2.2. Participants
The participant sample size followed the example of 20–30 subjects used in a similar study with isokinetic and unilateral countermovement jump comparisons [
17]. To reduce heterogenicity we limited our sample to players from the same soccer team with similar training regimes and nutritional intake. It included 32 right-side dominant, elite professional soccer players (12 midfielders, 6 strikers/forwards, 6 defenders, 4 goalkeepers, 4 backsides) who had been playing for at least five years on first and second Brazilian divisions, training regularly one to two sessions per day, six times per week.
In order to be included in this study, players had to be able to fully participate in team training sessions and match play at the time of the analysis. Players who had third-degree hamstring or quadricep muscular injury in the past 3 months, knee surgery in the past 12 months, or were currently in treatment for other muscular injuries or illnesses were excluded from our sample.
The purpose, experimental procedures, possible risks, and benefits of the study were explained to the athletes, who provided a written informed consent form to confirm participation in the study. Demographic and anthropometric information on the sample is presented in
Table 1.
2.3. Instruments
For isokinetic data collection, we used an isokinetic dynamometer (Cybex-CSMI, model HumacNorm 2009, Stoughton, MA, USA) with a signal acquisition rate of 500 Hz. To improve players’ test understanding, we used a modified 10-points Borg scale (BORG) [
18] for perceived exertion—where zero was no strength effort and 10 the maximum strength effort possible—and a visual analog pain scale, where zero was “no pain” and 10 was “worst pain imaginable” (VAS) [
19].
For the countermovement analysis, we used a uniaxial force plate (Biomec 400 v1.1®, EMG System Ltd.a®), consisting of four load cells with an internal circuit that changes in electrical resistance upon the application of force (dimensions: 600 mm × 400 mm; sampling frequency: 1000 Hz, A/D converter with 16 bits of resolution).
For data storage and processing was used a MacBook Pro Notebook (Cupertino, CA, USA) equipped with Microsoft Office software package for Mac (version 2011, Redmond, Washington, DC, USA) and Statistical Package for Social Sciences (SPSS) from IBM (Armonk, NY, USA).
2.4. Procedures
Participants were requested to eat according to their team’s nutritionist-prescribed diet in the 48 h preceding the assessment and then refrain from eating and drinking substances other than water for one hour before the session. All tests were carried out in January, a few weeks before Brazil’s Regional Championship season starts. Players were also instructed to refrain from strenuous activities 48 h before testing.
Upon arrival, participants were provided with appropriate explanations and demonstrations of all procedures. Anthropometric data were recorded before the session began, following the International Society for the Advancement of Kineanthropometry (ISAK) Level 01 certified anthropometrist protocol.
After an interview and physical assessment, all subjects were submitted to a testing protocol following the guidelines of the APTA (American Physical Therapy Association) and soccer-specific studies using isokinetic machines [
20,
21]. The same physiotherapist, with 10 years of experience, performed all isokinetic tests. All isokinetic tests were performed on both legs separately to compare their results with unilateral jump variables. The testing order was the same for all subjects. Initially, the vertical unilateral countermovement jump (UCMJ) test was performed, and after 5 min of rest, the isokinetic test was administered.
To execute the concentric isokinetic test, participants were positioned on the seat of the isokinetic dynamometer and executed 10 repetitions of concentric knee flexion and extension, both with a velocity of 90 degrees per second and a range of motion of 100 degrees, for warming-up purposes (Borg up to 5, VAS up to 1, or test interrupted), followed by a rest period of 120 s. The warm-up on the isokinetic machine was chosen to improve specificity and familiarization with the following test. The athlete then performed five concentric repetitions of knee flexion and extension at 60 degrees per second for familiarization with the exercise velocity, followed by another rest period of 120 s. Then, they performed three concentric repetitions of knee flexion and extension (velocity: 60 degrees per second and range of motion: 100 degrees) with maximum effort (Borg 10), while continuously receiving the standardized verbal encouragement: “Faster”. The presence of pain equal or superior to 05 on VAS interrupted the test, canceling it and excluding the subject from the sample. The repetition with a higher peak torque value of all three repetitions was used for statistical analysis.
To execute the eccentric isokinetic test, participants performed at 60 degrees per second and a range of knee motion of 100 degrees. The subject executed 5 repetitions of warm-up and familiarization, followed by 3 repetitions at maximum effort (Borg 10), while constantly receiving the standardized verbal encouragement: “Hold it.” The presence of pain equal or superior to 05 on VAS interrupted the test, canceling it and excluding the subject from the sample. Between each set of exercises, subjects had 120 s to rest. Between each limb’s test, participants had 120 s to rest. The peak torque was extracted from the isokinetic machine by its manufacturer’s dedicated software (HumacNorm 2009, CSMi Inc., Boston, MA, USA) and normalized by each subject’s body weight.
The unilateral countermovement jumps (UCMJ) were executed after 5 min of warm-up exercise, consisting of jogging with self-paced moderate velocity and 3 submaximal UCMJs. The subjects performed three UCMJs on the force platform, with a recovery interval of 60 s after each trial. The BIOMEC400 platform’s EMGLab2 software (Biomec 400 v1.1®, EMG System do Brasil Ltd.a®) was used to obtain the vertical force of the ground reaction. All force signals recorded by the platform were filtered with a 35 Hz second-order low bandpass filter (Butterworth filter) to eliminate electronic noise.
To execute each UCMJ, players stood on the test leg in the center of the force platform with their hands on their hips and the knee of the non-jumping leg slightly flexed, so that the hovering foot was positioned at approximately the mid-shin height of the jumping leg. Players performed a countermovement to a self-selected depth, before jumping as high as possible and landing on the same test leg. No swinging of the non-jumping leg or arms was allowed. The jump height of the highest attempt was recorded in centimeters (cm). Sixty seconds of rest was provided between all trials. The players were instructed to maintain balance with the testing leg after landing for at least 3 s. If players touched the ground with the hovering leg or their hands came off their hips, the jump was deemed void and retaken after 60 s of rest.
2.5. Data Analysis
Demographic data extracted were age, height, weight, and handedness.
Table 1 shows all demographic and anthropometric data of the sample. The descriptive analysis of the isokinetic and UCMJ data is shown in
Table 2 and was composed by means, standard variation, standard error of measurement, minimum, and maximum values.
The normality of all data was confirmed using visual inspection and the Kolmogorov-Smirnov test. The homogeneity of variance was assessed via Levene’s Test. The Pearson’s correlation was used to identify positive associations between all variables from the isokinetic test and unilateral countermovement jump. Linear correlation was conducted to better analyze variable inter-relationships and the isokinetic torque’s predictability from force platform data. Confidence Intervals (95%) are provided for all measurements. The degree of correlation was classified as Very High (higher than 0.9), High (between 0.7 and 0.9), Moderate (between 0.5 and 0.7), Low (between 0.3 and 0.5), or Very Low (between 0 and 0.3).
All data were processed using SPSS v.20 (IBM, Chicago, IL, USA) with a level of statistical significance set at alpha level p < 0.05.