rotation during the CMJ180. We hypothesized that the AP force couple would be significantly greater during the CMJ180 than during the CMJ0. Conversely, we hypothesized that the ML force couple would not be significantly different between the CMJ180 and the CMJ0 because the feet are medially aligned through the system's axis of rotation, minimizing the potential for any moment arm. Dependent variables were averaged across the 3 trials for statistical analysis between jumping tasks.
Statistical Analyses
Dependent variables included jump height, downward phase time, upward phase time, Fz net relative impulse and peak force, and ML and AP force couple values during the downward and upward phases. Paired-samples t-tests were used to examine task differences. Statistical significance was set a priori at α = 0.05. We recognized trends between 0.05 < α < 0.10.
Results
Jump Height and Time
Vertical jump height was influenced by task and was greater for the CMJ0 compared with the CMJ180 (41.6 ± 4.03 cm vs. 39.4 ± 4.05 cm, p = 0.002). Neither the downward time (p = 0.082) nor upward time (p = 0.137) was different between tasks. Jump height and the downward and upward times are given in Table 1.
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Table 1.: Jump height and phase times.*
Vertical GRF
During the downward phase, peak Fz during CMJ0 trended lower than the CMJ180 (19.8 ± 0.86 N·kg−1 vs. 21.0 ± 0.82 N·kg−1; p = 0.059). During the upward phase, peak Fz was significantly greater during the CMJ180 than during the CMJ0 (25.86 ± 3.13 N·kg−1 vs. 25.17 ± 2.73 N·kg−1; p = 0.049). Vertical net relative impulse was not significantly different between jumps during the downward phase (p = 0.167). Upward phase vertical net relative impulse trended higher in the CMJ0 compared with CMJ180 (5.12 ± 0.21 N·s·kg−1 vs. 4.66 ± 0.16 N·s·kg−1; p = 0.071). Mean and standard deviation values are given in Table 2.
Table 2.: Vertical peak force and net relative impulse.*
Rotational GRF Couples
The ML force couples during the downward phase (p = 0.463) and during the upward phase (p = 0.520) were not different between tasks. The AP force couple during the downward phase was significantly higher in the CMJ180 (0.12 ± 0.10 N·s·kg−1 vs. 0.06 ± 0.06 N·s·kg−1; p = 0.034) compared with the CMJ0. The AP force couple during the upward phase was significantly higher in the CMJ180 (0.32 ± 0.09 N·s·kg−1 vs. 0.06 ± 0.04 N·s·kg−1; p < 0.001) compared with CMJ0. The downward and upward phase force couples are illustrated in Figures 1 and 2, respectively.
Figure 1.: Downward phase force couple. The bar chart above displays the force couple averaged across the downward phase and separated into medial-lateral or anterior-posterior directions. The medial-lateral force couple uses Fx values, whereas the anterior-posterior force couple uses Fy values. Note that the force couple index is a unit-less value. *Significant difference between standard countermovement jump and countermovement jump with 180° rotation (p = 0.034).
Figure 2.: Upward phase force couple. The force couple averaged across the upward jump phase and separated into medial-lateral or anterior-posterior directions. The medial-lateral force couple uses Fx values, whereas the anterior-posterior force couple uses Fy values. Note that the force couple index is a unit-less value. *Significant difference between standard countermovement jump and countermovement jump with 180° rotation (p < 0.001).
Figure 3 depicts the 3-dimensional force data from a single CMJ180 trial.
Figure 3.: Clockwise countermovement jump with 180° rotation example.
Discussion
The most important findings of this study were a significantly reduced jump height (approximately 2.2 cm) and an increased AP force couple during the downward and upward phases of the CMJ180 vs. CMJ0. We computed average standard deviations for each condition per participant to describe the variability of jump height among trials. Participant standard deviations averaged ±1.66 cm (range, ±0.22 to ±5.64 cm) and ±1.99 cm (range, ±0.52 to ±4.91 cm) for the CMJ0 and CMJ180, respectively. In addition, when we analyzed the jumps using the maximum jump height trial of each condition, the conclusions remained the same. Therefore, the trial size of 3 seems to be sufficient to reveal a performance effect.
Although a 2.2-cm difference may be inconsequential to some players, coaches, and researchers, it demonstrates that a relatively simple modification of a vertical jumping task can decrease maximal effort performance as measured by jump height. The decrease in jump height during the CMJ180 could be related to inhibited capacity of the athlete to coordinate a modified jumping task, thus directing coaches and practitioners to consider additional skill practice at similar tasks during training. During the CMJ180, the lower extremity is tasked with 2 objectives simultaneously: (a) produce maximal force for height and (b) produce rotational forces to rotate precisely 180°. Perhaps, by methodologically isolating the dual objectives of maximum jump height and 180° rotations, the unique function of the lower extremities could be better understood. This could lead to specific training methods for coaches and athletes to key on rotational and vertical components of jumping. Athletes and coaches may benefit from additional rotational jumping tasks if it is not already incorporated into their training programs. Longitudinal training studies could help determine whether the decrease in jump height can be addressed through training.
From a mechanics perspective, a force couple is a term used to describe 2 vectors equal in magnitude and directionally opposed while acting at a perpendicular distance from an axis of rotation. A force couple causes a rigid body to rotate without center of mass translation. In this study, we developed a parameter to represent the magnitude of the force couple and thus the rotational forces in the ML (i.e., MLFC) and AP (i.e., APFC) planes while in contact with the ground. This allowed us to collectively examine all participants as 1 group regardless of their preferred direction of rotation.
When comparing the CMJ180 to the CMJ0, the force couple values indicated that no significant differences were present between jumping tasks for the MLFC during the downward and upward phases. However, the anterior-posterior force couple was significantly greater in the CMJ180 than the CMJ0 for the downward and upward phases. These data suggest that rotational GRFs are necessary for the CMJ180 task but differ from a CMJ0 in the AP plane only. Participants were instructed to stand with their feet square to the force platform; therefore, no differences in the MLFC throughout the jump should be expected because the force couple would be restricted to directionally opposed Fx values acting along the same line of application. Because MLFC was not different between the conditions, it confirms that the line of application of the ML force vectors from each foot did not create a force couple that contributed to rotation.
The significant difference in the force couple in the AP plane and not the ML plane confirmed our original hypothesis. But, it is important to realize that rotation could be manipulated independently of the AP force couple action at the ground. For example, an athlete could manipulate his or her angular velocity via segmental inertia. However, these actions can only modify existing angular velocity generated from the ground, and it cannot create or halt rotational energy. Repositioning the limbs closer to the body reduces rotational inertia, whereas extending the limbs away from the body increases rotational inertia. These capabilities allow midair adjustments to the body's angular velocity, allowing a more precise rotation for the task demand (e.g., 180°). We did not obtain kinematic data, restricting our ability to measure the contributions of segmental manipulations to rotational jumping. Future studies may find this topic of interest for sports demanding rotational jumping and could benefit from the addition of kinematics. Furthermore, evaluating the AP force couple during different jumping variations may provide insight into sport-specific movements that are more aligned with competition than a strict countermovement jump.
Reduced jump height and maximal force production can occur as a result of the use of an internal focus of attention (27,28). Conversely, using an external focus can increase jump height compared with an internal focus because of a greater jump net relative impulse and lower extremity joint moments associated with an external focus (27). In a recent study, 12-year-old gymnasts using an external focus (tape across the chest) were able to produce superior movement form (judged subjectively) and greater jump height than an internal focus (direction of the hands) (1). The rotational task may invite an internal focus due to its potential difficulty or novelty to an athlete. Therefore, the outcome of our study could have been influenced if we had given a jumping target (external focus) rather than the maximum effort (internal focus) instruction. Additionally, the combination of simplicity and previous experience in performing the CMJ0 could create an advantage for performance over the CMJ180. For example, brain activity and organizational thinking is significantly reduced in expert golfers compared with novice golfers during preshot routines (19). If athletes are more experienced with the CMJ0 than CMJ180, we can reason that the performance gap could be addressed with practice. Future studies would benefit from isolating a mode of attentional focus (i.e., external focus) and level of experience to avoid confounding factors, especially when participants perform a range of task difficulties.
Despite the reduced jump height observed during the CMJ180, peak Fz was significantly higher in the CMJ180 during the upward phase. Additionally, there were no differences in vertical net relative impulse during the downward or upward phases, despite different jump heights. Previous studies have investigated peak Fz, net relative impulse, and peak power to predict jump height, with conflicting results likely occurring as a result of task differences (i.e., squat jump vs. countermovement jump) (12,13). In our current study, peak Fz was greater in the CMJ180 but was accompanied by a decrease in jump height. Additionally, Fz net relative impulse was not significantly different between tasks, despite different jump heights. Similar Fz net impulses and different jump heights could be related to the differences between the downward vs. upward phases and the actual propulsive phase timestamps. During the countermovement jump, propulsive forces are applied during the late phase of the downward movement. Therefore, the upward phase cuts the propulsive phase short and could have affected the net relative impulses we found. Alternatively, jump height calculations may be the culprit for similar Fz net impulses and different jump heights. Using time in the air, it is possible that the CMJ180 led participants to contact the ground earlier with more extended joints than the CMJ0. In this case, kinematics would be required to analyze preparatory landing kinematics. In consideration of these findings, mechanical determinants of vertical jump height could be task specific rather than variable specific. As such, the presence of a universal determinant for jump height may be impractical when examining a variety of jumping tasks.
There are limitations when describing the rotational jump. Although center of pressure measurements would allow for the validation of the 180° rotation, we are confident that visual detection restricted any major deviations from 180°. Regarding the force couples, the axis of rotation cannot be determined by kinetics alone, and the human body is not a true rigid body. As mentioned previously, athletes are capable of developing and transferring rotational energy through the kinetic chain, which could accelerate or decelerate rotational movement (11,29–31). This can be observed in many sports such as gymnastics, extreme sports, acrobatics, and diving. However, without obtaining kinematic data, it is not possible to discern the contribution of segmental energy transfers to aerial rotation compared with GRFs. To our knowledge, triaxial GRF data have not been explored during rotational jumping tasks to determine the underlying mechanisms associated with producing an aerial rotation. Future research combining kinetic and kinematic analyses is necessary to improve our understanding of the way in which the body mechanically performs a rotational jumping task.
Practical Applications
In conclusion, results indicated that jump height was lower during a CMJ180 vs. CMJ0 task. The lower jump height could not be explained by differences in peak Fz, which was greater in the CMJ180, or Fz net relative impulse, which was not different between tasks. The rotation during CMJ180 was accomplished primarily through left-right AP force couples.
Athletes and coaches may consider including, or increasing the frequency of, rotational jumps into their training programs to maintain jump height under rotational demands. Future research in biomechanics and internal vs. external attentional focus may benefit from investigating AP GRF data in addition to kinematic and electromyographic analyses to further the understanding of how athletes complete rotational jump tasks.
References
1. Abdollahipour R, Wulf G, Psotta R, Palomo Nieto M. Performance of gymnastics skill benefits from an external focus of attention. J Sports Sci 33: 1807–1813, 2015.
2. Arnason A, Sigurdsson S, Gudmundsson A, Holme I, Engebretsen L, Bahr R. Physical fitness, injuries, and team performance in soccer. Med Sci Sports Exerc 36: 278–285, 2004.
3. Bedoya AA, Miltenberger MR, Lopez RM. Plyometric train effects athletic perform youth soccer athletes: A Systematic Review, J Strength Cond Res 29: 2351–2360, 2015.
4. Campo SS, Vaeyens R, Philippaerts RM, Redondo JC, de Benito AM, Cuadrado G. Effects of lower-limb plyometric training on body composition, explosive strength, and kicking speed in female soccer players. The J Strength Conditioning Res 23: 1714–1722, 2009.
5. Cronin J, Hansen K. Strength and power predictors of sports speed. J Strength Cond Res 19: 349–357, 2005.
6. Enright K, Morton J, Iga J, Drust B. The effect of concurrent training organisation in youth elite soccer players. Eur J Appl Physiol 115: 2367–2381, 2015.
7. Erkmen N. Evaluating the heading in professional soccer players by playing positions. J Strength Cond Res 23: 1723–1728, 2009.
8. Fédération Internationale de Football Association (FIFA), 2015, 2007.
9. Harry JR, Paquette MR, Caia J, Townsend RJ, Weiss LW, Schilling BK. Effects of footwear condition on maximal jumping performance. J Strength Cond Res 29: 1657–1665, 2015.
10. Haugen TA, Tønnessen E, Seiler S. Anaerobic performance testing of professional soccer players 1995–2010. Int J Sport Physiol Perform 8: 148–156, 2013.
11. Hauw D, Renault G, Durand M. How do aerial freestyler skiers land on their feet? A situated analysis of athletes' activity related to new forms of acrobatic performance. J Sci Med Sport 11: 481–486, 2008.
12. Kirby TJ, McBride JM, Haines TL, Dayne AM. Relative net vertical impulse determines jumping performance. J Appl Biomech 27: 207–214, 2011.
13. Kollias I, Hatzitaki V, Papaiakovou G, Giatsis G. Using principal components analysis to identify individual differences in vertical jump performance. Res Q Exerc Sport 72: 63–67, 2001.
14. LaPorta JW, Brown LE, Coburn JW, Galpin AJ, Tufano JJ, Cazas VL, Tan JG. Effects of different footwear on vertical jump and landing parameters. J Strength Cond Res 27: 733–737, 2013.
15. Loturco I, Nakamura FY, Kobal R, Gil S, Cal Abad CC, Cuniyochi R, Pereira LA, Roschel H. Training for power and Speed: Effects of increasing or decreasing jump squat velocity in elite Young soccer players. J Strength Cond Res 29: 2771–2779, 2015.
16. Loturco I, Pereira LA, Cal Abad CC, D'Angelo RA, Fernandes V, Kitamura K, Kobal R, Nakamura FY. Vertical and horizontal jump tests are strongly associated with competitive performance in 100-m Dash events. J Strength Cond Res 29: 1966–1971, 2015.
17. Gil SM, Zabala-Lili J, Bidaurrazaga-Letona I, Aduna B, Lekue JA, Santos-Concejero J, Granados C. Talent identification and selection process of outfield players and goalkeepers in a professional soccer club. J Sports Sci 32: 1931–1939, 2014.
18. Meylan CM, Nosaka K, Green J, Cronin JB. Temporal and kinetic analysis of unilateral jumping in the vertical, horizontal, and lateral directions. J Sports Sci 28: 545–554, 2010.
19. Milton J, Solodkin A, Hluštík P, Small SL. The mind of expert motor performance is cool and focused. Neuroimage 35: 804–813, 2007.
20. Ramirez JM, Nunez VM, Lancho C, Poblador MS, Lancho JL. Velocity-based training of lower limb to improve absolute and relative power outputs in concentric phase of half-squat in soccer players. J Strength Cond Res 29: 3084–3088, 2015.
21. Ramirez-Campillo R, Gallardo F, Henriquez-Olguin C, Meylan CM, Martinez C, Alvarez C, Caniuqueo A, Cadore EL, Izquierdo M. Effect of vertical, horizontal, and combined plyometric training on explosive, balance, and endurance performance of Young soccer players. J Strength Cond Res 29: 1784–1795, 2015.
22. Styles WJ, Matthews MJ, Comfort P. Effects of strength training on squat and sprint performance in soccer players. J Strength Cond Res 30: 1534–1539, 2015.
23. Thorpe RT, Strudwick AJ, Buchheit M, Atkinson G, Drust B, Gregson W. Monitoring fatigue during the in-Season competitive phase in elite soccer players. Int Journal Sports Physiology Performance 10: 958–964, 2015.
24. Wallace JL, Norton KI. Evolution of world cup soccer final games 1966–2010: Game structure, speed and play patterns. J Sci Med Sport 17: 223–228, 2014.
25. Williams CA, Oliver JL, Faulkner J. Seasonal monitoring of sprint and jump performance in a soccer youth academy. Int J Sport Physiol Perform 6: 264–275, 2011.
26. Wisloff U, Castagna C, Helgerud J, Jones R, Hoff J. Strong correlation of maximal squat strength with sprint performance and vertical jump height in elite soccer players. Br J Sports Med 38: 285–288, 2004.
27. Wulf G, Dufek JS, Lozano L, Pettigrew C. Increased jump height and reduced EMG activity with an external focus. Hum Mov Sci 29: 440–448, 2010.
28. Wulf G, Dufek JS. Increased jump height with an external focus due to enhanced lower extremity joint kinetics. J Mot Behav 41: 401–409, 2009.
29. Yeadon MR. The biomechanics of twisting somersaults. Part III: Aerial twist. J Sports Sci 11: 209–218, 1993.
30. Yeadon M. The biomechanics of the human in flight. Am J Sports Med 25: 575–580, 1997.
31. Yeadon M, Kerwin D. Contributions of twisting techniques used in backward somersaults with one twist. J Appl Biomech 15: 152–165, 1999.
