Effects of Short Rest Intervals on Body Composition, Hormonal Response and Strength During High-Load Resistance Trainings

Background: The modulation of rest intervals in resistance training could induce different muscular tissue and hormonal response. The aim of the present pilot study was to the long-term adaptations induced by short rest intervals in high-load resistance training. Methods: pulley, leg press, curl with dumbbell, french press, hummer curl, hummer curl with relaxation and pull ups) such as to move close to a ’classical’ training tab. Anthropometric parameters, strength and hormonal responses were taken in all the subjects before and after the training protocol. Results: Training increased arm (P<0.05), thigh (P<0.05) and chest (P<0.01) circumferences, while decreased waist circumference (P=n.s) with respect to baseline. Strength on RM was increased on bench press (P<0.05), pulley (P<0.05) and leg press (P<0.01) exercises. Blood analyses showed a decrease of IGF1 (P=n.s) and cortisol (P<0.05), while testosterone (P=n.s), DHT (P<0.01) and GH (P<0.001) were increased. In addition, we observed an increase of TEST/CORT ratio (P<0.05). Conclusion: The results of the present study showed that a single training program with short rest intervals associated to high-load strength exercises could induce an increase in both muscular mass and strength. Strength

mechanical stress [3], In contrast, high-volume programs (greater number of repetitions concomitant with the use of short rest intervals) elicit greater metabolic stress [2]. Among the different hormonal adaptations induced by physical exercise, Testosterone (TEST), Growth Hormone (GH) and Insulin-like Growth Factor-1 (IGF1) are the most studied hormones. The expression of these hormones is related to the intensity [4], rest interval and volume of the exercise [5]. The classic model of periodization, the systematic process based on the altering of one or more training program [6], is characterized by a high initial training volume and low intensity, and, with the progress of training, volume decrease and intensity gradually increase [2].
As concern rest periods, it was observed that exercise with long versus short rest periods (2-5min vs 30-40s) induce a greater strength increase [7]. In contrast, according to other studies, low intensity, high volume and moderate rest periods (2-3min) training program have been demonstrated to induce an increase of hypertrophy [8,9]. However, a few methodological limitations, including program design and hypertrophy assessment, raise several questions concerning the efficacy of each training program.
Moreover, the hormonal response associated with these training programs, remain unclear. The aim of the present pilot study was to investigate the effects of a specific resistance training program with short rest periods on the physiological adaptations of muscle strength and hypertrophy.

Experimental Design
Prior to the onset of the study, the participants were required

Strength Testing
Strength in the bench press, pulley and leg press was assessed pre-and post-training. A general warm up consisting of riding a cycle ergometer and abs exercise such as bicycle crunch and traditional crunch for 5 min. Subsequently, the general warm up was followed by specific warm up for 10 smith machine, 10 leg press and 10 pulley with 20% of RM each other. The determination of one repetition maximum (1RM) was executed as described by Mangine et al. 10. 1RM was estimated for the other exercise: curl with dumbbell, french press (dumbbell), hummer curl, hummer curl with relaxation, pull-ups pre-and post-training.

Resistance Training Intervention
Participants reported to the Pisa University Sports Centre (CUS) three times per week (M/W/F) to complete their first assigned training program. Briefly, the first training program was structured to increase strength and hypertrophy. Number of series and repetitions change for each exercise, but the basic concept that linked the structure of the training program is that every repetition must be carried out as if it were the last, bringing muscle exhaustion.
This was achieved by working with a weight that induced this condition. The participants performed at 87-100% of 1RM. The second training program was structured to increase strength. On Monday and Friday, participants performed pyramidal system to rise and fall for smith machine, leg press and pulley, starting from 92% until 100% of 1RM. On Wednesday, participants performed 3x1 at 100% of 1RM about smith machine, leg press and pulley plus 1xmax with 50% of RM and with a weight that allows the execution of 3 series to 6 reps for triceps and hummer curl. The gradual increase in weight for every exercise was calculated in order to obtain the muscular exhaustion before the completion of reps. The weight of exercise was increased in every training session. Training intensity was determined from 1RM testing (smith machine, leg press and pulley) and estimated 1RM (all other exercises) [10]. For each training program, the rest period between each series was 90 s, while the rest period between each exercise was 2min. Only in the step between upper and lower body recovery, rest period was 5 min. Both training programs had an average duration of 70±25 min.

Blood Sampling
Blood samples were obtained at two time points: baseline (BL) (after the two weeks of preparatory phase) and at the end of the resistance training intervention (E). Each blood samples were

Statistical Analysis
The values are expressed as mean ± standard deviation (SD).

Anthropometric and Morphological Changes
After the anthropometric evaluation, the subjects' BMI did not significantly differ before and after training program ( Figure 1).  Figure 2). In contrast, the waist circumference was decreased (78.51±3.30 vs 77.86±2.44cm, p=n.s, Figure 2). An     a) The data are expressed as mg/dl of glucose, total-cholesterol, HDL, LDL, triglycerides b) and insulin μU/ml before and after training.

Biochemical and Hormonal Parameters
These data represent the men ± SEM of pre-and post-training program. *P<0.05 vs pre-training program.
Blood parameters in response to training, during W3 and W10 are shown in Figure 4. As concern the lipid profile, the training

Discussion
Two features are remarkable in our protocol: 1) The recovery time used during training sessions (90 seconds); 2) The onset of an anabolic condition.
Considering them, there was not only a great strength and hypertrophy rise, but also an important metabolic and hormonal adjustment. According to literature data, many resistance training protocols have been already demonstrated to increase strength and hypertrophy, but they differ as far as concerned the recovery time used [11][12][13][14]. For example, in terms of chronic adaptations, De Salles, et al. [15] suggested that 3-5 minute rest periods between sets can produce greater increases in absolute strength. Also

Schoenfeld et al. (2016) argued that a longer inter-set rest period
could better enhance muscle Strength and hypertrophy [16]. The other remarkable feature of our protocol is the significant fat free mass increase after only 8-week training: in fact, the increase of armchest-thigh circumferences and the waist circumference decrease are usually observed in aerobic trainings programs or in longer resistance training ones [17]. It has also to be noticed that, in our study, participants performed a high-load/ low volume workouts as against other protocols that suggested exactly the opposite in order to obtain a protein accretion. For example, Burd et al. (2010) argued that low load/high volume resistance training is more effective in increasing muscle protein synthesis [18]. In the same way, other studies have demonstrated that the low load resistance, performed to muscular failure, induced a similar myofibrillar accretion [19].
From a metabolic point of view, the lipid profile improvement is huge, considering our 8-week high -load intensity training (87-100% of 1RM) : some studies even suggest that resistance training doesn't affect the lipid profile [20,21], while other authors argue that only low/moderate intensity resistance training can modulate LDL, triglycerides and HDL in a positive way [22].
Both the marked enhancement in muscle strength and the better lipid profile could result from the anabolic adaptations that happened in the present study (TEST-DHT-GH increase / IGF1-ACTH-DHEA-CORT decrease): in particular, testosterone administration is known to reduce insulin resistance, total cholesterol, LDL cholesterol and triglycerides and improve anthropometric parameters [23]. the longer rest promotes a long-lasting elevation for TEST [26].
In this case, the authors evaluated hormonal parameters after 30min maximum. In contrast, we measured them after 2 days, in order to observe the long-term adaptation. The anabolic condition achieved in our study is also confirmed by the DHT rise and the DHEA-s decrease. Dyhidrotestosterone, a free TEST intermediate synthetized by 5α-reductase is the most active TEST metabolite thanks to a higher receptor activity and a lower dissociation rate compared to TEST. It has been showed that 12 weeks of resistance training significantly restored, in older man, free testosterone, and DHT to levels seen in young subjects [27].
DHEA-S, instead, is a TEST precursor [28] and so its decrease correlates with increased TEST levels. Conflicting data concerning DHEA response after exercise have been reported: studies reported its increase after resistance exercise [29] or any change after training in men and women [31]. In all these studies, the evaluation has been conducted in acute, while we performed the analysis after 48 hours. In addition, these authors have used training protocols with different load, intensity and rest periods making difficult the comparison among them. In our protocol, the anabolic process activation is also supported by the evaluation of serum GH. It is a metabolic stressor related to body growth, hypertrophy and regulation of metabolism and its release seems to be dependent to rest intervals between sets [32]. Bottaro et al. (2009) observed that GH increases more significantly with short rest intervals (30s) than long-rest ones (60 or 120s) [33]. Similar results were obtained by Fink, et al. [34]. Even if we used similar short-rest intervals (90s), unlike them, who used low loads, our participants performed a high-load (87<>100%) with short rest period and medium repetitions. Moreover, the GH-hypertrophy effect is regulated by IGF1, but it has been shown that its local expression in skeletal muscle appears to act independently of any change in serum GH or IGF-1 [35]. Therefore, free fat mass increase and hypertrophy could be related to a local IGF-1 increase, since we did not observe a significant serum IGF-1 change.
In our hands, no significant changes in LH serum level. LH stimulates intratesticular TEST secretion [36] but its response to physical activity has been rarely investigated. Consistent with our data, Taipale et al. (2017) have observed no alteration of serum LH after resistance training even though they used lower loads than ours [37]. Of note, both cortisol and ACTH serum levels decreased in our study, therefore favouring the TEST/CORT ratio enhancement. This is a significant result considering the fact that cortisol is usually involved in the inflammatory response to exercise becoming an important parameter of the overtraining syndrome [38]. In this sense, Izquierdo et al. (2009) showed how cortisol level can increase after resistance trainings with a consequent high release of pro-inflammatory cytokines [39]. Probably, in our protocol, cortisol reduction depends on our high load/low volume training program: in fact, its greatest acute elevation has been demonstrated to occur in different protocols from our workouts, such as medium -load, high-volume protocols, usually preferred by body builders [40].

Conclusion
The aim of our study was to investigate the effects of 8 weeks of resistance protocol based on high load (over 87%RM), medium repetition, short rest periods between sets (90s) and short rest periods between exercises on physiological point of view. Overall, we observed a significant increase of strength, related to significant increase of DHT and a trend of increase of testosterone, the major responsible to strength increase. In addition, we observed an increase of free fat mass related to increase of GH and the ratio of testosterone cortisol. As concern the lipid profile, a decrease of cardiovascular risk factors typical of aerobic exercise were observed. All these adaptations could be related to the use of short rest periods with high load, which induce a not completely recovery as concern muscular (i.e. lactide acid restore) and heart rate.
Herein, we showed that a high load with short rest periods could induce typical adaptations of strength and hypertrophy training, as well as some typical adaptations of aerobic workouts. All authors have read and agreed to the final version of the manuscript.