In recent years, there has been an increase in the number of commercially available “extreme” fitness programs being marketed to the general public. Many of these programs advertise a functional approach to fitness, employing elements of traditional resistance exercise combined with variations of circuit weight training, using short rest periods and varying levels of intensity. These programs have gained a loyal following among athletes, fitness enthusiasts, and particularly the military and law enforcement communities, although only anecdotal evidence exists as to their effectiveness and safety, and according to some reports these programs create high levels of mechanical and oxidative stress, resulting in performance decrements and nonfunctional overreaching (3). The training methodology advocated by these programs is not new, as resistance exercise performed at higher intensities with very short rest periods was first described by Kraemer et al. (22), and circuit weight training employing short rest intervals has been described in the literature (2,10,11,16,32). However, the combination of resistance exercise at higher relative intensities with extremely short rest intervals is a more recent phenomenon.
This unconventional training approach seeks to combine the benefits of both short rest periods and relatively heavy loads, to capitalize on the benefits of each. High-volume, high-intensity resistance exercise not only increases metabolic stress but it has also been shown to cause greater anabolic hormonal stimulation and thus greater adaptation in terms of muscle hypertrophy (12,20,22,24), when compared with the lighter relative intensities used in a typical circuit protocol. Likewise, resistance exercise performed with shorter rest periods elicits a greater metabolic stimulus than resistance exercise incorporating longer rest periods (20). However; the demanding nature of high-volume, high-intensity, short rest protocols highlights the fact that there appears to be a fine balance between maintaining rest periods which are short enough to provide the metabolic stimulus necessary for muscle hypertrophy while allowing for a high enough relative intensity to recruit higher level motor units.
With resistance exercise, the need exists to create adequate levels of physiological stress to elicit favorable adaptations such as muscle hypertrophy and increased cellular buffering capacity, that is, increasing an individual's ability to tolerate fatigue. It has been demonstrated that both anabolic and catabolic hormones play a role in stimulating anabolic processes associated with tissue breakdown, repair and remodeling and ultimately, tissue growth (7,24). Resistance exercise causes a temporary increase in cortisol, an adrenal hormone (glucocorticoid) which inhibits protein synthesis, promotes protein degradation, and also plays a vital role in the body's fight or flight response. The highest cortisol increases have been observed in those resistance exercise protocols which are high in volume and intensity, combined with short rest intervals, and those which elicit the highest lactate responses (7,20,22,24). Normally, cortisol increases are mitigated by concurrent increases in the anabolic hormone testosterone, although resistance exercise-induced testosterone increases in women are dramatically lower (e.g., 20- to 30-fold) than those seen in men, and the cortisol response to acute resistance exercise appears to be attenuated in women (7,38). However, regardless of sex differences, circulating cortisol concentrations are a marker of the magnitude of adrenal cortical stress that is most likely precipitated by an adrenergic stress (i.e., high catecholamines) created by a particular protocol and are therefore useful in measuring the physiological stress response.
The high-intensity, short rest training approach of commercially available programs seems to contradict conventional periodized resistance training models, which have proven to be effective in maximizing physiological gains such as muscle strength and hypertrophy, while minimizing the risk of overtraining. The current American College of Sports Medicine position stand on resistance training sets forth detailed exercise prescriptions that address a broad spectrum of performance needs and provides guidelines for safe implementation of resistance exercise programs (35). There exists extensive, evidence-based support advocating the benefits of traditional resistance exercise performed at varying intensities combined with rest intervals that vary depending on the training level of the athlete (33,35). However, existing literature describing a circuit-type approach to weight training has been largely focused on lower intensity repetition maximum (RM) zones of ≤50% 1RM, and “short” rest periods ranging from 1 to 3 minutes. Even more concerning is the paucity of research on resistance training at higher intensities combined with extremely short rest periods. Thus, the physiological demands and potential benefits of resistance exercise performed at higher intensities, in combination with rest periods of <1 minute are largely anecdotal.
Because of the growing popularity of commercially available high-intensity, short rest protocols and the lack of research that exists in this particular area, we sought to determine the extent of acute physiological stress, as measured by lactate and cortisol response, total work, relative intensity, time to completion, heart rate, and rating of perceived exertion, created by a protocol which is representative of one of the most intense, physically taxing workouts likely to be encountered in a typical commercial extreme fitness program. We hypothesized that the extent of physiological stress created by this protocol would result in significant increases in lactate and cortisol in recreationally resistance trained men and women. We expected recovery values for cortisol to show no significant differences from baseline values. Furthermore, we hypothesized that men and women would exhibit differences in total work performed during the protocol, and that this would impact the magnitude of increase in lactate and cortisol, with greater volume of work in men resulting in greater lactate and cortisol response. We did not expect to see significant differences in time to completion, average relative intensity used, heart rate or rating of perceived exertion scores. Finally, we hypothesized that endurance performance would be a good indicator of time to completion on the acute high-intensity, short rest (HI/SR) protocol. Therefore, the purpose of this research study was to evaluate the acute physiological effects of an HI/SR resistance exercise protocol in recreationally resistance trained men and women.
Experimental Approach to the Problem
The study was conducted over the course of 6 experimental visits. During the first testing visit, the subjects reported to the laboratory for anthropometric measurements and familiarization with study procedures. The subjects were familiarized with the back squat, bench press, and deadlift exercises and were asked to demonstrate proper technique with each lift. Approximately 48 hours later, the subjects completed the second visit, the fitness test, which consisted of maximum repetitions of pushups in 2 minutes, followed by maximum repetitions of sit-ups in 2 minutes, followed by a timed 2-mile run on a 200-m indoor track. This was followed by the third and fourth testing visits, during which subjects' 1-repetition maximum (1RM) in each of the 3 lifts was determined. Approximately 1 week after 1RM testing, the subjects completed the acute HI/SR protocol, which consisted of a continuous circuit of barbell back squat, bench press, and deadlift exercises beginning with 10 repetitions of each, then 9 repetitions of each, then 8 repetitions and so on, following a descending pyramid scheme, until 1 repetition of each exercise was performed on the final set of the protocol, with no prescribed rest periods. All the subjects began the protocol with the same relative intensity (75% of previously established 1RM in each lift) and were asked to complete the protocol as quickly as possible, while attempting to maintain the same load throughout. Blood samples were collected at pre, immediate post (IP), +15 minutes, +60 minutes, and +24 hours postprotocol. The final recovery testing visit was scheduled 24 hours after the completion of the acute HI/SR protocol.
Nine recreationally resistance trained, healthy men (height = 172.4 ± 4.0 cm, weight = 77.8 ± 8.8 kg, age = 23.5 ± 3.5 years) and 9 women (height = 168.4 ± 9.4 cm, weight = 68.5 ± 10.4 kg, age = 22.9 ± 2.0 years) with at least 6 months of resistance training history (minimum 2× weekly) participated in this study (Table 1). The study was approved by the University of Connecticut's Institutional Review Board (IRB) for use of human subjects in research. The subjects read the consent form and were briefed as to the risks and benefits of the investigation and were given the opportunity to ask any questions regarding the study. Subsequently, all the subjects signed an IRB-approved informed consent document before participation in the study. Participation was voluntary, and the subjects were advised of their right to cease participation at any time. The subjects who were recruited and subsequently selected for inclusion in the subject pool had been involved with a resistance training program for a minimum of 6 months and were familiar with the back squat, bench press, and deadlift exercises. Before participation, the subjects completed a physical activity questionnaire indicating their weekly physical training activities and how long they had been training. On average, the subjects participated in some form of physical activity including resistance exercise 3–4 times per week, complemented by moderate amounts of endurance exercise (running or treadmill exercise 2–3 times per week). No subject reported having any exposure to high-intensity, short rest metabolic conditioning such as that advocated by “extreme” conditioning programs. The subjects were primarily recruited from the university student population: some had previous competitive athletic experience at the collegiate level; however, none of the subjects were competing in collegiate athletics at the time of their participation in the study. All subjects were medically screened to rule out any injuries or preexisting medical conditions which would preclude participation or confound the study results.
Familiarization, Fitness Test, and 1RM Testing Visits
Once approved for participation, the subjects reported to the laboratory for visit 1, familiarization with study procedures and anthropometric measurements. Subjects' height and weight measurements were taken in athletic clothing and no shoes, after which anthropometric data were obtained for each subject using hip, waist, and umbilicus circumference measurements and skinfold thickness (men: chest, abdomen, thigh; women: triceps, suprailiac, thigh) via calipers. Anthropometric data were measured in triplicate. Afterward, the subjects were familiarized with the verbal numerical CR-10 “Rating of Perceived Exertion” (RPE) scale (numbered 1–10, with extended scale, called magnitude estimation, which allowed subjects to choose a number >10), and were then familiarized with the blood pressure and heart rate measurement techniques to be used in each subsequent testing visit (inflatable blood pressure cuff and Polar heart rate monitor). This was followed by familiarization with the standard warm-up consisting of 5 minutes of light cycling (60 rpm) on a stationary bicycle followed by a series of dynamic stretches, which would be performed on each subsequent study visit. After familiarization with the warm-up protocol, the subjects were familiarized with the back squat, bench press, and deadlift exercises and were asked to demonstrate proper lifting technique on each of the lifts.
During the second study visit, the subjects performed a fitness assessment for the investigators to determine if endurance performance is a predictor of time to completion on the acute HI/SR protocol and to quantify each subject's fitness level for validation of subject selection. The fitness assessment consisted of maximum repetitions of pushups in 2 minutes, followed by maximum repetitions of sit-ups in 2 minutes, followed by a timed 2-mile run on a 200-m indoor track. The subjects were given 10 minutes of rest in between each test component and were instructed to rest, perform dynamic stretches, and drink water during that time. The subjects were not held to a performance standard during the fitness assessment but were encouraged to perform each component to the best of their ability.
Subjects' 1RM data were obtained during the third and fourth testing visits, separated by a minimum of 48 hours. Back squat and bench press testing were done on the first 1RM testing day, and deadlift 1RM was obtained on a separate testing visit so that the back squat would not compromise deadlift 1RM results. The 1RM testing was done according to the methods of Kraemer et al. (27). After the standard warm-up, the subjects performed 6–8 repetitions of the given exercise using light resistance (∼50% 1RM) followed by 2–5 repetitions at moderate intensity (75–85% 1RM). Finally, the subjects performed progressively heavier single repetition trials, until reaching the maximal 1RM within 3–5 attempts using correct lifting technique. The subjects were given 3-minute rest periods between each attempt.
The back squat was performed with the use of a standard 45-lb (20.45 kg) barbell and squat rack equipped with crash bars. The rack height and stance width were established and recorded to ensure consistency during the subsequent acute HI/SR protocol. Additionally, each subject's foot position was traced onto a rubber mat that was placed on the lifting platform during the 1RM testing visit and then the acute HI/SR protocol so that subjects could adhere to the same squat stance. During 1RM testing, the subjects were asked to squat to parallel (90° knee flexion). Spotters were placed at each end of the barbell during 1RM testing and the subsequent HI/SR protocol.
The bench press was performed with the use of a free-standing bench press rack, standard 45-lb (20.45 kg) barbell, and padded bench. Rack height was established and recorded to ensure consistency during the subsequent acute HI/SR protocol. The subjects were asked to maintain 5 points of contact during the bench press: both feet on the ground and hips, back and head in contact with the bench. A repetition was counted as long as the arms (elbows) were fully extended during the upward phase of the movement. The subjects performed the bench press 1RM test and subsequent HI/SR protocol with a spotter directly behind the bench, who assisted with the lift only if the lifter reached muscle failure.
The deadlift was performed with the use of a standard 45-lb (20.45 kg) barbell and a rubberized platform. The subjects could use either conventional or sumo style deadlift stance and were instructed to adhere to the same lifting stance for the subsequent acute HI/SR protocol. The subjects did not use weight belts or lifting straps during the deadlift but could use chalk if they desired. The subjects were instructed to maintain correct technique throughout the lift, beginning with a proper setup—shoulders directly over or slightly in front of the bar and neutral back position and finishing with full hip extension at the end of the upward phase of the lift. If excessive rounding of the back was evident during an attempt, the subject was instructed to cease that attempt. During the acute HI/SR protocol, if the investigators observed unsafe lifting technique, the weight used was reduced by 5% 1RM during the ensuing set.
Acute HI/SR Protocol
The subjects were asked to abstain from exercise for a minimum of 72 hours before the acute HI/SR protocol. Upon arrival at the laboratory, hydration status was obtained, and then the subjects were given a standardized breakfast with a 20/35/45 protein-fat-carbohydrate ratio. Approximately 1 hour after finishing the standardized breakfast, the subjects were seated, and an indwelling catheter was inserted into the antecubital vein. A baseline blood sample was taken 15 minutes after insertion, after which baseline heart rate and blood pressure measurements were obtained. Approximately 2 hours after breakfast, the subjects performed the standardized warm-up and were then given the opportunity to conduct their own warm-up exercises and a few light repetitions of the back squat, bench press, and deadlift if they so desired. Once warm-ups were complete, the subjects began the acute HI/SR protocol. Two stopwatches were started when the subject unracked the barbell from the squat rack to begin the first set (back squat) of the protocol, and time to completion was recorded upon the subject reaching full hip extension on the final repetition of the deadlift exercise. Throughout the acute HI/SR protocol, heart rate and RPE were monitored.
The acute HI/SR protocol consisted of the back squat, bench press, and deadlift exercises, beginning with 10 repetitions of each exercise, respectively. Each set decreased by 1 repetition until the final set of a single repetition (descending pyramid scheme), for example, 10 repetitions of back squat, 10 repetitions of bench press and 10 repetitions of deadlift, followed by 9 repetitions of back squat, 9 repetitions of bench press and 9 repetitions of deadlift. This repetition scheme continued until 1 single repetition of each exercise was performed on the final circuit. The weight used for the exercise protocol was set at 75% of the subject's previously established 1RM in each exercise. If a subject was unable to maintain the prescribed number of repetitions at 75% 1RM (broken set), or if the subject demonstrated unsafe lifting technique because of fatigue, the weight on that particular exercise was decreased by 5% 1RM on the subsequent set.
The subjects were asked to complete the prescribed number of repetitions in a given set as quickly as possible. There were no prescribed rest periods: The subjects were allowed volitional rest between sets, with strong verbal encouragement to continue to the next set as quickly as possible. Heart rate and RPE measurements were recorded immediately after each set of back squats to accurately capture peak data. Immediately after the completion of the acute HI/SR protocol, the subjects were seated for the IP blood draw. Upon completion of the protocol, the subjects were seated in a comfortable phlebotomy chair and blood draws were obtained at +15 and +60 minutes postprotocol from the indwelling catheter. The subjects could drink water ad libitum throughout the exercise protocol but could not eat until after the +60-minute blood sample was collected. Additionally, heart rate and blood pressure measurements were recorded +60 minutes postexercise.
As a safety precaution, the participants were instructed to refrain from donating blood within the time period 2 weeks before the start of the study through 2 weeks after the study. Blood samples were collected at baseline (pre), immediate postexercise (IP), +15 minutes, +60 minutes, and +24 hours postprotocol. As previously described, an indwelling cannula (catheter) was inserted into the antecubital vein 15 minutes before the baseline blood draw. The cannula was kept open with a saline lock. Before each blood draw, 3 ml of blood was extracted to avoid inadvertent saline dilution of the blood sample. For each blood draw, approximately 22 ml of blood was collected. Resulting serum or plasma was centrifuged at 3,000g at 4° C for 15 minutes, aliquoted, and stored at −80° C until subsequent analyses.
Additional Experimental Controls
Per exclusionary criteria, the subjects were asked to abstain from caffeine, alcohol, drugs, or antiinflammatory medications for 24 hours. The subjects were also asked to abstain from exercise for 72 hours before the acute testing visit and were asked to refrain from exercise 48 hours before 1RM and fitness testing preparatory visits. On the day of the acute HI/SR protocol, the subjects reported to the laboratory after an 8-hour fast and were then fed a standardized breakfast (20/35/45 protein-fat-carbohydrate percent ratio). The subjects were fed the same standardized breakfast on the day of the recovery visit. Additionally, the subjects were instructed to drink 2 cups of water the night before and the morning of the acute HI/SR protocol as well before the fitness test and 1RM testing visits. Hydration status was confirmed before all testing via urine spectrometry (urine specific gravity ≤1.025). The HI/SR protocol was conducted between 0800 and 1200 hours. Women were tested in the third week of their menstrual cycle (early Luteal phase), and 6 of the 9 women were on hormonal birth control.
For the analysis of plasma lactate, a disodium-ethylenediaminetetraacetic acid vacutainer was used for blood collection. Blood samples were analyzed in duplicate using a liquid lactate assay (Point Scientific #L7596) as reported by Gutmann et al. (13), with modifications. Intraassay and interassay variances were ≤5 and 10%, respectively. The assay wavelength was read at 546 nm on a Molecular Devices VERSAmax tunable microplate reader.
Serum cortisol was analyzed in duplicate using an enzyme-linked immunosorbent assay (CALBiotech, Spring Valley, CA, USA), with sensitivity of 11.1 nmol·L−1 intraassay coefficient of variation (CV) of 3.8% and interassay CV of 8.0%. The intraassay CV was 2.8%, and interassay CV was 4.4%. The assay wavelength was read at 450 nm on a Molecular Devices VERSAmax tunable microplate reader.
Data were analyzed with a repeated measures analysis of variance with sex as a between subjects factor and time (pre, IP, +15, +60, and +24) as a repeated measure. Appropriate post hoc analyses were used to determine pairwise differences between distributions when a significant F score was observed. Additionally, Pearson product-moment correlation coefficients were used to determine pairwise associations between selected dependent variables. An analysis of covariance with repeated measures was used to determine if total work influenced the response of selected dependent variables. Data are presented as mean ± SD with the alpha level for significance set at p ≤ 0.05 and 95% confidence interval (CI) calculations were determined.
Significant time effects were observed in lactate and cortisol response in both men and women. Significant sex effects were observed in lactate response but not in cortisol response. Total work was higher in men than in women and influenced magnitude of increase in lactate but not cortisol. No significant sex differences were noted in time to completion, average relative intensity, heart rate response, or RPE scores. Correlations were observed between peak lactate response (IP) and total work; however, no correlations were seen between peak cortisol response (+15) and total work. Protocol time to completion was not correlated with total work or rating of perceived exertion (RPE). However, time to completion correlated with previous 2-mile run performance.
Increased plasma lactate values were observed in both sexes, with significant increases from baseline noted at IP, +15, and +60 time points. Significant sex differences were observed at IP and +15 time points. The highest observed lactate value at IP was 17.3 mmol·L−1 among men and 13.8 mmol·L−1 among women (mean ± SD men: 14.2 ± 2.3 mmol·L−1; mean ± SD women 9.1 ± 2.2 mmol·L−1). As expected, lactate values decreased from IP values at the +15 time point (women: X[Combining Overline]diff = −3.7 mmol·L−1, sig = 0.000, 95% CI = −4.7 ≤ X[Combining Overline] ≤ −2.7; men: X[Combining Overline]diff = −4.6 mmol·L−1, sig = 0.002, 95% CI = −7.2 ≤ X[Combining Overline] ≤ −1.9). Significant sex differences in blood lactate response from baseline values were seen at IP (X[Combining Overline]diff men: 12.7 ± 0.8 mmol·L−1, 95% CI 9.8 ≤ X[Combining Overline] ≤ 15.6; X[Combining Overline]diff women: 7.7 ± 0.8 mmol·L−1, 95% CI 4.6 ≤ X[Combining Overline] ≤ 10.7), and +15 (X[Combining Overline]diff men: 8.2 ± 0.8 mmol·L−1, 95% CI 5.0 ≤ X[Combining Overline] ≤ 11.3; X[Combining Overline]diff women: 4.0 ± 0.6 mmol·L−1, 95% CI 1.6 ≤ X[Combining Overline] ≤ 6.4), with a greater magnitude of lactate increase observed in men. Lactate values remained elevated at the +15-minute time point and had still not returned to baseline concentrations +60 minutes postprotocol although there was a significant decrease from +15 values; indicative of the magnitude of metabolic stress created by the acute HI/SR protocol (Figure 1).
Study results show increased serum cortisol values in both men and women, with significant increases noted at IP, +15, and +60 time points (Figure 2). No sex differences in cortisol responses were observed at any time point. The highest mean cortisol values were observed at +15 minutes postexercise (mean ± SD men: 1,247.4 ± 364.0 nmol·L−1; mean ± SD women: 985.2 ± 438.1 nmol·L−1), with highest recorded values of 1,860.2 nmol·L−1 in the male subject group and 1,831.7 nmol·L−1 in the female subject group. No significant sex differences (X[Combining Overline]diff men IP: 738.6 ± 87.0 nmol·L−1, 95% CI 405.2 ≤ X[Combining Overline] ≤ 1,072.0; X[Combining Overline]diff women IP: 440.8 ± 74.4 nmol·L−1, 95% CI 155.5 ≤ X[Combining Overline] ≤ 726.1; X[Combining Overline]diff men +15: 938.6 ± 105.5 nmol·L−1, 95% CI 534.2 ≤ X[Combining Overline] ≤ 1,343.1; X[Combining Overline]diff women +15: 610.2 ± 106.3 nmol·L−1, 95% CI 202.7 ≤ X[Combining Overline] ≤ 1,017.7; X[Combining Overline]diff men +60: 753.9 ± 112.3 nmol·L−1, 95% CI 323.2 ≤ X[Combining Overline] ≤ 1,184.6, X[Combining Overline]diff women +60: 517.6 ± 92.9 nmol·L−1, 95% CI 161.6 ≤ X[Combining Overline] ≤ 873.7) were observed between groups at any time point. Cortisol remained elevated at the +60-minute time point, indicative of the strenuous nature of the HI/SR protocol. Cortisol values at the +24 time point were slightly lower than pre-exercise values in both sexes, although statistical analysis showed no significant differences between pre and +24 values in either experimental group.
As expected, significant differences were observed between men and women in total work performed during the acute HI/SR protocol (mean ± SD men: 14,384.6 ± 1,854.5 kg; mean ± SD women: 8,774.7 ± 1,612.7 kg). Both groups completed a total of 55 repetitions of the back squat, bench press, and deadlift exercises respectively, for a total of 165 repetitions performed during the entire HI/SR protocol. Both sexes began the protocol with the same relative intensity (75% of previously established 1RM in each lift), although men performed a greater amount of total work in terms of absolute load than did the women. The difference in total work performed can be attributed to men having, on average, greater muscle mass (greater muscle cross-sectional area [CSA]) than women, which results in greater force production capability in men. However, it is of note that one of the female participants matched one of the male participants for total work performed during the deadlift event and outperformed another male participant in that same event. Additionally, in terms of total work, another female participant outperformed one of the male participants in the bench press. Total work was highly correlated with lactate response at IP (peak values) (r = 0.83, p = 0.00); however, it was not correlated with cortisol response at +15 (peak values) (r = 0.38, p = 0.12). There was no correlation between total work completed during the protocol and rate of decline in % 1RM (r = 0.37, p = 0.13). Therefore, total work may not be a good predictor of rate of decline in the percentage of relative intensity in this protocol (Figure 3).
Average Relative Intensity
No significant sex differences were observed in average relative intensity used during the HI/SR protocol (Figure 4). In total, 4 men and 7 women were able to complete the protocol with a relative intensity of 70% 1RM or higher (average for all 3 lifts), which may be indicative of their training level coming into the study. The average relative intensity maintained throughout the entire protocol for all 3 lifts combined was 68.8 ± 3.1% for men and 70.1 ± 3.5% for women. When the average relative intensity was weighted for the total number of repetitions performed at a given intensity per set, the average weighted relative intensity used throughout the protocol was 70.4 ± 2.6% for men and 71.3 ± 2.7% for women.
None of the recreationally trained men or women were able to maintain loads corresponding with 75% 1RM in all 3 exercises (squat, bench press, deadlift) throughout the entire protocol, although some subjects were able to maintain 75% 1RM in 1 or 2 of the lifts. Both sexes experienced a significant decline from baseline % 1RM used beginning at set 3 (8 repetitions). One male participant had no drop in the percentage of 1RM in the back squat or deadlift but dropped to 60% 1RM in the bench press by the last set. One other male participant did not drop weight in the deadlift but dropped to relative intensities of 65% 1RM in the back squat and 60% 1RM in the bench press. Among the women, 4 were able to maintain 75% 1RM in the bench press event only and 2 others were able to maintain 75% 1RM in the back squat only. One woman maintained 75% 1RM in the deadlift, while dropping to loads of 70% 1RM in back squat and bench press. The lowest recorded relative intensity among women occurred in the bench press and deadlift, with 1 subject dropping to relative intensities of 50 and 55% 1RM, respectively. For men, the lowest recorded relative intensity occurred in the deadlift, with 1 subject dropping to a relative intensity of 55% 1RM by the last set of the protocol.
Women, on average, completed the protocol with a slightly higher, but statistically nonsignificant percentage of 1RM than did the men (midpoint % 1RM women: 69.1 ± 0.3%; midpoint % 1RM men: 67.4 ± 0.9% and final % 1RM women: 68.0 ± 0.3%; final % 1RM men: 66.1 ± 2.0%). Additionally, there was no significant difference between groups in the rate of % 1RM decline (average percentage drop) for all the lifts combined (average percentage decline women: 0.1 ± 0.1%; average percentage decline men: 0.1 ± 0.1%) or for each lift individually.
Time to Completion
Women, on average, performed the protocol faster than did men (mean ± SD women: 34:51 ± 10:85 minutes; mean ± SD men: 39:66 ± 14:83 minutes). However, statistical analysis showed no significant differences between groups. The fastest recorded time to completion for women was 21:28 minutes and for men, 21:57 minutes. The slowest recorded time for women was 50:15 minutes, and for men, 63:53 minutes. Among the fastest women tested included 2 former Division I collegiate athletes (rowing and swimming). The subjects who had slower times of completion but higher absolute loads in terms of 1-repetition maximums achieved, included a female rugby player and 2 men who reported that they had been training for powerlifting and bodybuilding. Overall, despite differences in individual performances, there was no observed sex effect in faster or slower times to completion.
There was no association between time to completion and total work performed (sets × reps × load) (r = 0.38, p = 0.12) or between time to completion and the rate of decline in percentage of 1RM (% drop in relative intensity) when male and female groups were combined (n = 18) (r = 0.28, p = 0.27). Therefore, total work may not be a good predictor of time to completion in this protocol. Time to completion on the HI/SR protocol was correlated with previous performance on the 2-mile run (r = 0.56, p = 0.02). Therefore, running performance as measured by the 2-mile run event may influence (∼31% of the shared variance) the time to completion of such an HI/SR protocol, when load used is equated for relative intensity.
Similar results were observed for both groups in terms of heart rate response. The highest average heart rate values were observed after set 2 (9 repetitions) (mean ± SD men: 177.6 ± 10.7 b·min−1; mean ± SD women: 175.6 ± 10.2 b·min−1), with peak values of 191 b·min−1 for men and 190 b·min−1 for women (Figures 5 and 6). Average heart rate values for men were slightly higher, although statistically nonsignificant, than average heart rates for women at all time points except set 4 (7 reps). Heart rate remained significantly elevated from baseline throughout the protocol, which can be attributed to the lack of rest periods between sets and the high relative intensity used, regardless of the fact that total work was decreasing with each set.
Rating of Perceived Exertion
No significant differences were observed between average RPE values for men and women at any time point (Figures 7A and B). Rating of perceived exertion increased significantly from baseline at set 2 (9 repetitions) in both sexes and remained significantly elevated through set 8 (3 repetitions) (Figure 6). The highest self-reported RPE scores on the CR-10 scale were 12 for men and 15 for women, both occurring at set 4 (7 repetitions), and the lowest recorded RPE score of 3 occurred on the final set (1 repetition) of the protocol. Results indicate that RPE and total work were not correlated, with RPE remaining elevated despite decreases in total work per set, as shown in Figure 7B.
A dramatic adrenal cortical stress was observed with the use of a higher intensity short rest resistance exercise protocol. Significant increases were observed in plasma lactate and serum cortisol concentrations as a result of the HI/SR protocol in both men and women. As expected, men experienced a greater magnitude of lactate increase than did the women; however, no significant sex differences were observed at any time point in cortisol response. Total work was found to have a main effect on lactate response but no effect on cortisol response in either experimental group. Both groups performed the HI/SR protocol with a starting relative intensity of 75% 1RM and experienced similar rates of decline in percentage of 1RM used. Men performed greater total work (sets × reps × load) during the protocol because of higher absolute 1RM loads than the women.
The results of this study show increased lactate values at IP, +15, and +60 time points in both men and women, with significant sex differences at IP and +15 time points. These findings are consistent with previous research of high-intensity, short rest exercise protocols. In this study, the highest lactate value observed was 17.3 mmol·L−1 in men and 13.8 mmol·L−1 in women. Kraemer et al. (22) reported a peak lactate value of over 21 mmol·L−1 in men, although a higher relative intensity of 80% 1RM was used in that study. Other studies have reported slightly lower lactate values ranging from 8 to 10 mmol·L−1, although the relative intensities and rest intervals used were lower and longer, respectively, that in the present investigation (12,18). The results of these prior studies confirm that although higher individual peak lactate values were observed in this study, average lactate values in both men and women are similar to those seen in other comparable protocols. Thus, it appears that the HI/SR protocol did not create an extraordinary stimulus in terms of metabolic response beyond that which has been previously reported. This might be because of the volitional rest period length used in the current investigation.
As expected, the average lactate values observed in this study are slightly higher than those reported in resistance exercise studies using rest intervals >1 minute. For example, Izquierdo et al. (17) reported lactate values of approximately 12.0 mmol·L−1 in men in response to a 5× 10RM protocol when using rest periods of 2 minutes. Other studies have shown similar results when intensities of 70–75% 1RM and rest periods ranging from 1.5 to 2 minutes were used (25,28). Lactate values observed in women in the present investigation are higher than those that have been previously reported in studies of hormonal responses to resistance exercise in women (7,20,30). However, some resistance training studies using longer rest periods have reported higher average lactate values than those observed in this study. For example, Häkkinen et al. (15) reported lactate values of >15.0 mmol·L−1 in resistance trained men performing a 10RM protocol at 70% 1RM, even though longer rest intervals of 3 minutes were used. Ahtiainen et al. (1) reported similar lactate values in a 10RM hypertrophy protocol with 2-minute rest periods in recreationally resistance trained men. Notably though, the highest lactate value observed in men in the present study was greater than average values reported by both Häkkinen (15) and Ahtiainen (1).
In comparison to high-intensity endurance exercise, the resistance exercise-induced lactate increases observed in this study are higher than lactate values seen during an acute bout of maximal cycling at 36% maximum leg power (cycle ergometer) measured 5 minutes postprotocol (23). Another recent study of maximal cycle ergometer testing in men and women at varying intensities also resulted in lower lactate concentrations than those observed presently (39). Our results were slightly higher than lactate concentrations seen in a 3-minute supramaximal rowing ergometer test (26) and a 6-minute maximal “all-out” rowing ergometer test (9). The high lactate values seen in these studies are likely because of the high power output generated during maximal rowing exercise in addition to the endurance (cardiovascular) demand. Finally, lactate values in men and women in response to a high-intensity flywheel ergometer protocol 5 minutes postexercise were lower than lactate values observed at IP in this study (5).
Given the high relative intensity, high total volume of work (165 total repetitions), and extremely short rest intervals used in this study, the lactate values observed were not as high as expected. This may have been because of the training level of some of the subjects, or likely can be attributed to volitional rest periods taken in between sets, allowing for some lactate clearance to occur, although the subjects were verbally encouraged to complete the HI/SR protocol as quickly as possible. Prior research has demonstrated that increases in lactate are strongly correlated with perception of fatigue (22,34). Based on peak RPE values reported in this study, it appears that the most strenuous segment of the HI/SR protocol occurred during sets 4 (7 reps) and 5 (6 reps), the time points at which theoretically the highest lactate values would have occurred. However, because blood samples were only obtained at IP and thereafter, it is possible that lactate values observed in our study did not reflect peak concentrations. If samples had been obtained at midpoint or at time points corresponding with the highest reported RPE values, higher lactate values might have been observed, given the high relative intensity, total volume of work and lack of rest periods in this protocol. The lactate values observed in this study; although not as high as expected, nonetheless confirm that in addition to relative intensity, rest period length is an important factor determining the magnitude of metabolic stress created by a particular resistance exercise protocol. The high concentrations of plasma lactate seen in this study are an indication of reduced opportunity for lactic acid clearance and thus increased metabolic stimulus.
Cortisol values observed in the present investigation showed significant increases from baseline at IP, +15, and +60 time points in both men and women. However, no significant differences were observed in cortisol response between men and women at any time point. Interestingly, the cortisol values seen in this study were higher than those reported by Kraemer et al. (22), despite higher peak lactate values observed in that study. This once again may be explained by the fact that our lactate measurements likely did not reflect peak values reached during the protocol. Previous research has shown that there is a strong correlation between lactate and cortisol increases, with the highest cortisol values seen in those protocols that produce the highest lactate (7,24). Likewise, high cortisol values have also been correlated with high relative intensity, high-volume, short rest protocols (7). The cortisol values observed in this study confirm results of prior studies, which have demonstrated that the combination of high relative intensity, high volume of work and short rest interval length elicits the greatest metabolic demand and creates a dramatic adrenergic and adrenal cortical stress response (20–22,24,30).
The cortisol values observed in this study are higher than the values reported in previous research conducted on traditional resistance exercise using rest periods of 1–3 minutes, with similar relative intensities as that used in the HI/SR protocol (75% 1RM). For example, Fragala et al. (8) reported lower cortisol values in both men and women in response to a heavy squat (6 × 5RM) protocol with 3-minute rest periods than those seen in this study. Similarly, other studies have reported cortisol values in recreationally active men (36) and women (30) that were lower than those presently reported. Kraemer et al. (20) also demonstrated increased cortisol concentrations in women in response to strength and hypertrophy protocols using 5RM and 10RM loads, respectively, and corresponding rest periods of 3 minutes or 1 minute. Interestingly, in the 5RM strength protocol, cortisol values were lower than those observed presently; however, in the 10RM hypertrophy protocol, cortisol values at the +15 minute time point were lower than what was observed at that same time point in our study. This once again points to the magnitude of adrenal cortical stress created by our protocol and potential recovery implications.
The cortisol values of this study were higher than those observed during maximal cycle ergometer testing in young men (14) and peak cortisol values obtained during maximal power and endurance events, such as a rowing ergometer endurance event (19). However, cortisol values reported by Caruso et al. (5) in response to a high-intensity flywheel ergometer protocol 30 minutes postprotocol were similar to the cortisol values observed at 60 minutes postexercise in this study. The higher cortisol values seen in that study can likely be attributed to the fact that flywheel ergometer exercise is more resistive in nature than traditional endurance exercise protocols.
Previous research has shown that cortisol values increase in response to athletic competition, psychological stress, or as a result of the anticipatory rise often experienced by athletes before competitive events such as Olympic weightlifting (6). Our results confirm these findings, as the cortisol values observed at the +24 time point in this study were somewhat lower than those observed at baseline, indicating that baseline values may have reflected an anticipatory rise in cortisol concentrations. Our findings showed that cortisol values remained elevated for one hour after completion of the protocol with average cortisol values at the +60 time point still more than double that of baseline values, highlighting the extent of the acute adrenal cortical stress of the HI/SR protocol. Similar responses were observed in Rugby players 30 minutes after competition, with cortisol concentrations at that time point nearly double those at baseline (29). Although cortisol remained significantly elevated 1 hour postprotocol in this study, values returned to near-baseline concentrations the following day, with no significant differences between baseline and +24 values in either men or women. This finding may demonstrate that the present HI/SR protocol, although extremely strenuous in nature, does not pose an immediate recovery problem in terms of circulating hormone concentrations. However, it is worth noting that the subjects were well rested before the HI/SR protocol, having been asked to refrain from physical activity for 72 hours before. This may have had an impact on both baseline cortisol values and recovery concentrations during the +24 recovery visit.
The lack of a significant difference in cortisol response between men and women is somewhat unexpected, as attenuated cortisol response to resistance exercise is well documented in women (8,20,24,30), but this may simply be a reflection of the magnitude of adrenal cortical stress (that may be mediated by a precipitous adrenergic stress) created by the acute HI/SR protocol in comparison with other protocols, and the training level of some of the women. Prior studies have shown differences in glucocorticoid receptor activity in women in response to resistance exercise, namely, that in women the glucocorticoid receptors in muscle tissue may be already saturated before the onset of resistance exercise (38). In other words, resistance exercise-induced cortisol increases will be taken up by other target tissues such as leukocytes (7). Fragala et al. (8) reported no cortisol increase in women in response to a heavy-resistance exercise protocol (6 × 5RM squat protocol with 3-minute rest periods) and also observed attenuated glucocorticoid receptor activity compared with men. Upregulation of glucocorticoid receptors in lymphocytes in women was first seen at the 1 and 6 hours postexercise time points (8). The high cortisol values observed in women in our investigation may be a reflection of saturation of the glucocorticoid receptors in the muscle tissue after exercise and a 60-minute delay for upregulation of the receptors on B-cell lymphocytes. Such prior findings and the results of this study raise concerns about the role of excess circulating cortisol in the body, with the saturation of so many target receptors having potential negative implications on muscle growth and repair processes and immune suppression, making the recovery time course less than optimal.
As expected, significant differences were observed in total work performed between men and women. Previous studies have demonstrated similar results, with men performing on average more work, in terms of total absolute loads lifted, when compared with women. This difference in total work may be attributed to greater muscle CSA in men and thus the presence of a higher number of muscle contractile units. However, prior research has shown that although men typically have greater muscle CSA than women, women are able to achieve the same type of muscle fiber type conversions as those seen in men, and can achieve strength and hypertrophic gains similar to those observed in men in response to resistance exercise (4,36,37). However, regardless of total work performed, both groups used the same relative intensity throughout the protocol, and experienced similar rates of decline in average percentage of 1RM used. Women have been shown to fatigue more slowly than men (7), which may be why they were able to complete the protocol slightly faster than men (on average) with a slightly higher percentage of 1RM, even though these differences were not statistically significant.
No significant differences were observed in heart rate or RPE values for either experimental group. Previous studies have shown that both men and women experience similar heart rate and RPE responses to strenuous exercise (32,34). As mentioned previously, prior research has demonstrated that women fatigue more slowly than men when matched for muscle strength (7). Women are also able to recover quicker than men, which may be partly because of attenuated inflammation response in women and the protective effects of estrogen (7). The slight variations observed in RPE responses in this study, namely, that women reported peak RPE scores later in the protocol than did men when loads used were equated for relative intensity, seem to confirm these findings.
In summary, our findings indicate that lactate values in men at the IP and +15 time points were significantly different than those seen in women, with both sexes showing significant increases from baseline at the IP, +15, and +60 time points. No sex differences were observed in cortisol response in men and women, with significant increases in both sexes at IP, +15, and +60 time points. No sex differences were observed in relative intensity, heart rate response, rating of perceived exertion, or time to completion, indicating that this protocol provided a similar metabolic stimulus for both sexes, eliciting similar levels of motor unit recruitment. Although limited sex differences were observed in the present investigation, our data showed significant increases in all observed physiological markers, reflecting the magnitude of adrenal cortical stress (precipitated by a potentially very dramatic adrenergic stress) created by this HI/SR protocol, irrespective of sex of the participants. Furthermore, research on glucocorticoid receptor activation in men and women shows that there are sex differences in the acute response to resistance exercise and that there may be differences in the time course of glucocorticoid uptake in the tissues. As shown in the current investigation, high-intensity, short rest protocols elevate cortisol concentrations well beyond what is typically seen in response to resistance exercise protocols using lighter intensities and longer rest intervals. This has potentially serious implications in muscle tissue growth, recovery, and immune processes because of the catabolic effects of cortisol.
Although the results of this study provide an insight into acute responses to HI/SR resistance exercise, more research is needed to determine the nature and extent of chronic adaptations to such protocols. Thus, the primary implication for future research is the need to incorporate HI/SR protocols into a periodized resistance training program. The repeated us of such protocols in a sequential manner without programmed rest days should be examined to determine whether a chronic elevation of resting cortisol concentrations occurs. Chronically elevated resting cortisol concentrations that were much higher than normal were observed in U.S. Army Ranger school trainees after three months of Ranger School training, during which time they endured extreme mental and physical stresses (31).
The findings of this study demonstrate that this particular HI/SR protocol produced a much greater cortisol response than traditional resistance training protocols using similar relative intensities (≥75% 1RM) and longer rest intervals. When high-intensity resistance exercise is performed with little to no rest periods, as in this study, the metabolic demands of the workout rise and the risk for chemical and mechanical damage increases. The findings of this study suggest that HI/SR protocols may be best suited to the well-trained athlete who already has a level of fitness commensurate with the demands of this protocol. Strength coaches must exercise caution when implementing these protocols with less-experienced athletes and individuals wishing to incorporate short rest metabolic training should be advised to gradually reduce rest period length with concurrent increases in exercise intensity. Special concerns such as rhabdomyolysis must be taken into consideration, as high-intensity, short rest protocols could induce excessive muscle damage in athletes. Additionally, because of the lack of existing research on long-term implications of HI/SR protocols, coaches should carefully monitor athletes when performing HI/SR to minimize injury risk and the potential for nonfunctional overreaching. Although short rest protocols have benefit and certainly have their place within an athlete's training arsenal, the findings of this study suggest that their greatest benefit may arise when incorporated into a periodized strength and conditioning program, which allows for adequate train-up, rest, and recovery.
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