Participants
Twelve participants with visual impairment (B1 category) agreed to participate in this study. This included six soccer players practicing for 13 to 20 years and six sedentary men who did not participate in any kind of physical exercise for at least six months. Six participants had congential visual impairment and six had an acquired visual impairment (3 = athletes, 3 = sedentary). For those players who had developed blindness of acquired nature, the age of onset was 9.3 ± 2.1 years but they had not played soccer before their loss of sight. The evaluation of the severity of vision loss was based on an ophthalmological examination. The inclusion criteria were as follows: visual impairment (less than 20/200 and the field of vision limited to 20o legal visual impairment [11]; no medical contraindications to participate in the research; and male sex. Exclusion criteria were as follows: the coexistence of other disabilities and diseases, and taking medications that could affect the results of the analyses. Soccer players have been involved in the sport for at least 4 years in a local soccer team. All participants met their daily obligations for the study.
Procedures
All the players who participated in the present research completed the full time of the match, which lasted 2 × 25 min, while all goalkeepers were excluded from the study. The soccer players participated in 5 matches. The soccer players participated in two training sessions with the team on the field (for 75 min each training). Each of the training involved physical condition exercise, technical and tactical exercises. Also, they participated in two strength training sessions in the gym every week (for approximately 90 min each training).
All measurements were conducted under field conditions on natural or artificial grass of the 5th generation. Also, the matches were performed on non-raining days where the wind speed did not exceed 1 m/s. The soccer trainings were performed every Tuesday and Thursday. The trainings at the gym were performed every Wednesday and Friday.
All procedures and any possible risks and discomforts were fully explained in detail to participants, before the start of the study. There were no caffeine beverages, smoking consumption, or meals for at least 3 h before the testing. All the participants signed a consent form for their participation. The study was approved by the ethical committee of the Department of Physical Education and Sports Science, according to the ethical standards in exercise and sports research with disabilities. Participants’ characteristics are shown in Table 1.
Table 1
Participants’ characteristics.
| Soccer players (n = 6) | Sedentary people (n = 6) | | | |
| | Percentiles | | Percentiles | | | |
| median | 25th | 75th | median | 25th | 75th | p | η2 | U |
Age (y) | 27 | 21 | 35.5 | 24.5 | 21.5 | 34 | .936 | < .001 | 17.5 |
Height (cm) | 169.5 | 164 | 176.5 | 173 | 167 | 178.8 | .173 | .169 | 9.5 |
Weight (kg) | 70 | 64.4 | 78.3 | 81.1 | 69.9 | 93.2 | .023 | .457 | 4 |
BMI | 25.5 | 22.5 | 27.7 | 29.2 | 27.1 | 37.2 | .038 | .394 | 5 |
Body Fat (%) | 21 | 19.8 | 22.3 | 24.5 | 20.5 | 27.8 | .002 | .766 | < .001 |
Lean body mass (kg) | 54.6 | 51.5 | 62.5 | 61.8 | 53.4 | 69.3 | .032 | .282 | 7 |
Congenital /Acquired | 3 / 3 | 3 / 3 | | | |
Experimental Approach To The Problem
A cross-sectional protocol approached the problem of the study. The VO2max was measured in the lab and running and cardiorespiratory parameters were measured during five soccer matches. The 12 participants of the study were divided into two groups of six (soccer players and sedentary). All of them performed anthropometric and VO2max measurements. The players participated in five soccer matches where physiological indicators and match running distance were measured. These indexes checked whether they were correlated to the VO2max measured in the laboratory.
One week before the beginning of the study the players familiarized themselves with the tests. Testing was incorporated within the laboratory and teams’ field. More specifically, the anthropometric and VO2max measurements were completed during the first visit to the laboratory. At the beginning of the VO2max measurement, all participants performed a 10-min warm-up and stretching, and at the end a 10-min cool-down period. In the field, the soccer players before each soccer match performed a standardized 25 min warm-up consisting of five min sub-maximal running, five min of stretching exercises, ball-handling exercises for five min, three repetitions of 15-m run-outs at approximately 90% of maximal speed, and four min of active recovery. VO2max measurements were performed in the same conditions and the players avoided intense exercise in the preceding 24-h. The soccer matches were performed on a soccer field with synthetic grass.
Anthropometric Measurements
Body mass was measured using an electronic digital scale with the participants in their underclothes and barefoot. The height was measured to the nearest 0.1-cm (Seca 220e, Hamburg, Germany). Body mass index (BMI) was calculated (Kg/m2). Body fat (%) was assessed with skinfold measurement (4-fold method): biceps (S1), triceps (S2), suprailiac (S3), and subscapular (S4) by specific (Lafayette, Ins. Co., Indiana)
The percentage of body fat was estimated by the equation of Siri (1956) [24]:
BF (%) = [(4.95/D) − 4.5] × 100
Maximal Oxygen Consumption (Vo) Assessment
VO2max is the maximum rate of O2 uptake as measured during large muscle mass exercise such as running [1], which is widely considered to be the gold standard measurement of integrated cardiopulmonary muscle oxidative function. This test was carried out in the laboratory and its results show the level of aerobic capacity of the athlete. The most accurate measurement of VO2max, which is referred to as the “gold standard”, consists of maximal laboratory testing on the treadmill [5]. We use the treadmill to measure VO2max because this kind of motion (walking-running) is similar to the movements in soccer matches. Τhis index is used in assessing players’ aerobic capacity. VO2max of soccer players was assessed in the morning. The room temperature was around 23°C and the relative humidity was 50%. The cardiorespiratory VO2max test was performed on a treadmill (Pulsar; h/p/Cosmos, Nussdorf-Traunstein, Germany) using a continuous protocol until exhaustion, consisting of eight two-minute stages. The initial grade was 0% and the speed was at 4 km/h for warm-up. In the basic phase of the test, the grade was set to 3% and speed at 5 km/h. Speed was constant throughout the test, while grade was being increased by 2% every 2min. After the final stage, a cool-down session took place for 2min, at 2 km/h and 0% grade. Before study measurements, a pilot study was conducted. VO2max and cardiorespiratory indices were measured via a breath-by-breath ergospirometric system (Oxycon Pro; Jaeger, Wurzburg, Germany). The analyzers had been calibrated before measurements. The highest VO2max value recorded was accepted as the VO2max, after achieving the stabilization of VO2max for at least five measurements (steady-state). Moreover, during the testing, the following parameters were recorded: heart rate (HR), maximal HR, and respiratory exchange ratio (RER).
The VO2max was achieved when at least two of the four criteria following, were met: a) HR exceeded 95% of the expected maximal HR predicted by formula 220-age; b) leveling off (plateau) of VO2max despite the increase in treadmill grade; c) a respiratory exchange ratio (RER) higher than 1.1; d) the subject was no longer able to continue walking despite verbal encouragement.
Global Positioning System (Gps) Analysis
HR data from the match were recorded using the Polar Team Pro (10Hz) (Polar Electro Oy, Kempele, Finland). The recording of the HR began when the athlete wore his monitor. All the values were taken directly from the Polar Team2 Pro software. In matches, each player wore the same GPS device to avoid any existing interunit variation, and at the end of the match, data was analyzed, and 4 indices were used for the total match (total distance, mean and maximum movement velocity, mean heart rate).
Statistical analysis
All the statistical analysis was performed using SPSS (version 24.0; SPSS Inc., Chicago, IL, USA). Mann-Whitney U test were used to evaluate differences between sedentary and soccer players’ anthropometric characteristics and performance. Effect sizes were estimated by calculating partial eta squared and were classified as small (.01 to .058), medium (.059 to .137) or large (.138 or higher) according to Cohen (1988) [4]. Finally, the correlation analysis by Pearsons’ method was used to examine relationships between VO2max and the rest of the physical values during the match. The level of statistical significance was set at p < .05.