There are several parameters in strength training that have a significant influence on strength training responses. Among the most important are the size of the load and the number of repetitions and sets (10). In hypertrophy-type training, increasing power production capacity is not the main focus of the training. Therefore, multiple sets and a high number of repetitions have been used (4), which not only causes hypertrophy but also develops high-intensity muscular endurance (16) and aerobic performance (11). In this form of training, 1- to 2-minute rest periods between sets are typical (5), to maintain the high intensity of the training. In the initial phase of training, the development of force has been shown to be predominantly neural, whereas hypertrophic changes may occur later (33).
Testosterone (TES), which is an anabolic hormone, has been shown to play a major role in strength development (19,20,22). However, some studies have shown no elevation in resting TES levels during strength training (15,29), possibly because of uptake by androgen receptors (26). On the other hand, Ahtiainen et al. (3) and Kraemer et al. (21) reported significantly higher TES levels at rest after 10-14 weeks of strength training, highlighting the importance of training volume. Hormonal responses may also be related to training intensity and the training background of the subjects (6, 8). In previously trained men, different recovery times do not seem to influence hormonal responses or neuromuscular performance during several months of strength training (4).
Training may also be too strenuous, and cortisol (COR) has been used as one of the indicators of training load (23). As with TES, it has been shown that COR may elevate (12), decrease (20), or stay at the prephase level (13) after strength training. It has been suggested that chronic changes in COR may be involved in tissue homeostasis and protein metabolism (25). The COR and TES have been shown to be mediated via binding to the glucocorticoid receptor and the androgen receptor, respectively (36). The regulation of these receptors may be dependent on nutrition or design of the training program (36). The TES/COR-ratio has been used to indicate the anabolic-catabolic balance of the body (2). If COR levels increase more than TES levels, or TES levels even decrease, the body may go into a catabolic state, which may decrease functional and structural capacity of the body. Shorter rest periods appear to be beneficial for producing anabolic hormone responses. However, if the rest period is too short, the total loading may be too high, which may then cause nonfunctional overreaching, leading to the possibility of overtraining syndrome (31) during strength training over a longer period of time. The use of a constant recovery time may not be ideal, because the optimal recovery time may be highly subject dependent. One possibility is to use heart rate (HR) as a tool to evaluate training load and optimal recovery. The HR and thus blood flow increase during exercise (28). Therefore, HR is directly proportional to the intensity and duration of the exercise. There have been only a few studies that have examined HR recovery after strength training exercise. Otsuki et al. (34) showed that in strength trained men, the HR decreased more rapidly back to baseline level after a single exercise set as compared with sedentary controls. They also suggest that increased vagal nerve reactivation might be a primary factor for the fast recovery of HR after the exercise. Heffernan et al. (14) also showed faster recovery after 6 weeks of resistance training. They also investigated the effects of detraining and showed that HR recovery was regressed after 4-week detraining period. These results suggest fast adaptation in HR recovery in the beginning or after the training period. Polar Electro has developed a new hear rate monitor (FT80) for strength training which informs when the person is sufficiently recovered to continue the training session. The purpose of this study was to examine whether this HR recovery program designed by Polar Electro would lead to better responses in different physiological and biomechanical parameters than a program with constant recovery times.
To best of our knowledge, no previous studies have examined the effects of accurate HR dependent recovery times on neuromuscular function, aerobic performance, and basal hormone responses during hypertrophy-type strength training. The aim of this study was to evaluate differences between HR-based recovery time and constant recovery time in terms of hormonal and neuromuscular responses to 7 weeks of strength training. The TES and COR were used as indicators of anabolic and catabolic body states, respectively. Strength development (1 repetition maximum [RM] and 10RM) and central activation ratio (CAR) were used to examine neuromuscular performance and muscular and neural development. Maxo2 was measured to evaluate possible changes in aerobic performance and also monitor possible decrement of the common performance level caused by overuse. It was hypothesized that individually determined recovery times would optimize the recovery period and would therefore lead to more effective responses to hypertrophy strength training, without causing symptoms of overreaching.
In total, 24 male subjects participated in this study. Subjects were divided into 2 equal groups according to their level of voluntary physical activity, which was determined by the International Physical Activity Questionnaire (9). One main criterion for subjects was that they did not have any strength training background during the past 12 months. From a total of 24 subjects, 21 (31 ± 9 years) completed the entire study period (FT80 study group, n = 12; CONTROL, n = 9). Subjects were instructed to continue normal daily exercises as they had done before and to use an HR monitor during each physical training session. These exercises were mostly different types of endurance exercises (e.g., jogging). Additional strength training was not allowed. All subjects signed an informed consent and were aware of the protocol and possible risks of the study. Subjects were also advised of their right to withdraw from the study at any time. The study was conducted according to the declaration of Helsinki, and the methods were approved by the ethics committee of the University of Jyväskylä.
At the beginning, subjects were familiarized with all test and training equipment. Subjects trained 3 times per week over a 7-week period. The test group used a Polar fitness trainer FT80 during the training. The FT80 HR monitor gave a signal after each set, which notified the subject when recovery, according to the algorithm of Polar, was sufficient to perform the next set. The control groups used constant 2-minute recovery periods after each set, which was measured with a Polar F6 HR monitor. The training program consisted of programs A and B, which are presented in Table 1. David's strength training devices were used except in abdominal, low back trainings and in shoulder press presented in program A. Ten-RM loads were measured during the first training session. Subjects increased the loads during the 7-week period if the first 10RM load was not heavy enough and they were able to do 11 or more repetitions. In each program, each movement was performed 3 × 10RM, excluding program A abdominal and low back, which were performed 3 × 15-20 reps. Programs A and B were performed in turn making total of 3 training sessions per week at least 48-hour recovery between the each session. After the 7-week training period, 10RM loads were remeasured. Both groups also used HR monitors during other exercises, which was a different type of endurance exercises (e.g., jogging). Therefore, it was possible to evaluate subjects' total training volume during the 7-week training period.
Basal serum hormone concentrations were taken before the first training session (0 week) and 2 days after the last training session (7 weeks). Blood samples were drawn from an antecubital vein using a needle into 5 ml Venosafe (Terumo Medical Co., Leuven, Belgium) vacuum-gel serum tubes after an overnight fast (basal) at rest seating position between 7:00 and 8:00 am. Samples (10 ml) were centrifuged for 30 minutes from collection at 3,500 rpm for 10 minutes to separate the serum. Serum samples for the hormonal analyses were kept frozen at −20° C until analysis. Serum TES and COR hormone concentration were analyzed by Immulite 1000 (Diagnostics Products Corporation, Los Angeles, CA, USA) using commercial chemiluminescent enzyme immunoassays (IMMULITE®/IMMULITE 1000; Diagnostics Products Corporation). The intraassay coefficients of variance for these assays were <5.7, <4.0, and < 4.8%, respectively, and sensitivity of variance values were 0.5 nmol·L−1, 0.026 mIU L−1, and 0.2 nmol L−1, respectively.
Body composition measurements (body mass, skeletal muscle mass [SMM], and percentage of body fat [F%]) were performed at the same time points as blood samples using 8-polar bioelectrical impedance (Inbody720, Biospace Co. Ltd, Seoul, Korea). Eight-polar bioelectrical impedance has been shown to be a reliable method to detect changes in body composition when performing under strict standardized conditions (18). Accordingly, ΔSMM and ΔF% during the study were calculated using this method.
Subjects started HR recording (FT80 and F6) at the beginning of the each training session. In both strength training programs A and B, leg exercises were performed first. Leg exercise average and maximum HR are presented in Table 2. Total leg exercise time was measured for both FT80 and control groups. At the time of the measurements, FT80 was a prototype, which did not yet allow recording of the recovery times. However, because the recovery time of 2 minutes among control group was always the same, the time used for 1 leg exercise set for control group could be calculated. Assuming that total working time for 1 set was the same in control and test groups, the recovery time for the test group was calculated using the following formulas:
where TFT80 is the total leg exercise time for group FT80 (6 sets), S is the number of sets, Ct is the time taken to perform 1 set by the control group, and R is the number of the recovery period. Ct was calculated using the following formula:
where Tcontrol is the total leg exercise time for control group (6 sets) and TR recovery time between sets among control group (always 2 minutes). Total training time during the entire 7 weeks' training period was calculated = strength training + other physical exercises.
Concentric 1 Repetition Maximum and 10 Repetition Maximum
Concentric 1RM knee extension was measured using a knee extensor dynamometer (Hurlabs Oy, Tampere, Finland) at the beginning, middle and after the training period. Subjects were seated with a hip angle of 110° and a knee angle of 90°. Pneumatic load was increased until subjects were not able to extend their legs to a knee angle of at least 170°, starting from 90°. Load was increased progressively and subject performed 4-6 trials until the maximum was achieved. Subjects had 45-second rest between the attempts. Pneumatic 1RM load was analyzed.
Ten-RM loads were measured in the gymnasium at the beginning and end of the study period. Loads were measured from all exercises, but only knee extension and flexion are reported because CAR was measured from the quadriceps muscles. Effect of loads was visually evaluated, and load was increased if the first 6 repetitions were too easy. Performance was approved if the number of repetitions was between 8 and 12, and 10RM was then estimated (30). Total volume was calculated (load × repetitions) as some subjects were able to perform 10RM with maximal load during the 7th week measurement.
Central Activation Ratio
Central activation ratio was measured using the superimposed twitch method from the quadriceps muscle group (32) at the beginning and end of the training period. This ratio represents the level how fully subjects are able to activate their muscles. Stimulation is given during MVC and will cause additional force, if the muscle has not been fully activated by voluntary commands. Subjects sat with a hip angle of 110° and a knee angle of 120°. Six stimulation electrodes (5 × 10 cm) were placed over the quadriceps muscle. Measurements were performed during knee extension MVC with supramaximal stimulation intensity (intensity of max passive twitch response + 50%). Stimulation was induced using 0.2-milliseconds double pulses with 10-millisecond intervals using a DS7A stimulator (Digitimer Ltd.; Welwyn Garden City, United kingdom). Torque data were collected using a 1401 Power A/D-converter (CED, Cambridge, United kingdom) and Spike2-software (CED. Central activation ratio (17) was analyzed using voluntary torque and superimposed stimulation torque responses.
To determine whether strength training and different interset rest periods would affect o2max (ml·kg−1·min−1), the subjects performed a maximal treadmill test before and after the 7-week training period. The start of the test involved walking for 3 minutes at 4.5 km·h−1 (1% slope) as a warm-up. Thereafter, exercise intensity was increased every 3 minutes to induce an increase of 4 ml·kg−1·min−1 in women and 6 ml·kg−1·min−1 in men, which correspond to the theoretical o2max demand of running (1). This was achieved by increasing the initial running speed of 4.5 km·h−1 by a mean of 0.9 km·h−1 (range 0.6-1.4 km·h−1) and by increasing the initial grade of 1° by a mean of 0.5° (range 0.0-1.0°) up to the point of exhaustion (1). Pulmonary ventilation and respiratory gas exchange data were measured online using the breath-by-breath method (Jaeger Oxygen Pro, VIASYS Healthcare GmbH, Hoechberg, Germany), and mean values were calculated at 1-minute intervals for statistical analysis. The HR was continuously recorded at 5-second intervals using a telemetric system (Polar810i, Polar Electro Oy, Kempele, Finland). The criteria used for determining o2max were as follows: a lack of increase in o2max and HR despite an increase in grade and speed of the treadmill, a respiratory exchange ratio >1.1 and a postexercise blood lactate value (determined 1 minute after exercise completion from a fingertip blood sample using a lactate analyzer; LactatePro®, Arkray, Kyoto, Japan) that was >8 mmol·L−1 (1). All participants fulfilled these criteria.
Repeated-measures analysis of variance (ANOVA; group × training) with Bonferroni post hoc tests were used to identify significant differences between and within the groups. One-way ANOVA was used to identify significant differences between the groups in recovery times, total training time, and relative changes in measured variables. All data are presented as mean ± SD. Where assumptions for normality were not met, the data were log transformed before statistical analysis. The untransformed values are shown in the text, tables, and figures for more meaningful comparison. Statistical analyses were performed using SPSS (Version 15.0.1. 2005; SPSS Inc., Chicago, IL, USA). The level of statistical significance was set at p ≤ 0.05.
Recovery Times and Total Training
Recovery times between the test and control groups were significantly different. Test group (FT80) recovery time at the beginning was 54.5 ± 18.6 seconds, whereas control group (CONTROL) used a constant 120-second recovery. There were no significant differences in any group recovery times after 4 or 7 weeks of training. The recovery times are presented in Figure 1. There were no significant differences in total exercise times between the groups during the 7 weeks of training.
At the beginning of the training period, body composition characteristics were similar between the groups. Weight remained at the same level after 7 weeks of training (FT80 0.7 ± 2.5%, n.s.; CONTROL 0.7 ± 2.7%, n.s.). There was a main effect of training for SMM (p < 0.001) and F% (p < 0.001). The SMM increased in both groups (FT80 2.3 ± 1.1%, p < 0.001; CONTROL 2.4 ± 2.6%, p < 0.05). Fat% decreased in both groups, although significance was not observed (FT80 −6.1 ± 8.4%, n.s.; CONTROL −5.5 ± 10.9%, n.s.) (Table 3).
Concentric 1 Repetition Maximum
Before training, the 1RM was similar in the FT80 and control group. However, there was a main effect of training for concentric 1RM (p < 0.001), which increased significantly in both groups during the 7-week strength training period. After 4 weeks of training, 1RM increased by 24.0 ± 11.8% (p < 0.001) in FT80 and by 18.0 ± 12.9% (p < 0.001) in CONTROL. During the last 3 weeks of training, 1RM continued significant increment (CONTROL 19.6 ± 13.4%, p < 0.001; FT80 13.1 ± 8.1%, p < 0.001) (Figure 2).
Ten Repetition Maximum Loads
As with concentric 1RM, 10RM knee extension and flexion load exhibited a main effect of training (p < 0.001), increasing after 7 weeks of training. At the beginning, the highest total volume during 1 knee extension set was observed in CONTROL (871 ± 159 kg), whereas FT80 total volume was slightly lower (667 ± 200 kg). After 7 weeks of training, 10RM increased in both groups (FT80 63.3 ± 33.6%, p < 0.001; CONTROL 32.7 ± 21.3%, p < 0.001). However, the relative increase in FT80 was significantly higher (p < 0.05) than in CONTROL. Regarding knee flexion, FT80 and CONTROL values were 462 ± 83 kg and 529 ± 107 kg, respectively. After the training, the greatest increase was observed in FT80 (53.8 ± 21.6%, p < 0.001), whereas the increase in CONTROL was 24.8 ± 25.9% (p < 0.01; Figure 3).
Central Activation Ratio and Aerobic Performance
There was a main effect of training for CAR (p < 0.01) increasing in both groups toward the end of the training period. However, a significant increase was only observed in FT80 (1.6 ± 2.3%, p < 0.05) after training. Aerobic performance (o2max) remained the same, and there were no differences between or within groups. Central activation ratio and aerobic performance data are presented in Table 4.
Basal Serum Hormone Concentrations
There was a main effect of training (p < 0.001) and group × training interaction (p < 0.01) for TES. At the beginning of training, TES levels did not differ between the groups (FT80 15.2 ± 4.5 nmol L−1; CONTROL 12.7 ± 5.0 nmol L−1, n.s.). After 7 weeks of training, FT80 TES level increased in FT80 by 22.8 ± 15.3% (p < 0.001), whereas the increase was only 7.3 ± 20.4% (n.s.) in the control group. In addition, TES level was significantly higher in FT80 than in CONTROL (p < 0.01) after 7 weeks of training (Figure 4).
There were no significant differences in serum COR levels at the beginning or after the 7-week training period between or within the groups, nor were significant changes observed during the training period (FT80 −5.3 ± 21.9% n.s.; CONTROL −3.2 ± 14.9% n.s.) (Figure 5). There was also a main effect of training (p < 0.001) and group × training interaction (p < 0.05) for TES/COR ratio. However, the increase was only significant in the FT80-group (FT80 36.3 ± 39.0%, p < 0.001; CONTROL 12.4 ± 25.4%, n.s.; Figure 6).
This study showed that the recovery period after each set in strength training may be <1 minute if it is based on individual HR (FT80 54.5 ± 18.6 seconds). Thus, the intensity is much higher and the total workout can be performed in less than half the time when compared to the traditionally used 2-minute recovery period. Previously, it has been shown that strength training will cause improved HR recovery (14), which might be caused mainly by increased vagal nerve reactivation (34). In this study, the recovery time in the study group was slightly decreased after the 7 weeks of training although the decrement was not significant. Nonsignificant finding might be partly be caused by large deviations in individual HRs. Shorter interset recovery led also to significantly higher TES levels (FT80 22.8%) after 7 weeks of strength training. The TES is an anabolic hormone, which is needed for development of muscle mass and thus muscular strength. Muscular strength increased in both groups, but the greatest increases, accompanied by an increased CAR, were observed in FT80, where recovery time between sets was shorter. The COR level and oxygen uptake did not decrease, suggesting that the intensity of the training was not too high to cause overreaching (31). These findings indicate that HR recovery program by Polar Electro may be a powerful tool in strength training to optimize the recovery periods between sets.
One RM increased significantly in the FT80 and control groups during the first 4 weeks. Moritani and deVries (33) have shown that during the first 4-5 weeks of strength training, neural adaptation plays a major role in force development. In this study, the results may indicate a similar process of neural development in the early phase of training. Ten RM and CAR were only measured at the beginning and after the 7-week training period. Despite rather high initial levels, both 10RM and CAR increased more in FT80, which supports the concept of greater complete training responses with shorter recovery periods. Increases in force can be attributed to neural adaptations, such as synchronization of active motor units or higher motor unit firing rates (35) after 7 weeks of training.
In body composition, SMM increased significantly in both groups, but differences between the groups were not observed. It has been suggested that high-volume strength training may induce more hypertrophy than low-volume training (24,27). Same increment in both groups might indicate that shorter rest period did not cause higher hypertrophy response and that the advanced development might be more neural based. It should be noted that a 7-week training period may be too short to observe hypertrophy responses, so the effects of rest periods on this parameter may not be so evident. This could suggest that longer training periods (e.g., 12-16 weeks) are needed to evaluate the possible role of rest periods in hypertrophy responses. More direct measurements of hypertrophy, such as magnetic resonance imaging or ultrasound, could have revealed possible muscle specific responses.
Ahtiainen et al. (4) showed that shorter vs. longer rest periods did not cause any difference in neuromuscular or hormonal responses after 14 or 28 weeks of strength training in previously strength trained men, when using 2- and 5-minute rest periods. Recently, similar results have been reported in previously untrained men with 1- vs. 2.5-minute rest periods (7). In this study, the group with the shorter rest period had higher TES responses compared to the control group. The average rest period based on individual HR was approximately 50-55 seconds. In the FT80 group, variations in rest times were large (23-133 seconds). The average rest times also changed after 7 weeks of training (12-122 seconds). This indicates that optimal recovery times are most likely individual for each subject, and thus, a constant recovery period may not be optimal for all subjects. This was especially evident in FT80, where the increase in 10RM load was relatively higher than in controls after 7 weeks of training.
Shorter recovery, and thus higher intensity, may lead to greater responses, but excessive intensity may lead to overreaching. The COR response has been used as indicator of the catabolic state of the body (23). In this study, COR responses did not change during the training period, indicating that overreaching was not evident. The TES/COR ratio also supports this finding. Because TES level was higher and COR levels did not change, the TES/COR ratio was higher, indicating an increased anabolic state of the body. This was observed in FT80 but not in the control group. These results support the suggestion that HR can be used as a rest period indicator in hypertrophy strength training, optimizing the endocrine responses to training in both men and women. Maximal oxygen uptake capacity was not expected to increase much after strength training. The lack of change in o2max indicates that strength training did not lead to negative aerobic performance and further supports the suggestion that overreaching did not occur (31) during this 7-week training period.
It can be concluded that an HR-dependent resting period using Polar Electro FT80 HR monitor may be more optimal in hypertrophy strength training than a constant resting period. The former method leads to shorter resting periods, and thus to shorter training sessions, but induces similar or even superior training responses in the early phase of training. Because training is intensive, the use of short rest periods without some degree of planning should be avoided to reduce nonfunctional overreaching and further development of overtraining syndrome, particularly during long-term training periods. The protocol in this study may be considered as a metabolic training cycle that can be used by coaches and trainers within a longer periodized training program.
The authors wish to express their gratitude to Polar Electro Ltd., Mr. Heikki Piirainen from Snowpolis who was responsible for project management, and Mr. Risto Puurtinen who was responsible for hormone analysis. There are no conflicts of interest related to this study.
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