The anaerobic threshold (AT) has been frequently used for the evaluation of aerobic fitness in different populations by analyzing the response of blood lactate (3,13,19,33), ventilation (28,32), and blood glucose concentration [Gluc] (25-27). The AT reflects an exercise intensity above which a disproportional increase in the rate of anaerobic glycolysis occurs, leading to an accumulation of blood lactate and to hyperventilation, which indicates the transition from aerobic metabolism to a condition in which the contribution of the anaerobic glycolytic pathway rises proportionally as a function of the increase in exercise intensity (9,17,28,31).
The studies that have identified the AT from [Gluc] responses (25-27) demonstrate that during incremental tests (ITs), this variable decreases until the AT, and from this intensity, its response increases and enables the identification of an exercise intensity at which a balance between blood glucose output and its uptake may occur. The identification of such intensity would have clinical applications for individuals with type 2 diabetes mellitus (DM-2) since they would better select exercise intensities (e.g., below or at the glucose threshold [GT]) to optimize the [Gluc] uptake and thus improve the [Gluc] control, avoiding a drastic increase in the [Gluc] due to the high intensity of the exercise (5).
Studies have demonstrated the application of AT for functional evaluation on cycling (19), swimming (10), and running (33). However, this practice often has been applied to running and cycling, with only a few studies investigating the identification of lactate threshold (LT) (4) and GT (22) in resistance exercises and no studies analyzing this possibility in individuals with DM-2.
Insulin resistance, which leads to chronic hyperglycemia, characterizes DM-2. This disease is one of the main causes of death and functional incapacity in many countries all over the world (1,2), and both aerobic and resistance exercises have been recommended in the treatment of the disease (5,12,34). However, further investigations about the exercise intensities that would optimize blood glucose control for these patients are needed, mainly regarding resistance exercise. Therefore, since the anaerobic threshold has been suggested as an important reference for exercise prescription for individuals with DM-2 (15,16), the purpose of the current study was to identify and compare different methods of determining the LT and GT in resistance exercise for individuals with DM-2.
Experimental Approach to the Problem
In order to verify if the LT and GT would be identified in resistance exercise for individuals with DM-2, an incremental test (IT) in resistance exercise was performed on the leg press (LP) and bench press (BP) with blood lactate and glucose measurements. Also, the relationships between LT and GT identified with resistance exercise and LT (33) identified with the cycle ergometer were assessed.
Nine men diagnosed with DM-2 through a previous medical screening [i.e., fasting blood glucose ≥126 mg·dL-1 (1) or glycosylated hemoglobin >7% (2)] volunteered to participate in the study. All participants were involved in training programs for at least 12 months (i.e., aerobic and resistance exercises), consisting of approximately 40 minutes of exercise per day, 3 times a week. The subjects gave their written informed consent to the experimental procedures after having the possible benefits and risks of participation in the study fully explained to them. The protocol was approved by the university's ethics committee. The subjects' characteristics are presented in Table 1.
Data collection was performed at the Laboratory of Strength Studies at the Catholic University of Brasilia. The participants were initially submitted to cardiovascular examinations that included resting electrocardiogram (ECG) and blood pressure measurements. Those who presented with cardiovascular, neurological, or orthopedic complications were excluded from the study. On different days, the subjects performed an IT on a cycle ergometer until volitional exhaustion in order to evaluate an effort ECG, to determine the lactate threshold in cycling (LTc) (33), and to determine the peak oxygen uptake (2peak) (14); one-repetition maximum (1RM) test (18) on the LP and BP (Righetto, mod., Power Tech, Brazil); and an IT in resistance exercise (LP-ITLP and BP-ITBP). With the exception of the 1RM, all ITs were performed in a fasted state, between 8:30 and 9:00 am, and in a room with a temperature of about 25°C.
Incremental Resistance Exercise Test
The ITs for the identification of the LT and GT on the LP and BP were performed in this order, on the same day, and with a 30-minute recovery interval between them. The selected intensities during these tests were 10, 20, 25, 30, 35, 40, 50, 60, 70, 80, and 90% of 1RM. Each repetition cycle lasted about 2 seconds, with 1 second for the concentric phase and 1 second for the eccentric phase. Each stage lasted 1 minute, and the subjects performed 30 repetitions. Two minutes of passive recovery were allowed between the stages to increase the intensity (i.e., % 1RM) and for blood sampling and blood pressure measurements. In addition, during the IT, the heart rate (HR) and the rate of perceived effort (RPE) were continuously measured. The rhythm was controlled by visual and verbal commands, and the end of the test was determined by the subject's incapacity to perform the movement within the correct biomechanics previously established or by the incapacity to perform the number of repetitions established for the stage. Moreover, other criteria for testing interruption would be an RPE score between 19 and 20 on the Borg scale (6) and a sudden increase in blood pressure immediately after the exercise stages (automatic method) to values of about 250/115 mm Hg (2). However, none of the participants even reached a blood pressure above 220/110 mm Hg.
Blood Collection and Laboratory Analysis
During each between-stage interval of the ITLP-BP, 25 μL of capillary blood were collected from the earlobe by using heparinized capillaries previously calibrated, for blood lactate concentration [Lac] and glucose concentration [Gluc] measurements. The blood samples were deposed in Eppendorf tubes containing 50 μL of sodium fluoride at 1%. The [Lac] and [Gluc] were determined by a lactate and glucose analyzer (YSI 2700S).
Lactate Threshold and Glucose Threshold Identification
The LT was identified by an exponential increase in [Lac], and the GT was identified through [Gluc] kinetics during the IT and defined as the exercise intensity at which blood glucose stopped to decrease and began to increase during the test (Figure 1).
Besides that, the ratio between [Lac] and the relative exercise intensity (QLac = [Lac]/% 1RM ratio) was plotted against the relative intensity (i.e., % 1RM) for each stage of IT in order to make possible, through a mathematical adjustment, the use of a second-order polynomial function. This one originated a second-order equation that, after being derived, identified the exercise intensity at which there was a disproportional increase in [Lac] in relation to the % 1RM, allowing for the identification of the LT. The QLac technique was used to identify the polynomial lactate threshold in both resistance exercises (i.e., LTpLP and LTpBP). The identification of the LTp for a single person with DM-2 is presented in Figure 2.
An exploratory analysis was used to verify the data normality, and then descriptive statistics were performed. Data are presented as mean, SD, and SEM. The mean results of the ITs were selected at relative test moments for all the participants, as follows: “begin,” first stage of the IT; “mid1,” midpoint between the first stage and the LTLP or LTBP; “LTLP” or “LTBP,” lactate threshold identified for the LP or BP; “mid2,” midpoint between the LTLP or LTBP and the last stage or exhaustion; and “end,” last stage or moment of exhaustion. The comparisons between the variables related to LT and GT identified by the different methods either with the cycle ergometer or with resistance exercise were made through a one-way analysis of variance (ANOVA) with repeated measures and a Tukey-Kraemer post hoc test. A Pearson product moment correlation coefficient was used, and the agreement between the protocols was confirmed by the technique of Bland and Altman (6). The adopted α level for significance was p < 0.05 (Statistica version 5.0; StatSoft, Inc., Tulsa, OK).
The mean [Lac], [Gluc], and QLac in the relative intensities for all participants are presented in Figure 3 for the LP and in Figure 4 for the BP. After achieving the LT, the [Gluc] responses were similar to the [Lac] responses for both exercises (Figures 3A and 4A).
No significant difference was observed among the % 1RM, [Gluc], and HR corresponding to the thresholds identified by different methods for the LP and the BP (Table 2). On the other hand, the absolute intensities (kg) corresponding to the thresholds were different between the BP and the LP (p < 0.05) and between the LTpLP and the LTLP (p < 0.05). The results for [Lac] and RPE at the threshold intensities in resistance exercise are presented in Table 2.
Significant correlations were observed between the threshold intensities identified by the different methods in resistance exercise (Table 3), with the agreement between these methods being confirmed by the technique by Bland and Altman (6) (Figure 5). Besides that, a significant correlation was observed between the thresholds identified in resistance exercises by the different methods (Table 3) and with the threshold determined with the cycle ergometer (i.e., LTc).
The main findings of this study were the possibility of identifying the LT and GT in resistance exercise for individuals with DM-2 as well as the similarity between [Gluc] and [Lac] responses after the LT was attained during ITs on the LP and BP (Figures 3A and 4A). Moreover, the polynomial adjustment also enabled the LT identification in both exercise types, from the responses of QLac (i.e., [Lac]/% 1RM ratio) using a second-order polynomial function (Figures 3B and 4B) with no differences to the thresholds determined by other methods (Table 2). Significant correlations occurred between the thresholds identified by different methods in both resistance exercises as well as between these methods to the aerobic capacity (i.e., LT) identified on the cycle ergometer (i.e., LTc) and to the 1RM previously tested (i.e., 1RMLP and 1RMBP). The results also demonstrated the agreement between the LTLP and LTBP, considered a gold standard, with other methods used for threshold identification (Table 3; Figure 5), suggesting the validity of these protocols in the functional evaluation of individuals with DM-2.
In the current study, the intensities corresponding to the LT and GT in resistance exercise were between 30% and 32% 1RM for the LP and the BP. These results are in agreement with the results from Barros et al. (4) in a sample of healthy young individuals, in whom the LT occurred around 28% and 32% 1RM for the biceps curl and LP, respectively, as well as the thresholds identified from blood lactate and glucose in a study by Oliveira et al. (22), which occurred around 30% 1RM, also for healthy young individuals. Petrofsky et al. (23) demonstrated in an animal model that in static contractions above 20% 1RM, there is an increase in the intramuscular pressure, according to the exercise intensity, which elicits a small arterial blood flow, whereas for dynamic contractions, the literature suggests that in intensities above 30% 1RM, this reduction in blood flow would occur (35). In both conditions, the anaerobic glycolysis activity would be increased and allow for the identification of a metabolic transition of the energetic pathways. It is speculated that this phenomenon (i.e., LT) should be mainly marked by a hemodynamic factor, as evidenced in the literature (23,35) in which the higher % 1RM promotes an intramuscular pressure that increases muscular tension and that is higher than the capillary pressure, causing them to be blocked. This blockage thus leads to muscular hypoxia and diminishes oxygen availability, increasing both the activity of the glycolytic pathway and the blood lactate levels, enabling the identification of the LT.
Some studies have identified the GT in the cycle ergometer (26) and running (25), by the method of glucose minimum in active, healthy, young individuals and athletes, whereby the participants, after recovering from a supramaximal exercise, performed an incremental exercise series, eliciting [Gluc] kinetics similar to those of individuals with DM-2 in the current study (Figures 3A and 4A). Berg (5) postulated that high-intensity resistance exercises may stimulate the hypothalamus-pituitary-adrenal axis to secrete stress hormones and increased glucose output from the liver and the blood. Therefore, the possible explanation for the increase in [Gluc] above the LT intensity in resistance exercise is based on a high increase in catecholamine and glucagon responses, which stimulate a higher hepatic glycogenolysis and gluconeogenesis increasing the [Gluc] output in relation to its uptake (5,11,30,36,37).
The use of the thresholds proposed in the current study to prescribe resistance exercise intensities becomes important to individuals with DM-2, as hyperglycemia is the main sign of this disease and incremental exercises performed below these thresholds (e.g., GT) result in the reduction of [Gluc] (Figures 3A and 4A). This suggests that an acute exercise performed at intensities related to GT and LT would contribute to better blood glucose control for these patients, as demonstrated in the laboratory during exercise (21) and until 75 minutes of recovery after resistance exercise performed at 23% 1RM (20).
The intensity (kg) corresponding to LTLP was between 46% and 60% of the body weight, whereas the intensity corresponding to LTBP was between 18% and 26% of the body weight for individuals with DM-2 and a mean age of 47.2 ± 12.4 years, who participated in this study. Additional studies about the practical applications of these thresholds in resistance exercise training for individuals with DM-2 are still necessary.
The training intensity control may also be possible by using the RPE (7), once it delimits exercise intensities for which there should be a predominance of glucose uptake (i.e., <11-14 on the 15-point Borg scale) or in its hepatic production, leading to an increase in [Gluc] (i.e., >15 on this same scale) (Table 2). However, additional studies must be performed regarding the use of RPE in predicting the metabolic transition in resistance exercises as well as its application in training prescription for special populations, such as individuals with DM-2. Despite the higher [Lac] and RPE for the BP than for the LP, it was verified that the metabolic and cardiovascular stress, from [Gluc] and HR, respectively, were not different when comparing the threshold intensities for both incremental resistance exercises (Table 2). On the other hand, when the absolute intensities (kg) corresponding to LT and GT were compared between the 2 exercises, higher values were observed, as expected, for the LP than for the BP for all methods (p < 0.05). This can be explained by the smaller muscular mass involved in the BP, as reflected by the mean 1RM previously obtained (65.3 ± 16.8 kg [BP] versus 159.2 ± 41.5 kg [LP], p < 0.05) (Table 1).
An important aspect related to the benefits of resistance exercise for individuals with DM-2, who may present with cardiovascular complications, is the increase in strength and muscular resistance (1,2,5). Takarada et al. (29) showed that resistance training performed at lower intensities results in an increase of muscle mass and strength in middle-aged women. Therefore, this effect would include an increased absolute load (kg) corresponding to the GT and/or LT in resistance exercise for DM-2. Possibly the benefits associated to this would include a smaller number of motor units being recruited in parallel to a decreased cardiac overload during sub-maximal activities of daily living which, in turn, would reduce the risk for cardiovascular events for DM-2 (8,24).
The results demonstrated the possibility of identifying the LT and GT in the LP and BP as well as the LTp through QLac for both exercises in individuals with DM-2. New studies are necessary to elucidate the meaning of these thresholds and their possible clinical applications in the evaluation and prescription of resistance exercises based on glycemic and hemodynamic control of these patients.
Based on the results, it can be concluded that it was possible to identify the LT and GT in resistance exercise performed by individuals with DM-2, through the presented methods, and that [Gluc] responses above LT were similar to those of [Lac] for the LP and BP. The meaning of these thresholds still needs to be established, and additional studies are being developed in order to analyze their clinical applications in exercise prescription and evaluation for the glycemic and hemodynamic control of individuals with DM-2.
Both the LT and the GT seem to delimit an exercise intensity above which the blood glucose production may be higher than blood glucose uptake, eliciting an increase in blood glucose levels. In the current study, the LT and GT intensities were beyond 30% and 32% 1RM for the individuals with DM-2. In addition, the intensities (kg) corresponding to these thresholds were between 46% and 60% of the body weight on the LP and between 18% and 26% of the body weight on the BP.
Exercise prescription would be done in 3 sets of 20 to 30 repetitions each, with 1 minute of rest and alternating the muscle groups. These suggestions would have practical applications for blood glucose control for individuals with DM-2 with characteristics similar to the participants and mainly individuals with DM-2 who present with unbalanced blood glucose, because these exercise intensities are low to moderate and thus secure in terms of cardiovascular and endocrine stress.
Alternatively, individuals with DM-2 who present with well-controlled blood glucose may be able to perform high-intensity resistance exercise training (e.g., above LT until 80% 1RM or more). However, the relative risks and benefits for patients while performing a high-intensity resistance exercise should be done on an individual basis. The training control would also be done by using the RPE with the 15-point Borg scale. The data showed that the optimal resistance exercise intensity for blood glucose diminution is an RPE around 11 to 14, while exercise performed above on RPE of 15 may elicit the blood glucose to increase for the patients and thus should be used with caution.
We are grateful to CNPq (475575/2004-0) and SIGEP/UCB for the financial support. Also, our acknowledgments extend to LAFIT/UCB, LABEF/UCB, Micromed-DF, and SalvaPé-SP for technical assistance.
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