ORIGINAL ARTICLE
J Appl Biomater Biomech 2011; Vol. 9 no. 1, 40-46 DOI: 10.5301/JABB.2011.7732
Evaluation of counter movement jump parameters in young male soccer players Livio Quagliarella1, Nicola Sasanelli1, Giuseppe Belgiovine1, Domenico Accettura2, Angela Notarnicola3, Biagio Moretti3 Biomedical Engineering Section, Department of Clinical Methodology and Medical-Surgical Technologies, University of Bari, Bari - Italy 2 Institute of Sports Medicine, Bari - Italy 3 Orthopedics and Traumatology Section, Department of Clinical Methodology and Medical-Surgical Technologies, University of Bari, Bari - Italy 1
ABSTRACT Purpose: The Counter Movement Jump (CMJ) is widely used as a field test to evaluate the muscular explosive power of lower limbs in various sports, including soccer. The aim of the present study was to verify the possibility to discriminate the effect of training level and age on young adult male soccer players, adopting the flight time, the peak force, or the peak power obtained from a countermovement jump. Methods: One hundred and twenty three young male soccer players were enrolled in the study; the participants were split into groups according to Tanner’s stages (pre- and post-pubertal) and training level (high and low activity). A piezoelectric force plate was used to acquire the ground reaction force used to calculate the flight time, the peak value of the vertical component of the ground reaction force, and the peak value of the developed power. Results: As expected, the best performances were obtained by the post pubertal highly-trained soccer players. All performance parameters presented statistically significant differences between ages, while only the flight time exhibited statistically significant differences between training levels. After normalization of the experimental data to body height and weight, there were no statistical differences between ages; statistically significant differences were found only for the flight time and for peak power between training levels. Conclusions: The flight time was the parameter more sensitive to detect differences in the jump performance related to training and age. Adopting a normalization procedure it was possible to highlight that only the flight time and the peak power are sensitive to training effects on young adult male soccer players. Key words: Counter movement jump, Soccer, Age, Training level Accepted: January 31, 2011
JABB11_1504_quagliarella
INTRODUCTION Soccer is one the most popular sports in the world. It is a fast moving field game that requires a mix of physical, technical, mental, and tactical skills. While most soccer research has so far been focused on injury incidence, treatment, and prevention, one the most interesting lines of research focuses on performance. The best way to describe performance is to analyze player movement patterns during competition, but it is a rather prohibitive task in view of the large number of uncontrolled variables involved. Therefore, it is more convenient to study a specific ability by means of quantitative measures of a single motor task within the context of motion analysis. Some kinetic variables play a role in terms of performance level since greater muscle capability has been suggested to be linked to a higher percentage of fast-twitch 40
muscle fibers (1-4). While this may be attributable to a natural disposition, it also implies that muscular strength and the rate of strength development play a significant role in achieving the best performance along with two other factors: training and age can remarkably affect jump performance in young athletes (5-7). Plyometrics are training techniques used by athletes in all types of sports to increase strength and explosiveness. Plyometrics consists of a rapid stretching of a muscle (eccentric action) immediately followed by a concentric or shortening action of the same muscle and connective tissue. The stored elastic energy within the muscle is used to produce more force than can be provided by a concentric action alone (8). Standing Vertical Jump (SVJ) has been widely used in sports to monitor the level of plyometrics performance of both elite and recreationally active athletes (9-14) as well
© 2011 Società Italiana Biomateriali - ISSN 1722-6899
Quagliarella et al
Fig. 1 - In the execution of a CMJ, the jumper starts from an upright standing position, makes a preliminary downward movement by flexing at the knees and hips under the effect of gravity, then extends the knees and hips again (Push-off phase). The force measured by the platform becomes lower than the body weight, reaches the minimum value and starts to increase when the jumper makes the downward movement. During the Push-off the force is higher than the body weight and the power is positive. The maximum force (Fmax) and the maximum power (Pmax) occur after the lowest point of the countermovement. The acceleration of the jumper’s center of mass (indicated by ) is zero when the force drops to become equal to body weight and the jumper reaches the maximum upward velocity, then leaves the ground when the force first becomes zero (Take-off phase). At the middle of the Flight phase, the jumper reaches the maximum height of the jump, then starts to move downward with increasing speed. After the first contact with the ground (Landing), the force has several ‘‘impact peaks’’ and eventually becomes equal to the body weight when the jumper recovers his or her standing balance.
as to investigate lower limb functional ability under different conditions (15-17). Several literature studies have explored SVJ performance features. One line of research has focused on the effect of strength training programs on jump performance (18-22). The studies in question have been able to demonstrate a relationship between leg strength and jump performance. Among the different ways to perform a SVJ, the Counter Movement Jump (CMJ) is widely used as a field test of the efficacy of strength training because it is simple to perform and it is closely related to actions made during game play (22). The main difference between CMJ and the sport jumping technique occurs when performers are required to execute a CMJ without an arm swing. The restriction of the
use of the arms has been widely shown to result in a 10% reduction in performance (23-25). The CMJ allows leg power to be assessed under both slow stretch-shorten cycles and low stress load conditions since, starting from an upright standing position, the jumper makes a preliminary downward movement by flexing at the knees and hips, lowering his/her center of mass, and then immediately extending the hips and knees again to jump vertically up off the ground through a “stretch-shorten movement” (10, 26-28). The maximum height reached by the center of mass accounts for the most common performance parameter used to assess CMJ. The direct measurement of this parameter requires stereophotogrammetric systems, but, under the hypothesis of ballistic movement, it can easily be inferred from the flight time (Tf) (14, 17, 29, 30). Tf is defined as the time between the body take-off and the first subsequent contact with the ground. It can be directly and accurately measured using a contact plate or from the Ground Reaction Force (GRF) produced by a force platform, where Tf is the time interval between take-off and landing, when GRF is equal to zero (Fig. 1). The peak value of ground reaction force (Fmax) is significantly related to the muscle activity, particularly to that of the vastus lateralis (31) and for some authors is a reliable method of evaluation of lower extremity functional strength (32). The developed power (Pmax) is defined as the rate of doing work, and can be calculated as the product of force and velocity. Pmax could also be understood as the rate of energy development, providing supplementary information on CMJ beyond Tf which is linked to the total amount of developed energy. The aim of the present study was to verify the possibility of identifying the effect of the training level and age on young adult male soccer players by adopting three parameters obtained from a CMJ, namely, Tf, Fmax and Pmax. MATERIALS AND METHODS One hundred and twenty three young male soccer players were enrolled in the study (Tab. IA). Inclusion criteria were: (i) overall good general health and (ii) absence of injury or surgery to the lower extremities. A questionnaire was administered to all the subjects enrolled to assess their history of injuries along with their training practices. The number of training sessions per week was recorded for each subject. On the basis of these data, the population studied was split into two categories: high- and low-activity levels of training. A high-activity level of training was defined as participating in soccer teams involved in a local championship, with a training program at least three days per week or more. A low-activity level was defined as participating in recreational soccer teams and training programs less
© 2011 Società Italiana Biomateriali - ISSN 1722-6899
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Evaluation of counter movement jump parameters in young male soccer players
than three times per week. The subjects were also split into two groups pre-pubertal (pre-p) and post-pubertal (post-p) according to Tanner stages III-IV and V (33) respectively (Tab. IB). Each subject was asked to give his/ her informed consent prior to participation in the study and informed parental consent was obtained for children participating in the study. The study was approved by the local Ethics Committee. After a standard warm-up routine, consisting of moderate jumping and stretching, each subject was asked to perform a series of five maximum height CMJs. All jumps were performed barefooted and the subjects were asked to keep their hands placed on their hips. A Kistler 9286A piezoelectric force plate (Kistler Instrumente AG, Winterthur, Switzerland), connected to an Elite system (BTS S.p.A., Milan, Italy), was used to acquire GRF components. The platform data were recorded and sampled at 1000 Hz for 4 s. All calculations were carried out using custom software (34) created with MatLab® (Release 14, The Mathworks Inc., Natick, MA, USA). Tf and Fmax values were directly obtained from the platform output because Tf accounts for the time interval when the GRF vertical component was equal to zero; Fmax was the peak of the GRF vertical component, and Pmax was the maximum of the values obtained, for each jump, by multiplying the vertical GRF component of each sampled
TABLE IA - P ARTICIPANTS GROUPS: ANTROPOMETRIC DATA CATEGORIZED BY AGE
Classification
Age [years]
Body mass [kg]
Height [m]
Mean
sd
Mean
sd
Mean
sd
Pre-p
13.1
1.3
50.7
12.6
1.69
0.12
Post-p
18.3
1.6
68.2
10.6
1.76
0.07
The mean values and standard deviations (sd) are referred to pre-pubertal (Pre-p) subjects and post-pubertal (Post-p) subjects
TABLE IB - P ARTICIPANTS SUBDIVISION BY AGE AND TRAINING LEVEL Age range [years]
Low-level training
High-level training
Total
Pre-p
11-14
29
36
65
Post-p
15-20
26
32
58
52
68
123
Classification
Total
The number of subjects participating to the study is reported, with the subdivision respect to age (Pre-p = pre-pubertal; Post-p = post-pubertal) and training level
42
datum by the vertical velocity of the subject’s center of gravity. Vertical velocity was determined by subtracting body weight from the force-time curve, dividing by body mass, and integrating with respect to time, using the trapezoidal rule for numerical integration (35). All statistical analyses were carried out using Minitab14® (Minitab Inc., State College, PA, USA). Prior to statistical analysis, dependence of the parameters on anthropometric data was evaluated through the Pearson Correlation Coefficient (PCC) in order to obtain a body size-independent evaluation. The statistical procedures were adopted in accordance with the result of the normality test (Kolmogorov-Smirnov). The alpha-level for significance was set to 0.05. RESULTS The experimental data presented a statistical significant correlation (Pearson) with body height and weight, therefore, they were simultaneously normalized with respect to these parameters using a detrending method based on a second-order polynomial model (36). The normalization process produced data uncorrelated with body size, as well as magnitudes and units comparable with the original data. This normalization procedure simplified the interpretation of the performance parameters. The examined parameters were distributed normally, then parametric methods were adopted; data mean values and standard deviations referred to the age-based and the training level-based groups are shown in Table II. The best results were obtained by the post-pubertal, highly-trained soccer players. Through the Generalized Linear Model ANOVA test (Tab. III) applied to the original data, all parameters presented statistically significant differences (SSDs) between ages, while only Tf exhibited SSDs between training levels. After detrending, SSDs were found between training levels for Tf and statistically probable differences (SPDs) were found for Pmax ; as expected, there were no statistical differences between age. DISCUSSION For various sports, including soccer, success has been related to anthropometric, physiological, and motor skill attributes (37) that vary among age groups as demonstrated by the Ghent Young Soccer Project (38). In soccer, crucial importance is attached to both the force and coordination of the agonist/antagonist muscle of the lower limbs (39), whose power can be tested by assessing vertical jump performance (1, 40, 41). While it is known that CMJ performance depends on body size (and hence on age) (5, 6, 42) and on training level (7, 8, 19, 20, 29, 43), there are
© 2011 Società Italiana Biomateriali - ISSN 1722-6899
Quagliarella et al
Fig. 2 - The scatterplots of Tf versus Fmax (left side) and Tf versus Pmax (right side) evidenced that the same jump performance (Tf value) corresponds to different values of Fmax or Pmax. The wide range of variation of these two parameters could be related to differences in muscular fiber composition (particularly Type IIa and Type IIb fibers), which determines the generation of force and power during the push-off phase of the jump.
no indications about the CMJ parameters the can further elucidate the influence of these factors. Therefore, three parameters were analyzed: Tf, Fmax and Pmax, to evaluate CMJ performances obtained from the four tested groups. The evaluation of players, through CMJ is made even more complex by individual differences (44, 45), and training level. A normalization procedure was adopted to exclude the impact of height and weight due to either growth or inter-individual differences. The obtained values of Tf, Fmax and Pmax were similar to those reported in the literature (17, 30, 46-50); the differences among the maximum coefficient of variation of Tf (15.06%) and those of Fmax (29.99%) and Pmax (38.85%) were also similar. These differences are equally present in normalized data, meaning that independently of weight and height, the same Tf can be obtained with a wide range of Fmax or Pmax values (Fig. 2). In other words, different motor strategies and muscular capacity (both in terms of strength features and fiber composition) could be exploited to reach the same take-off energy.
TABLE II - PARAMETER VALUES BASED ON AGE AND TRAINING LEVEL CATEGORIES BEFORE AND AFTER DETRENDING Pre-p Parameter
Post-p
% level
Training level
% age Mean
SD
Mean
SD
Low
398
56
474
37
19.1
High
445
67
489
73
9.9
Low
412
59
428
51
3.9
High
452
56
456
71
0.9
Low
1540
334
2037
405
32.3
High
1527
458
2196
407
43.8
Low
1586
226
1613
397
1.7
High
1631
265
1810
216
11.0
Low
2200
575
3614
736
64.3
High
2494
969
3688
883
47.9
Low
2390
389
2589
485
8.3
High
2719
561
2739
861
0.7
Before detrending Tf [ms] After detrending
Before detrending Fmax [N] After detrending
Before detrending Pmax [W] After detrending
Pre-p
Post-p
11.8
3.2
9.7
6.5
-0.8
7.8
2.8
12.2
13.4
2.0
13.8
5.8
Mean values and standard deviation (SD) of flight time (Tf), peak force (Fmax) and peak power (Pmax), based on age (Pre-p = pre-pupertal; post-p = postpubertal) and training level. The percentage variations of each parameter between age groups (% age column) are calculated with respect to the Pre-p group and the percentage variations between training level groups (% level) are calculated with respect to the Low group
© 2011 Società Italiana Biomateriali - ISSN 1722-6899
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Evaluation of counter movement jump parameters in young male soccer players
TABLE III - P-VALUES OBTAINED FROM ANOVA GLM BEFORE AND AFTER DETRENDING Tf
Fmax
Pmax
Age
< 0.0005
< 0.0005
< 0.0005
Level
0.007
0.897
0.148
Age
0.597
0.411
0.589
Level
0.004
0.854
0.030
Before detrending
After detrending
Tf = flight time; Fmax = peak value of the vertical component of the ground reaction force; Pmax = peak value of the power output. The ANOVA General Linear Model (GLM) was applied adopting Age and Level as fixed factors.
Moreover, our results suggested that there were no training-traceable significant difference between the age groups, whereas training led to statistical differences in both age groups in terms of Tf and Pmax. This finding was in line with the results of Reilly and colleagues who have been able to show significant differences in vertical jump height among soccer players of different levels (51). The mutual influence of the development of the musculoskeletal structure (5, 6, 42, 44) and training programs (13, 38, 43, 44, 51) is supported by experimental data where the energy, muscular strength, and rate of developed energy are expressed by the values of Tf , Fmax and Pmax, respectively. By comparing the increases in the original data with post-normalization increases, where the prevailing effect was that of training, it was possible to define age as the factor likely to have the greatest impact on the jump performance. The analysis of Tf in Table II seemed to disprove what Christou and colleagues suggested (52), namely, that training-induced performance improvement (column % level) was greater than age-induced performance improvement (column % age), and confirmed the results of Viitasalo et al (6). By reducing the age influence by normalization, it was possible to show a different trend between pre-p and post-p in reaching the best performance. In both cases there was an obvious increase in Tf
linked to a higher increase in power for the pre-p, and to a higher increase in the force for the post-p. This effect is likely to be the consequence of both growth and training which can lead to different ways of performing a CMJ. As far as the aim of this paper is concerned, while before normalization the three parameters presented high SSDs related to age, Tf was the only one able to detect a SSD related to the training level. After normalization between training levels, only Tf exhibited a SSD, while Pmax showed a PSD and Fmax showed no difference at all. In conclusion, Tf was the most sensitive parameter for detecting differences in CMJ performance related to training and age.
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Financial support: This study was supported and funded by the University of Bari (Italy), with no sponsorship from the industry. Conflict of interest statement: None of the authors have any financial or personal relationships with other people or organizations that could inappropriately influence their work. Address for correspondence: Livio Quagliarella Sezione di Ingegneria Biomedica Policlinico p.zza G. Cesare, 11 70124 Bari, Italy
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