In the present study, a longitudinal, parallel and controlled design was used. The subjects were randomly allocated into the balance group (BAL, n = 11), which performed a balance training intervention twice a week for 30 min for 12 weeks, and the isotonic group (ISO, n = 12), which performed traditional strength training with isotonic machines (i.e., the leg extension and leg press) twice a week for 30 min for 12 weeks.
Twenty-three young male basketball players of two different Under Fifteen Basketball Excellence teams participated in this study (participants characteristics are given in Table 1). Before participation, subjects and parents were informed about the aim and the design of the study, and they signed a written informed consent. Only subjects who did not report any injuries in the 6 months prior to the study were recruited.
All the subjects began the pre-season period in September, and this study began in October at the beginning of the in-season period. Pre- and post-testing was performed on Tuesday starting at 4:00 p.m., which was not close to the game days. The games were played on Sunday. Anthropometric data were measured before testing.
Twenty-three subjects were evaluated on balance ability using a balance test (Libra board, Easytech, Prato, Italy) and on vertical jump ability using a CMJ Test (Optojump, Microgate, Bolzano, Italy). At the end of each specific training intervention, post-testing occurred 12 weeks after pre-testing. In post-testing, the same procedures and sequences were used as in the pre-testing.
After a revision of the test protocols and the survey procedures, subjects participated in a standard warm-up consisting of 5 min of jogging at a comfortable speed followed by 2 min of static stretching of the lower extremity muscles. This warm-up procedure was selected to avoid any variation in the usual warm-up of the players. A balance test was performed first, while the countermovement jump test was performed after. Sustained bipodalic and monopodalic (both left and right) positions were evaluated.
Balance ability was evaluated through the balance test using the Libra board (Easytech, Prato, Italy). This instrument (42 × 42 cm, weight 2.7 kg) consists of a balancing board with a wide antislip area. A Libra board is connected to a personal computer, and balance ability can be assessed with appropriate software (Libra software, version 2.2). The wide support area of the Libra board consists of three interchangeable plugs, making it possible to have different degrees of structural difficulty (40 cm = high; 24 cm = moderate; 12 cm = easy). Subjects were asked to fixate on a point on the wall, in the eye plane, at a distance of 3 m. A moderate degree of difficulty was set, with the board plug placed at 24 cm.
The balance test was performed using the Libra board on the lateral plane in three stance positions, including bipodalic and monopodalic (both left and right) positions. Three trials per position were executed, with each position held for 30 s and a rest of 30 s between successive trials. The mean result of the three trials was used as the test outcome. The Libra board software returns the outcomes in arbitrary units. The scores range between 0 and 100, and the better results are closer to 0. The same instrument and the same protocol have been used in other studies [12–14].
Countermovement jump test
The muscular power of the lower limbs was assessed using Bosco’s procedures , specifically the countermovement jump test (CMJ) with a free arm-swing, which is performed under three different conditions, namely, bipodalic and monopodalic (both left and right) positions. Subjects started from an upright standing position, and they were instructed to flex their knees (approximately at 90°) as quickly as possible and then to perform a vertical jump as high as possible, with a free arm-swing. The test was performed using the Optojump system (Microgate, Bolzano, Italy), an optical infrared validated device that can assess the height of a vertical jump . The height of the CMJ is indirectly estimated as the following: 9.81 × (flight time)2 × 8−1 . The height of the vertical jump correlates with the power output of the leg extensor muscles during natural motion [15, 17], with the 1 RM in half-squat and 10 m shuttle test performance .
In the pre-test condition analysis, no significant differences were found between the BAL and ISO groups (p > 0.05) in terms of their anthropometric characteristics and the balance and CMJ tests. All of the subjects were assumed to be in the same pre-intervention condition.
The BAL and ISO groups started the training protocol on the same day; it was performed for 12 consecutive weeks. Both the BAL and ISO groups performed the specific training on Tuesdays and Fridays (Table 2). Each specific training session lasted approximately 30 min. Before starting the training session, subjects completed their usual warm-up. The warm-up was composed of three phases: (1) 5 min of jogging at a comfortable speed; (2) 2 min of static stretching for the lower limb muscles; and (3) 5 min of shooting, from both sides of the court.
ISO group conducted a specific training session using the leg press and leg extension isotonic machines (Technogym Inc.). Each ISO subject performed the following: 5 sets of 12 leg press repetitions at 70 % 1 RM with 3 min of recovery and 4 sets of 10 leg extension repetitions at 70 % 1 RM with 3 min of recovery. This training regime was primarily focused on increasing the strength of the lower limbs. The strength training protocols were in accordance with the ACSM’s 2009 position stand on resistance training. ACSM recommended that novice to intermediate individuals train with loads corresponding to 60–70 % of 1 RM for 8–12 repetitions .
The main purpose of the specific balance training was to improve balance ability through training with unstable surfaces, such as the Swiss-Ball (Perform Better Inc.) and the Trial-T1 half-sphere (TRIAL s.r.l., Forlì, Italy, 45 × 23 cm). The following balance training was performed by all of the subjects. First, subjects performed 8 sets of 20 s of Swiss-Ball kneeling hold balancing with 30 s of recovery; subjects were told to maintain balance on the Swiss-Ball while keeping their knee at an approximate angle of 90°. Then, they performed 6 sets of 20 repetitions of the two-handed chest pass balance exercise with 30 s of recovery; subjects were instructed to maintain balance on the Trial-T1 half-sphere while performing 20 chest passes to a teammate in front of them at a distance of approximately 10 m. Finally, subjects performed 10 sets of 30 s per limb of the single-leg balance, with 10 s of recovery between repetitions and alternating the supporting lower limb. Subjects were instructed to maintain balance on the Trial-T1 half-sphere while keeping a knee angle of approximately 120°.
In addition to balance or isotonic training, both the BAL group and the ISO group completed 3 technical-tactical training sessions with their head coach, two of which occurred at the end of specific training, for a total time of approximately 4 h and 30 min per week. The athletes’ weekly schedule is shown in Table 2.
Statistical analyses were performed with SPSS software (version 13.0; SPSS Inc., Chicago, IL, USA). All data are presented as the mean ± standard error of the mean (SEM). To check the reliability of the balance and CMJ tests, the intraclass correlation index (ICC) was applied  with 95 % confidence intervals (CI) and coefficients of variation (CVs). ICC shows an acceptable reliability for all of the tests (Table 3).
A paired t test was used to evaluate the differences between the pre- and post-tests, while an unpaired t test was used to compare the pre-test conditions of the BAL and ISO groups. The significance was set at an alpha level of 0.05.