ACUTE PHYSIOLOGICAL AND MECHANICAL RESPONSES DURING RESISTANCE EXERCISE AT THE LACTATE THRESHOLD INTENSITY MANUEL V. GARNACHO-CASTAN˜O,1 RAU´L DOMI´NGUEZ,2 PEDRO RUIZ-SOLANO,2 JOSE´ L. MATE´-MUN˜OZ2

AND

1

Department of Physiology and Immunology, Faculty of Biology, University of Barcelona, Barcelona, Spain; and 2Laboratory of Biomechanics and Exercise Physiology, Department of Physical Activity and Sports Science, Alfonso X El Sabio University, Madrid, Spain ABSTRACT

Garnacho-Castan˜ o, MV, Domı´nguez, R, Ruiz-Solano, P, and Mate´-Mun˜ oz, JL. Acute physiological and mechanical responses during resistance exercise at the lactate threshold intensity. J Strength Cond Res 29(10): 2867–2873, 2015—The purpose of this study was to examine acute metabolic, mechanical, and cardiac responses to halfsquat (HS) resistance exercise performed at a workload corresponding to the lactate threshold (LT). Thirteen healthy subjects completed 3 HS exercise tests separated by 48-hour rest periods: a maximal strength or 1 repetition maximum (1RM) test, an incremental load test to establish the % 1RM at which the LT was reached, and a constant load test at the LT intensity. During the last test, metabolic, mechanical, and cardiac responses were monitored respectively through blood lactate concentrations, height (H), average power (AP) and peak power (PP) recorded in a countermovement jump test, and heart rate (HR). During the constant load test, lactate concentrations and HR remained stable whereas significant reductions were detected in H, AP, and PP (p # 0.05). Only low correlation was observed between lactate concentrations and the H (r = 0.028), AP (r = 0.072), and PP (r = 0.359) losses produced. Half-squat exercise at the LT elicits stable HR and blood lactate responses within a predominantly aerobic metabolism, although this exercise modality induces significant mechanical fatigue.

KEY WORDS half-squat, metabolism, hear rate responses, mechanical fatigue, jump height, muscular power

Address correspondence to Dr. Manuel V. Garnacho-Castan˜o, [email protected]. 29(10)/2867–2873 Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association

INTRODUCTION

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good knowledge of acute physiological and mechanical responses to resistance training (RT) is essential to understand the adaptive mechanisms produced by different exercise stimuli (9). Such responses are determined in large measure by stimulus duration and intensity along with other factors related to the specificity or type of exercise (28). Recent research efforts in the field have examined the transition point between aerobic and anaerobic metabolism, or so-called lactate threshold (LT) (6), during incremental resistance tests (11,12). The LT is defined as the workload during incremental exercise at which blood lactate concentrations start to exponentially rise, coinciding with eventual muscle fatigue (35). This intensity of exercise marks the start of lactic acid build-up or metabolic acidosis (10) and provides valuable information on the main physiological systems that are active during exercise. Working at the LT intensity has been attributed benefits in sports modalities such as cycling (24) and running (40), and this strategy has been also described as a suitable indicator of performance in endurance athletes (23). In the healthcare field, this intensity of exercise has been reported to improve cardiorespiratory fitness in recreational sports (39) and has been also used for cardiac rehabilitation purposes (26). However, only a few studies have identified the LT in resistance exercise (11,12), maybe because strength training at low (.40% of 1 repetition maximum [1RM]) or moderate loads relies mainly on anaerobic metabolism (22), increasing blood lactate levels and eliciting a drastic increase in muscle glycogen as the main energy source (8). Lactic acidosis is usually associated with other factors, for example, factors related to diabetes such as cardiovascular disease in which there is excess lactic acid production and hypoxia (17). An RT program at LT intensity would help maintain low and stable lactate concentrations in blood, regularizing blood glucose levels and optimizing muscle glucose uptake (3). Hence, performing a high number of repetitions at the LT intensity could be a useful way to improve cardiorespiratory fitness and VOLUME 29 | NUMBER 10 | OCTOBER 2015 |

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Lactate Threshold in Resistance Training local muscular endurance (13) both in athletes and patients with cardiorespiratory and/or metabolic disorders. To date, the few studies addressing the effects of RT performed at an intensity corresponding to the LT (11,32) have mainly focused on acute cardiorespiratory and metabolic responses to different exercises (e.g., leg press, bench press, and biceps curl). However, no study has focused on RT exercises in a standing position involving greater muscle mass. We selected the half-squat (HS) for our study because it is one of the most popular exercises in physical fitness programs, and physiological responses to constant load HS exercises performed at the intensity of LT are still unknown. Some studies have revealed significantly larger elevations in oxygen consumption and energy expenditure for large vs. small muscle mass exercises (21). This would likely compromise the stability of acute metabolic responses to a load executed at the intensity of LT. Furthermore, resistance exercises such as squats show greater neuromuscular specificity and corresponding data may be appropriate for transfer to other sports modalities (18). These studies (11,32) have revealed stable cardiorespiratory and metabolic responses; however, the muscle fatigue induced by such an exercise protocol remains to be determined. Muscle fatigue is recognized as a complex, multifactorial, task-dependent factor whose etiology has been much debated among experts in the field (15). Any prolonged physical activity performed at sufficient muscle intensity will produce fatigue, and this will impair the capacity of the neuromuscular system to generate force (19), leading to a drop in velocity or power (31). Muscle fatigue has been correlated with a loss in power and height during the countermovement jump (CMJ) and can be assessed by measuring these variables before and after exercise (31). There is a need to create different models of fatigue by assessing variables such as force and power during repeated dynamic muscle contractions (31). By controlling these variables, valuable information can be gained about the real mechanical demands of resistance exercises performed at an exercise intensity corresponding to the LT. This study was designed to examine acute cardiac, metabolic, and mechanical responses to a HS protocol performed at the intensity of the LT. Although prior data point to stable metabolic responses to RT (11), our working hypothesis was that such responses would not largely correlate with height and average and peak power losses produced in the CMJ.

METHODS Experimental Approach to the Problem

Subjects completed 3 different tests at the same time each day (63 hours) under the same environmental conditions (temperature: 21–258 C, atmospheric pressure: 715–730 mm Hg, and relative humidity: 40–50%) with a rest period between each test of 48 hours. The order of the tests was: (a) a 1RM HS test to establish the % loads to be used in the incremental

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test, (b) an incremental load HS test to establish the % 1RM corresponding to the LT, and (c) a constant load HS test at the LT. Before and after the constant load test, jump height (H), average power (AP), and peak power (PP) were determined in a CMJ test. Blood lactate concentrations were measured at the start of the constant load test and after each HS exercise set. Also, after each set of HS repetitions, subjects recorded their rating of perceived effort (RPE) using the Borg’s scale. Subjects

Thirteen healthy subjects were recruited among the students of Physical Activity and Sports Sciences, Alfonso X el Sabio University (Madrid, Spain) (age: 21.23 6 1.83 years, height: 180.54 6 5.31 cm, weight: 81.21 6 8.98 kg, body mass index: 24.87 6 1.99). Participants all had experience (2.69 6 1.25 years) in HS RT. Exclusion criteria were (a) the use of any medication or performance-enhancing drugs, (b) any cardiovascular, metabolic, neurological, pulmonary, or orthopedic disorders that could limit exercise performance, (c) being an elite athlete, and (d) less than 150 kg recorded in a 1RM test in HS. Before the study outset, participants were informed of the tests they would be taking and written consent was obtained from each subject. During the course of the study, the participants were instructed to refrain from other exercises or RT. In addition, during the 2 hours before the test, they avoided eating or smoking, and drinks were restricted to water. The study protocol was approved by the Ethics Committee of the Department of Physical Activity and Sports Sciences of the Alfonso X el Sabio University (Madrid, Spain) and adhered to the tenets of the Declaration of Helsinki. Half-Squat Technique

The subjects positioned themselves under the barbell in a standing position with the knees and hips fully extended and legs spread at the shoulders’ width. The barbell was placed on the upper back (trapezius muscle), approximately at the level of the acromion. The subject flexed the knees and hips (eccentric action) to lower the barbell in a controlled manner, until 908 flexion of the knees. From this position, the propulsive (concentric) muscle action was initiated until fully extending the knees and hips. One Repetition Maximum Test

Maximal strength (1RM) was determined for the HS test following the guidelines and recommendations described by Baechle et al. (2). A Smith machine was used to ensure controlled movements especially at work rates approaching 1RM. The test protocol included a standard warm-up for all subjects involving 10 minutes of joint movements and ballistic stretching. This was followed by a specific warmup consisting of 1 set of 3–5 HS repetitions at a relative

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Journal of Strength and Conditioning Research intensity of 40–60% of the maximum perceived maximum. After a 2-minute rest period, participants started the 1RM test. This involved 3–5 lifting attempts using increasing weights. The 1RM was defined as the last load lifted by the subject completing a knee extension to the required position. The rest period between each attempt was 2–4 minutes. Incremental Resistance Test

The protocol started with a standard warm-up consisting of 5 minutes of light trotting followed by 5 minutes of joint mobility drills and ballistic stretching of the lower and upper limbs. The incremental test was conducted in steps at relative intensities of 10, 20, 25, 30, 35, and 40% 1RM (11), selected according to the results of investigations that have located the aerobic-anaerobic threshold for RT at 25–35% of the 1RM (32). Each step lasted 1 minute and involved 30 repetitions of 2 seconds each (1 second for the concentric phase and 1 second for the eccentric phase). This rhythm was monitored with a metronome while an observer provided visual and verbal cues. A passive recovery period of 2 minutes was established between steps (11). During this time, the relative load (% 1RM) was increased and blood samples were collected for lactate determination. The test was terminated when the subject did not correctly perform repetitions or was unable to continue executing repetitions at the rhythm set for each step at the corresponding relative training intensity. Blood samples (5 mL) were obtained by finger pricking 30 seconds after the end of each step (11), and lactate levels determined using a portable lactate analyzer (Lactate Pro LT-1710; Arkray Factory Inc., KDK Corporation, Shiga, Japan). The reliability of this device has been reported by others (27). The LT was established using the algorithm adjustment method (12) based on the procedure described by Orr et al. (29), as the work intensity at which lactate concentrations start to increase in an exponential manner (38). The LT was located through computerized 2-segment linear regression by fixing the 2 linear regression equations emerging for each segment at the point of intersection between a plot of blood lactate concentration and relative intensity. Data treatment was performed using the software package Matlab version 7.4 (MathWorks, Natick, MA, USA). Constant Resistance Test

The constant load HS test was performed at the intensity corresponding to the LT as determined in the incremental test. The test initiated with a standard warm-up consisting of 5 minutes of light trotting and 5 minutes of joint mobility drills and ballistic stretching of the upper and lower limbs. The constant load HS protocol was conducted as 21 sets of 15 repetitions of 2 seconds (1 second of concentric exercise and 1 second of eccentric) guided by visual and verbal cues. The duration of each set was therefore 30 seconds. Recovery between sets was passive and of 1-minute duration. The whole constant load test took 31 minutes.

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Blood samples were obtained at rest and 30 seconds after the end of the exercise sets (S) S3, S6, S9, S12, S15, S18, and S21, when lactate concentrations were determined as described above for the incremental test. Heart rate (HR) was recorded by telemetry (RS-800CX; Polar Electro OY, Kempele, Finland) every 5 seconds. For data analysis, mean HR values were calculated for the 1-minute recovery period and the work accomplished in the set. Muscle Fatigue

Mechanical fatigue in the lower limbs was assessed in a CMJ test (20) using a force plate (Quattro Jump model 9290AD; Kistler Instruments, Winterthur, Switzerland) before and after the constant load test performed at the LT. The test starts with the subject standing with the legs extended and arms on the hips. The subject initiates the jump by bending the knees to ;908 and immediately and synchronously then starts to extend the knees (concentric action) in an explosive movement to attain the maximum height possible. During the jump, the knees should be fully extended and contact with the ground is first made with the toes. Subjects were instructed to keep their hands on the hips during the jump and to avoid any sideways or backward/forward movements. Subjects performed 3 jumps separated by a rest period of 30 seconds, and the mean values of H, AP, and PP recorded in the 3 jumps used in the subsequent analyses. Perceived Effort

The Borg’s scale was used to monitor the RPE (5) at the time points established for blood lactate determination. Statistical Analyses

The Shapiro-Wilk test was used to check the normal distribution of data, which are provided as mean values and their standard deviations (SDs). Heart rate, lactate, and

TABLE 1. Data concerning the characteristics of the participants, 1RM, load (kg), relative intensity (% 1RM), and HR at LT intensity in HS exercise.* Mean 6 SD

Variables Subjects Age (y) Weight (kg) Height (cm) Body mass index (%) 1RM in HS (kg) HS LT load (kg) HS LT intensity (% 1RM) HS LT HR

n = 13 21.23 6 1.83 81.21 6 8.98 180.54 6 5.31 24.87 6 1.99 188.78 6 35.64 43.72 6 11.87 22.8 6 3.54 139 6 13.54

*1RM = 1 repetition maximum; HR = heart rate; HS = half-squat; LT = lactate threshold.

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Lactate Threshold in Resistance Training statistical tests were performed using the software package SPSS Statistics version 19.0 for Mackintosh (SPSS, Chicago, IL, USA). Significance was set at p # 0.05.

RESULTS Descriptive data for the subjects and data corresponding to the loads and relative intensities (1RM and LT) of the HS tests are provided in Table 1. In each subject, the LT was identified in the incremental load test using the algorithm adjustment method to give a mean exercise intensity for the LT of 22.8 6 3.54% 1RM. Heart Rate, Blood Lactate, and Rating of Perceived Effort

Significant changes were observed in the HR response (F = 9.39, p , 0.001, ES = 0.406). Heart rate values varied significantly between S3 and the remaining sets (p , 0.01) and S21 and S6 (p = 0.042), indicating HR stabilization from S6 until the test end (Figure 1A). Significant changes in lactate concentrations (F = 10.76, p , 0.001, ES = 0.440) were observed between S3 and the remaining sets (p , 0.001) with levels stabilizing as from S3 (Figure 1B). The statistical power for both the HR and Figure 1. Heart rate (A), blood lactate concentrations (B), and RPE (C) recorded during the constant load HS lactate changes produced tests at an exercise intensity corresponding to the lactate threshold. *Significant difference vs. resting values (p , 0.01). †Significant difference vs. set 21 (S21) (p # 0.05). HR = heart rate; RPE = rating of perceived effort. was 1. No significant differences in the RPE were recorded throughout the test (F = 1.651, p = 0.143) (Figure 1C). IntraRPE responses were compared between the constant load class correlation coefficients (ICC) were 0.969 (confidence exercise sets by one-way analysis of variance. When signifinterval [95% CI]: 0.936–0.989) for HR and 0.959 (CI: icant differences emerged, the post hoc Tukey’s test was 0.894–0.982) for lactate. used to determine between which sets these occurred. A Student’s t-test for paired samples was used to examine Countermovement Jump Height and Average and Peak CMJ height and power losses produced in response to the Power Losses tests. Correlations between H, AP, or PP losses and lactate In response to the constant load HS test performed at the concentrations were assessed by linear regression. We also LT, significant losses were produced in jump height (t = 8.99, determined the effect size (ES) and the statistical power of p , 0.001) (Figure 2A), AP (t = 6.08, p , 0.001) (Figure 2B), and PP (t = 3.63, p = 0.003) (Figure 2B). the differences observed (12b $ 0.8 and a = 0.05). All

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DISCUSSION The main finding of this study was that a prolonged HS test performed at a constant exercise intensity corresponding to the LT leads to significant reductions in jump height and mean power and PP, despite stable lactate, HR, and RPE responses. Blood lactate concentrations increased significantly from the test start to the third exercise set (S3), from which point a steady state was reached that persisted until the end of the test. A lactate steady state seems to be characteristic of resistance exercise (11) and is the result of a balance produced between lactate production and clearance processes (4), thus maintaining low blood lactate concentrations. According to the findings of RT studies that have examined blood lactate concentrations, a steady state is usually produced between minutes 10 and 15 of exercise (11). The different time point for the lactate steady state recorded in our study (S3 per minute 4) may be attributable to methodological differences such as the exercise modality, exercise protocol, blood sampling method (16), and instrument used to determine lactate (27). The data provided by these investigations indicate that RT could be an adequate training method for work under a predominantly aerobic metabolism, provided low or moderate loads close to those corresponding to the LT Figure 2. Differences in jump height (A) and AP and PP (B) measured in a CMJ test before and after the constant are used. More work is, howload HS tests at an exercise intensity corresponding to the lactate threshold and correlations between relative CMJ height and mean and peak power (C) losses and blood lactate concentrations. *Significant difference vs. ever, needed to confirm these pretest value (p , 0.01). AP = average power; PP = peak power; CMJ = countermovement jump. preliminary findings. The HR response to the constant load test performed The correlations detected between the losses produced in H at the LT involved a significant increase from the test start (r = 0.028, p = 0.587), AP (r = 0.072, p = 0.504), and PP (r = until the end of the first set, thereafter remaining stable until 0.359, p = 0.076) and blood lactate concentrations indicate no the end of the test. This increase in HR produced at the start likely effects of lactate levels on H, AP, and PP losses (Figure of exercise is a common response observed in RT (14) driven 2C). The ICC for the CMJ was 0.987 (CI: 0.957–0.996). by increased sympathetic activity of the heart, which leads to VOLUME 29 | NUMBER 10 | OCTOBER 2015 |

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Lactate Threshold in Resistance Training the redistribution of blood flow and to reduced vein capacitance. This physiological response induces the necessary increase in cardiac output for metabolic regulation (36) while avoiding the build-up of high lactate concentrations in the active musculature during exercise. Our analysis of the mechanical fatigue produced in response to the constant load HS exercise performed at the aerobic-anaerobic threshold revealed significant reductions in H, AP, and PP. So far, 2 essential components of models of fatigue have been described: induced fatigue and the quantification of fatigue (25). In this study, fatigue was assessed in terms of power and height losses produced in the CMJ. Other authors have also used pretest and posttest vertical jump height measurements to determine the extent of fatigue produced (31,34). For example, Sa´nchez-Medina and Gonza´lez-Badillo (31) detected jump height reductions of 19 and 11% at 70 and 90% 1RM, respectively, in the back squat. In a study by Rodacki et al. (30), different knee flexion and extension exercises were performed in a weight machine at relative exercise intensity closer to those used here at ;50% 1RM (for the extension exercises) and ;40% 1RM (for the flexion exercises). These authors observed jump height losses close to 6% in response to flexion and 14% in response to extension exercises. In our study, the H reduction in the CMJ observed after the HS test was ;6.4%. This suggests that the differences observed with respect to these studies are the consequence of the different objectives pursued, exercise modalities, relative intensities, protocols, muscle actions, and movement velocities involved in the test exercises. Notwithstanding, the data reported in these studies seem to indicate that CMJ height, AP, and PP losses could be an adequate measure of the neuromuscular fatigue induced by different forms of exercise. More work is needed to confirm the results of these investigations. In several studies addressing the neuromuscular fatigue induced by different resistance exercise modalities, jump ability (31) and power (1) reductions have been frequently described and associated with high blood lactate levels. In contrast, we were unable to detect strong correlation between jump ability losses and lactate concentrations as the number of sets and repetitions of HS exercise increased. This seems to indicate that although there was no significant metabolic stress and the RPE of the study participants was lowmoderate, HS induced substantial mechanical fatigue at low relative exercise intensity of approximately 23% 1RM. We suggest that this reduced capacity of the legs to generate force in the CMJ test was likely attributable to muscle damage because participants showed late-onset muscle pain 24 hours after completing the knee extension tests, as described by others (7). This muscle pain was, however, not quantified in our study. Muscle pain is the most frequently used marker of muscle damage induced by exercise in human studies (37). Jump height and power losses, besides indicating modified muscle contractility, probably reflect a chronic reduction in resting adenosine triphosphate concentration (20) and

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increased production of superoxide radicals, which may induce muscle damage (33). Accordingly, we cannot rule out the possibility of intramuscular depletion of phosphocreatine reserves produced during an exercise session (especially type II fibers), impairing jump ability. Our study has several limitations that need to be considered. The 1RM of the participants may vary from 1 day to the next such that a test-retest would have been useful to avoid in greater measure the effects of such a systematic error. Also, in preliminary tests, we established the durations of muscle actions and recovery times, but these were different from those used in other RT studies. This suggests a need for a different protocol depending on the resistance exercise modality if the objective is to maintain stable blood lactate concentrations at the LT. The issue would need to be confirmed in further studies involving different resistance exercise protocols. According to our findings and those of prior investigations, RT is an adequate form of exercise to maintain a stable HR and a blood lactate concentration within that corresponding to a predominantly aerobic metabolism. However, despite these stable variables and a low relative working intensity close to 23% 1RM, HS exercise induces muscle fatigue. Surprisingly, muscle fatigue measured in terms of H, AP, and PP losses in the CMJ could not be correlated with increased blood lactate concentrations. The implications of this finding are that successive HS resistance exercise repetitions or sets performed at a constant exercise intensity corresponding to the LT could be maintained over time without demanding the anaerobic metabolism characteristic of RT, provided rest periods were established between sets. The muscle fatigue induced would probably lead to increased local muscle endurance in the absence of appreciable metabolic stress.

PRACTICAL APPLICATIONS Resistance training involving relatively light or medium loads corresponding to the LT and a high number of repetitions, besides ensuring a predominantly aerobic metabolism, could serve to improve local muscle resistance and cardiorespiratory fitness in healthy adults. The muscle fatigue induced (reflected by power and jump height losses) probably indicates that this RT methodology could be used for conditioning purposes at the start of an RT program, to develop both strength (local muscle endurance) and endurance capacity in healthy adults. For example, in sports modalities such as running and cycling (long distance), it is less usual to train the upper limbs. The LT intensity would allow for training both the lower and upper extremities using a specific metabolism (aerobic) and also lifting light loads avoiding the muscle hypertrophy, unnecessary in this type of athlete. A further possible application could be the prescription of this form of exercise in subjects with some sort of disorder. This type of exercise would be suitable for such patients because energy is provided mainly by aerobic metabolism. For example, diabetes mellitus (type 2) is associated with

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Acute Physiological and Mechanical Responses During Resistance Exercise at the Lactate Threshold Intensity.

The purpose of this study was to examine acute metabolic, mechanical, and cardiac responses to half-squat (HS) resistance exercise performed at a work...
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