There are a variety of strength training methods that are used to develop athletic performance, such as sprinting and jumping. These methods include; 1) heavy strength / hypertrophic training, 2) plyometric training and 3) explosive weight / power training using various loads associated with different parts of the power-velocity curve. Heavy strength training typically induces increases in maximum force production and muscle mass (Chelly et al., 2009; Cormie et al., 2010; Hakkinen, 1989; Sale, 1988; Tesch, 1989; Wilson et al., 1993), while plyometric training primarily increases rapid force production (Impellizzeri et al., 2008; Matavulj et al., 2001; Markovic et al., 2007). Explosive weight training may be considered as a hybrid of these two methods as it has been shown to develop all of these aspects of neuromuscular performance (Cormie et al., 2010; Harris et al., 2008; Lamas et al., 2012; McBride et al., 2002; Winchester et al., 2008).
One example of explosive weight training that has been the focus of several research papers is the jump squat exercise. Here, a loaded bar is held on the shoulders and the individual squats down prior to rapidly extending the legs and torso to finally leave contact with the floor. The load used during jump squat training seems to be an important consideration for training outcomes. Following the law of specificity, training with lighter loads improves power at the high velocity end of the force-velocity curve, whereas higher loads improves power at the high force end of the force-velocity curve (McBride et al., 2002; Smilios et al., 2013).
However, the efficacy of jump squat training to simultaneously improve different athletic performance remains an element of contention within the literature. For example, use of light load jump squat training (e.g. 0-30% 1-RM load) has not led to improved maximum strength in some studies (Cormie et al., 2010; Wilson et al., 1993). Conversely, heavy load jump squat training (e.g. 60-90% 1-RM) has been shown to be ineffective in improving countermovement jump performance/rapid force production (Newton et al., 1999). The above examples have used fixed loading protocols that were determined as a percentage of maximum squat performance (i.e. 1-RM) without assessing the individual's power curve during the jump squat.
In studies where the load that maximizes mean power output (calculated without the inclusion of body mass) has been calculated there seems to have been more consistent simultaneous improvement in maximum and rapid force production, as well as sprint and jump performance (Lamas et al., 2012; Newton et al., 1999; Smilios et al., 2013; Wilson et al., 1993). Nevertheless, use of loads that maximize mean power output has not been studied in depth. Given that maximum mean power output may shift slightly during training with different loading schemes (McBride et al. 2002; Smilios et al., 2013), it may be pertinent to assess what load maximizes power output and train according to that load in order to develop global improvements in neuromuscular performance.
Furthermore, due to the use of; 1) different training protocols, 2) different test protocols, 3) different subject training background (i.e. trained and untrained), and 4) low sample sizes used in the literature, it is not certain whether jump squat power training with individualized loads can indeed lead to simultaneous improvements in several athletic performance tests. Consequently, the purpose of the present study is to determine whether jump squat power training with individualized loads can simultaneously improve maximum strength, rapid force production, as well as vertical jump and sprint performance using a large cohort of physically active men.
Healthy, young men performed 8 weeks of loaded jump squat training. During training, each repetition was monitored and real-time feedback provided to the subjects to ensure maximum effort and appropriate termination of the set. The subjects were tested for maximal isometric squat strength, vertical jump performance and maximum sprinting speed pre-, mid- (after 4 weeks), and post-training (after 8 weeks). The tests were performed on 2 separate occasions, the first test day included countermovement jump (CMJ), squat jump (SJ) and the jump squat diagnostic series (described below). On the second day of testing, subjects performed isometric half squat and 50m sprint trials. Prior to all test sessions a standardized warm-up was performed consisting of 5 min jogging followed by 5 min dynamic stretching. Before the vertical jump tests the subjects performed 2 sets and 8 reps of jump squats (body weight only) separated by 1 minute of rest. Isometric testing was preceded by 2 submaximal trials over duration of approx. 4 seconds separated by 2 minutes of rest. Maximal running speed (50 m) was preceded by two 50 m sprints with submaximal effort.
Sixty eight male students of the Faculty of Physical Education and Sport completed the study (age 21.9 [+ or -] 2.5 years, body height 1.80 [+ or -] 0.06 m, and body weight 75.3 [+ or -] 9.5 kg). Subjects were moderately trained athletes with at least 2 years' experience in strength training. After being fully informed on all possible risks and discomfort they signed an informed consent form. The University's ethical committee approved this study, which was carried out according to the Declaration of Helsinki. At the beginning of the study subjects were pair-matched and randomly divided into the experimental group (EXP; n = 40) and the control group (CON; n = 40). However, due to dropouts (e.g. unrelated injuries, non-attendance in training, failing to attend test sessions) the final sample size included in this study was 36 in the experimental group and 32 in the control group.
As part of the study, subjects completed a familiarization session. Subjects were informed about the correct (unloaded and loaded) jump squat technique. Each subject practiced this exercise and was subsequently instructed through a strength training specialist how to improve their individual technique. All subjects were familiar with the devices used in the study, as well as with the measurement methodology.
Determination of the load that maximizes average power output--diagnostic series
Before the start of the experiment, subjects performed a diagnostic series of jump squats with maximal effort in the concentric phase of the movement. During the jump squat, each subject squatted down to a knee angle of approx. 90[degrees] (180[degrees] = full extension), which was controlled through the use of foam cubes, while supporting the barbell on the shoulders (Vanderka et al. 2016). The instructions for subjects were as follows: squat down in a controlled manner and then immediately jump straight up as quickly as possible. The series consisted of two trials with each load and gradually increased in 10 kg steps with 3 min rest between trials. The external load began with 20 kg (i.e. bar only) and the test was terminated upon reaching a plateau (or even decrease) in maximum average power output (Pmax). To confirm this finding, each participant performed 2 attempts above the Pmax load (+20 kg) as a control measurement.
Determination of the number of repetitions above 90% of maximum power output--pilot study
Based on findings of Baker and Newton (2007), who stated that maintaining power output of >90% maximum power output was possible for 2-3 repetitions with 45-60% 1-RM (full squat) and 5-6 repetitions with 35% 1-RM, it was important to determine how many repetitions would be possible with the chosen loads in the present study. Therefore, one week before pre-training measurements, a sub-set of the subjects (n = 20; age 21.5 [+ or -] 1.4 years; body height 1.78 [+ or -] 0.02 m; body weight 75.3 [+ or -] 8.7 kg) were randomly chosen to perform 1 set with Pmax load and 1 set with a lighter load corresponding to 90% of maximum power output (approx. 80% Pmax load according to the diagnostic series described above). The subjects performed these loading sets in a counterbalanced, crossover design. The subjects were instructed to jump as high as possible until the power output fell below 90% of the highest power output (typically rep 2 or 3) of the set. The subjects could perform, on average, 8.3 [+ or -] 2.8 repetitions maintaining the power output above 90% with lighter external load, and 4.4 [+ or -] 1.5 repetitions with Pmax load. The determined load and repetition series formed the basis of...