A previous study detected local increases in arterial diameter and blood flow after eight weeks of resistance exercise (Okamoto et al., 2009), resulting in blunted hypoxic muscle stimulation. Van Wessel et al. (2010) reported that the increase in muscle oxidative capacity produced by resistance training has a negative impact on muscle hypertrophy. Furthermore, Adams et al. (1993) reported that motor unit recruitment is reduced in people who do not regularly perform resistance training. In order to ensure successful muscular strengthening and hypertrophy, it is necessary to assess how much mechanical and metabolic stress was induced to the muscle by the exercise protocol.
The drop-set method (DS) and the reverse dropset method (RDS) are exercise methods, which athletes often use for increasing muscle strength and hypertrophy. The DS is an exercise protocol, in which resistance exercise is initially performed with a higher load and then gradually decreased. On the contrary, the RDS is an exercise method in which the load used is gradually increased. The DS is based on the physiological phenomenon that high-threshold motor units can be activated more easily once they have been recruited (Gorassini et al., 2002). Strong muscle contraction during the DS leads to mechanical capillary compression, resulting in restricted blood flow to muscles and the induction of acute intramuscular hypoxia. It is assumed that marked muscle hypertrophy is induced when muscles are simultaneously subjected to metabolic and mechanical stimuli (Schoenfeld, 2013). However, no previous study has compared the effects of the DS and RDS on intramuscular hypoxia and motor unit recruitment among trained and untrained individuals.
Therefore, the aim of this study was to verify the influence of different load exercise to the muscle activity during subsequent exercise with 75% of one repetition maximum (RM) load among trained and untrained individuals. The triceps brachii muscle, which plays a major role in the bench press exercise, was used in the study. The bench press is essential for many athletes looking to increase upper body strength (Dunnick et al., 2015). For the assessment of the effects of exercise training protocols, non-invasive and practical evaluation methods such as near-infrared spectroscopy (NIRS) and surface electromyography (EMG) are useful alternatives to muscle biopsy and magnetic resonance spectroscopy (Tanimoto et al. 2006). We hypothesized that the DS might be more effective at activating high-threshold motor units and inducing intramuscular acute hypoxia.
Sixteen resistance-trained men who were involved in a resistance training [trained group; mean age: 21.9 [+ or -] 2.6 years; mean height: 1.73 [+ or -] 0.05 m; mean body weight: 68.2 [+ or -] 9.1 kg; mean 1RM during bench pressing with a narrow grip (BPN): 61.5 [+ or -] 14.8 kg; mean triceps brachii thickness: 4.7 [+ or -] 5.1 cm] and 16 healthy young men who did not exercise regularly (untrained group; mean age: 22.7 [+ or -] 2.9 years; mean height: 1.74 [+ or -] 0.04 m; mean body weight: 62.6 [+ or -] 8.3 kg; mean 1RM during BPN: 35.4 [+ or -] 7.5 kg; mean triceps brachii thickness: 3.6 [+ or -] 0.9 cm) were recruited from among the students at Aino University. The inclusion criteria for the trained group consisted of at least 1 year's experience of resistance training, participating in a resistance training program at least 3 days a week, and performing triceps brachii exercises at least once a week. Subjects who reported any musculoskeletal injuries of the upper extremities in the year before the test were excluded. All subjects were instructed to refrain from vigorous physical activity within 12 hours of each session (Maehlum et al. 1986). Before participating in the study, the subjects were informed about the study procedures and any possible risks both verbally and in writing before signing informed consent forms. A priori data and a power analysis were used to detect the sample size. A minimum sample of 14 subjects was calculated based on detecting a difference of concentric muscle power, area under the Oxy-Hb curve, and RMS of EmG with 80% power and 5% significance. The sample size was calculated with the G Power software (version 3.1.4). The ethics committee of Aino University approved the study, which was conducted according to the most recent declaration of Helsinki.
Exercise protocols of the DS and RDS
All 1RM and BPN testing were performed using a press bench and a standard 20-kg Olympic barbell. Each subject lay with their back on the press bench and both feet on the floor. An electrogoniometer (DTS2D goniometer; Noraxon, Arizona, USA) was used to prevent compensatory horizontal abduction in the shoulder joint during the DS and RDS. The electrogoniometer was attached to the radial side of the right forearm and the lateral side of the upper right arm. They were asked to place their upper arms so that they were perpendicular to their body, flex their elbow joints to 90 degrees, and grasp the barbell, which was held in a fixed stand. They lifted the barbell from this starting position to full extension and then returned to the starting position. This triceps brachii concentric/eccentric contraction cycle was performed at a metronome-controlled tempo of one second per concentric contraction and one second per eccentric contraction. The subjects were instructed to perform the concentric phase of each repetition as fast as they could by pushing the barbell to complete extension as rapidly and explosively as possible. More than one week later after 1RM testing, the subjects performed BPN exercises using two different training set methods, the DS and RDS. The DS and RDS exercise were separated by intervals of at least 1 week. The order of the DS and RDS exercises was randomized for each subject. The DS and RDS protocols are shown in Figure 1. In order to achieve identical higher-volume workloads during both training set methods, load and the number of sets were determined (Schoenfeld 2010).
Triceps brachii concentric contraction power measurements
In both the DS and RDS, the peak power of each repetition during the concentric phase of 75% 1RM load exercise was assessed with a FiTROdyne Powerlyzer (Fitrodyne; Fitronic, Bratislava, Slovakia), which was attached to the barbell using a tether. The FiTROdyne unit uses the tether displacement time and manually entered load data to calculate power values. Jennings et al. demonstrated that this procedure exhibited high test-retest reliability (intraclass correlation coefficient; ICC: 0.86) when it was used to assess muscular power during a multiple-joint exercise (Jennings et al., 2005). The first of the 10 repetitions was different from other nine in that the barbell was lifted from the fixed stand during this repetition, so the data for the first repetition were excluded. The mean peak power was calculated from the remaining 9 repetitions. On two...