Type-2 diabetes is a well-recognized medical condition of increasingly epidemic levels that has been projected to be the seventh leading cause of death worldwide in 2030 (42,43). With an estimated prevalence of roughly 350 million (6), type-2 diabetes constitutes a major global health burden because it contributes to other complications, such as cardiovascular disease, neuropathy, nephropathy, retinopathy, physical disability, and premature death (27,33,43,44). Lifestyle factors such as physical inactivity and excess energy intake at least partly account for the development of type-2 diabetes, as does genetic predisposition, age, and racial makeup (13,25,46). The hallmark features include hepatic and peripheral insulin resistance, particularly in the skeletal muscle and adipose tissue compartments (9,16).
Currently, the therapeutic management of type-2 diabetes involves some form of lifestyle modification (28). In particular, regular exercise is recognized as an effective method of improving disease status (4,14,39). The basis for exercise therapy relates simply to the ability of skeletal muscle contractile activity to enhance insulin sensitivity and promote glucose uptake and clearance (15). Regular participation in structured aerobic, resistance, or concurrent exercise has been reported to have generally favorable effects on glycemic control, body composition, endothelial function, physical work capacity, and self-reported well-being (5,15,18,19,23,32,35,38). Much progress has also been made toward optimizing the program variables that define the exercise prescription, including intensity, duration, and single vs. multiple daily sessions (8,21). Overall, there seems to be a strong rationale for the inclusion of programmed exercise in patients with type-2 diabetes.
As an alternative to conventional resistance training, muscular overload may be accomplished with elastic bands, which offer variable resistance depending on their thickness and the extent to which they are stretched. Progressive overload can be achieved using thicker bands or elongating them beyond their resting length. Elastic bands are a convenient alternative to traditional loading methods because they do not require access to machines or free weights; therefore, they can circumvent some of the logistical issues associated with conventional resistance training. Although not extensively studied in type-2 diabetes (24), muscular overload through supervised elastic band resistance training has been reported to significantly lower glycosylated hemoglobin (HbA1c), decrease adiposity, reduce low-density lipoprotein cholesterol, enhance balance, and increase strength (1,18). Consequently, these adaptations may improve disease status and the quality of life.
Although exercise therapy for type-2 diabetes has a sound basis and scientific support, an issue that has not been thoroughly investigated is the extent to which diabetes duration (i.e., time since diagnosis) influences the response to exercise. Understanding the exercise response as a function of disease duration is important for program design specific to type-2 diabetes because it may provide insight on the ideal time to initiate exercise during a patients' disease trajectory. In this regard, some previous reports found a limited or specific time frame to obtain the benefits of a given therapy in several chronic, debilitating conditions (2,22,31), whereas others did not (11). With specific reference to type-2 diabetes, one study reported that 6 months of exercise training improved glycemic control, lipid profile, and function in patients with long-standing disease (36); however, the magnitude of change in comparison with patients with shorter disease duration could not be addressed because of the absence of this specific group.
Furthermore, previous work has documented differences in physiological features on the basis of the duration of type-2 diabetes. For instance, Park et al. (29) reported that in comparison to patients with shorter disease duration, those with a long history of diabetes (>6 years) exhibited significantly less leg muscle quality, an indicator of contractile apparatus integrity. De Feyter et al. (7) also found several indices of physical work capacity to be significantly lower in patients with long-standing type-2 diabetes compared with newly diagnosed patients. These findings suggest that the duration of diabetes is associated with differences in physiological and functional status and that there could be a divergent response to exercise training. Therefore, the purpose of this study was to evaluate the adaptive responses to elastic band resistance training in patients with short- vs. long-duration type-2 diabetes.
Experimental Approach to the Problem
To investigate whether the existing duration of type-2 diabetes influenced patient responses to progressive resistance training, a mixed between-within groups design was used in which patients were assigned to either a short-duration diabetes group (<7 years; n = 12) or a long-standing diabetes group (≥7 years; n = 14) in accordance with the previous work that investigated the impact of diabetes duration (7,29). Diabetes duration was determined based on the reported age at diagnosis (0 years) and the elapsed time until entrance into the study. Glucose control (i.e., blood analytes), body composition, and physical function were evaluated before and after training to determine if adaptive responses depended on disease duration.
Twenty-six untrained women with type-2 diabetes were recruited from the Eulji General Hospital and assigned to 1 of the 2 disease duration groups (short duration: <7 years; n = 12; long duration: ≥7 years; n = 14). Age range of subjects was 46–65 years. All subjects from both the groups participated in the resistance training program. Subjects were considered to have type-2 diabetes on the basis of fasting plasma glucose concentration of ≥7.0 mmol·L−1 or if they were taking oral hypoglycemic medication or insulin. Subjects who participated in the study also did not have any secondary complications. Exclusion criteria included uncontrolled hypertension, type-1 diabetes, cardiac arrhythmias, severe valvular heart disease, congestive heart failure, malignant tumors, or preexisting musculoskeletal injuries that would be contraindications for participation in exercise. Subjects prescribed medications were maintained on their current dosing schedule throughout the study. All participants provided informed consent, and all procedures were approved by the Eulji Hospital Review Board.
Before the start of the experimental period, all subjects reported to the Eulji Hospital Diabetes Center for familiarization with the resistance band exercises used during training. Patients used the lowest resistance available (i.e., yellow band) and were instructed on proper exercise technique under the supervision of a clinical exercise physiologist. The frequency (5 d·wk−1), exercise selection, and order were identical to the experimental training sessions. All subjects completed 1 month of familiarization before experimental training. After completing the familiarization period, subjects underwent pretest assessments including standard anthropometrics (height, body mass, body mass index [BMI], and waist circumference), evaluation of body composition, and a blood draw. Height and body mass were measured in minimal clothing, and BMI was subsequently calculated by dividing mass (in kilograms) by height squared (in square meter). Waist circumference measurements were taken around the narrowest portion of the trunk between the inferior aspect of the ribs and the superior portion of the iliac crest. Blood pressure was obtained after subjects were seated for 10 minutes to ensure a resting measurement. A graded exercise test was also administered to assess physical work capacity, and various field tests were performed to evaluate functionality. Resistance training began the week after baseline assessments, and 12 weeks later, the same measurements were repeated.
Resistance overload was implemented using elastic bands (Theraband; The Hygenic, Co., Akron, OH, USA). This method of resistance loading was selected for its ease of use and its suitability for previously untrained patients with chronic disease. The degree of resistance load was manipulated using bands with varying degrees of difficulty according to their color code (e.g., yellow, red, green, and blue in order of increasing resistance provided). The initial resistance band selected for each exercise was based on which band a patient could perform 15–20 repetitions maximum (RM). The RM was reevaluated every 4 weeks, and if a patient could complete at least 30 repetitions for a given exercise, the resistance was increased to the next most difficult band according to the manufacturer's guidelines.
Each training session consisted of 10 exercises performed in the following order: seated row, standing trunk rotation, leg press, sit-ups, crunches, bilateral bicep curl, lateral trunk flexion (i.e., side curl), standing overhead triceps extension, chest flys, and unilateral dorsiflexion (both limbs). The complete sequence of 10 exercises represented 1 circuit. For each exercise, the band was extended to 100% of its resting length to provide standardized resistance, with a cadence of 4 seconds each for the eccentric and concentric phases. Patients performed training twice per day (morning and afternoon) in circuit-training fashion, with 2 to 3 circuits of exercise distributed among these daily sessions (e.g., 2 in morning and 1 in afternoon). The frequency of training was 5 d·wk−1 for a total of 12 weeks. A typical session required approximately 40–60 minutes to complete (2–3 minutes per exercise and 20–30 minutes per circuit).
Both groups of patients received dietary education and counseling once per week by a medical doctor certified as a dietitian. They were given general instructions to limit their intake of carbohydrates and sugar-rich snacks. Patients were also asked to record their daily food and beverage consumption in a dietary log. The types and quantities of food documented in these logs were reviewed before each training visit. Diet logs were not collected or formally analyzed.
Blood Collection and Analysis
Blood was collected pre- and posttraining from the antecubital vein of each subject after an overnight fast of at least 10 hours. The samples were fractionated by centrifugation at 3,000 repetitions per minute (rpm) for 15 minutes, and the serum was subsequently collected and stored as aliquots at −80° C. Fasting blood glucose, 2 hours postprandial glucose, and hemoglobin A1c (HbA1c) were determined by the glucose oxidation method (glucose oxidase [GOD]-peroxidase [POD]) and high-performance liquid chromatography (ion exchange), respectively. Fasting insulin levels were quantitated by radioimmunoassay, whereas triglycerides, cholesterol, high-density-lipoprotein cholesterol (HDL-C), insulin, C-peptide, aspartate aminotransferase (AST), and alanine aminotransferase (ALT) were assayed using an automated chemistry analyzer (Hitachi 7170; Hitachi, Tokyo, Japan) and commercially available enzymatic-based kits according to the manufacturer's guidelines.
Bioelectrical Impedance Analysis
Body composition was determined at baseline and posttraining using 8-polar bioelectrical impedance analysis (InBody 3.0; Biospace Co., Seoul, South Korea). This device has been previously used to evaluate body composition in clinical populations (17). Subjects stood on the device in minimal clothing, with scan time being less than 2 minutes. Software accompanying the device provided values for muscle and fat mass and waist-to-hip ratio (WHR).
Graded Exercise Testing
A graded exercise test was administered to each patient after a 12-hour overnight fast using a cycle ergometer (Jaeger ER900, Germany) and metabolic cart. The test protocol consisted of an initial 20-watt load at 50–60 rpm, with subsequent 20-watt increases in workload every 2 minutes at the same pedaling frequency. The test continued in this manner until oxygen consumption failed to increase with a greater workload or until volitional failure. Expired gases, heart rate, and blood pressure were monitored and recorded during the graded exercise test. Ventilatory threshold (VT) was defined as the oxygen uptake value immediately preceding the point at which ventilation increased over time rather than attaining a steady state, and it was determined using the v-slope method. Maximum oxygen consumption (V[Combining Dot Above]O2max), VT, load at VT, time at VT, peak exercise load, peak exercise time, and hemodynamic measurements (maximum heart rate, peak systolic and diastolic blood pressure) obtained during and after the exercise test were used for subsequent analysis.
Physical Function Assessments
Field tests were administered at baseline and posttraining to evaluate indicators of physical function such as strength, muscular endurance, flexibility, and mobility. Strength was assessed for each arm using a grip strength device (30). Subjects were instructed to maintain a stable grip for 3 seconds before release. Three trials were performed for each arm, and the peak value was used for analysis. Upper- and lower-body muscular endurance was measured by the number of repetitions performed in the unilateral bicep curl, abdominal curl-up, and sit-and-stand exercises in 30 seconds (12,20). Additionally, shoulder and trunk flexibility were evaluated using the back scratch test (distance or overlap between middle fingers of each hand measured behind the back) and the chair sit-and-reach test, respectively (10). Finally, mobility was determined by the 8-foot up and go test, which required subjects to stand from a seated position, walk 8 feet around a cone, and return to the chair (37,40).
All data are presented as mean ± SD and expressed as absolute values unless otherwise indicated (e.g., relative V[Combining Dot Above]O2max). Sample sizes were determined based on an anticipated HbA1c difference of 0.30 HbA1c units between the groups in the change from baseline to posttraining similar to Bacchi et al. (3). Using α = 0.05 and 1 − β = 0.80, samples sizes of 20 were calculated per group, which also took into account a 10% attrition rate. Normality of the data was evaluated and confirmed by Shapiro-Wilk tests. Baseline data between the 2 groups (i.e., pretraining values) were analyzed by independent t-tests. All remaining variables measured before and after training were analyzed using a 2 × 2 (disease duration × training) repeated-measures analysis of variance. In the event of a significant interaction, follow-up testing was conducted using t-tests to localize the effect. For significant main effects of intervention, paired t-tests within each group were used to localize the pre-to-postintervention effects. Level of significance was accepted at p ≤ 0.05.
An enrollment target of 20 subjects per group was not achieved, possibly because of protocol demands and the time commitment required to perform twice daily exercise sessions on 5 consecutive days for 12 weeks. Nevertheless, patient compliance and retention were excellent, as all 26 enrolled participants successfully completed the study without complication (short duration: n = 12; long-duration: n = 14; 60–65% of target enrollment). Baseline characteristics of patients with short- and long-duration diabetes are presented in Table 1. Age and height were not different between the groups (p = 0.086 and 0.981, respectively); however, the disease duration in patients with long-standing diabetes was greater by 7 years (+233%; p < 0.001). There were no significant differences in body mass (p = 0.217) or BMI (p = 0.185) before training; however, waist circumference (+10%, p = 0.033) and WHR (+5%, p = 0.02) were greater in patients with a long-duration diabetes. There was also a tendency for greater fat mass (+18%, p = 0.11) and percent fat (+10%, p = 0.097) in patients with long-standing diabetes, but this was not the case with muscle mass (p = 0.55). No baseline differences were observed for indices of glucose control and lipid profile (p = 0.23–0.81). There were tendencies for lower performance in right (−14%; p = 0.086) and left (−10%; p = 0.149) grip strength, sit-and-stand repetitions (−15%; p = 0.124), shoulder flexibility (+52%; p = 0.058), sit and reach (−35%; p = 0.095), and 8-foot up and go time (+8%; p = 0.134) in patients with a longer history of diabetes. Relative V[Combining Dot Above]O2max tended to be lower with long-standing disease (−9%; p = 0.09).
Analytes for glucose control, lipid profile, and liver enzymes are presented Table 2. There were no significant diabetes duration × training interactions for any blood marker assayed. However, significant main effects of training were observed for HbA1c in which post hoc tests showed similar pre to post reductions in both the groups (short: −13%; p = 0.01; long: −18%; p = 0.001). The same pattern of training-induced reductions was also found with respect to fasting glucose (short: −23%; p = 0.005; long: −31%; p = 0.003), postprandial glucose (short: −36%; p = 0.002; long: −40%; p = 0.001), and insulin (short: −40%; p = 0.007; long: −34%; p = 0.001). For the lipid profile, there were significant main effects of training on total cholesterol and HDL-C but not triglycerides. Follow-up testing determined significant pre to post cholesterol decreases in both the groups (short: −11%; p = 0.035; long: −15%; p = 0.006). Regarding HDL-C, follow-up analysis found comparable increases in both the groups (short: +13%; p = 0.07; long: +9%; p = 0.05). Finally, there was a main training effect on C-peptide levels that follow-up testing indicated significant pre-to-post reductions in both the short-duration (−38%; p = 0.004) and long-duration (−51%; p = 0.002) groups. No significant effects were observed for either AST or ALT (p > 0.05).
Anthropometrics and Body Composition
All anthropometric and body composition data are illustrated in Figures 1A–C. There were no significant diabetes duration × training interactions (p > 0.05) for any anthropometric or body composition variable. However, there were significant main effects of training (p < 0.01) on body mass, BMI, waist circumference, WHR, fat mass, and percent fat. Post hoc analyses revealed significant body mass reductions (p < 0.01) in both the groups (short, pre: 57 ± 7 vs. post: 52 ± 6 kg; long, pre: 61 ± 9 vs. post: 58 ± 9 kg). Likewise, BMI was significantly lower (p < 0.01) in both the groups (short, pre: 23.4 ± 2.8 vs. post: 21.3 ± 2.3 kg·m−2; long, pre: 25.0 ± 3.1 vs. post: 23.4 ± 3.2 kg·m−2). This was also the case with respect to waist circumference (short, pre: 78.8 ± 6.7 vs. post: 73.5 ± 7.1 cm; p < 0.01; long, pre: 86.6 ± 8.2 vs. post: 80.7 ± 8.2 cm; p < 0.01) and WHR (short, pre: 0.88 ± 0.04 vs. post: 0.84 ± 0.04; p < 0.01; long, pre: 0.92 ± 0.05 vs. post: 0.89 ± 0.04; p < 0.01).
Regarding body composition, 12 weeks of training caused a significant reduction (main effect, p < 0.01) in fat mass and percent fat. Post hoc analysis indicated significant pre-to-post fat mass reductions (p < 0.01) in both the groups (short, pre: 16.8 ± 4.0 vs. post: 12.4 ± 3.2 kg; long, pre: 19.9 ± 5.2 vs. post: 16.2 ± 5.1 kg). Similarly, percent fat also decreased significantly (p < 0.01) in both the groups (short, pre: 29.2 ± 4.2 vs. post: 23.3 ± 4.0 kg; long, pre: 32.2 ± 4.7 vs. post: 27.7 ± 5.5 kg). Muscle mass determined by bioelectrical impedance analysis (BIA) was not significantly different between or within groups (p > 0.05).
Performance indices for each cohort before and after training are shown in Table 3. There were no significant diabetes duration × training interactions (p > 0.05) for any physical function parameter. However, significant main effects of training were observed for all functional tests. Follow-up analysis for the main effect of training on right and left limb grip strength showed significant pre-to-post gains in patients with long-standing diabetes (right limb, +13%; p = 0.02; left limb, +11%; p = 0.001) rather than patients with a shorter-duration disease (right limb, +2%; p = 0.56; left limb, +5%; p = 0.16). In contrast, the main effect of training on maximum bicep curl repetitions showed improvements in both the short-duration group (+15%; p = 0.001) and the long-duration group (+33%; p < 0.01). This was also the case for maximum abdominal curl-up performance (short, +75%; p = 0.005; long, +167%; p = 0.004) and sit-and-stand repetitions (short, +45%; p < 0.01; long, +47%; p < 0.01).
For the flexibility and mobility measurements, significant main effects of training were observed for the chair sit and reach and 8-foot up and go. Post hoc analysis for the main training effect on chair sit-and-reach performance showed significant pre-to-post improvements (p < 0.01) in patients with both short- (+38%) and long-duration diabetes (+85%). Likewise, follow-up tests on 8-foot up and go time showed significant pre-to-post improvements (p < 0.01) in both the short- (−20%) and long-duration (−20%) groups.
V[Combining Dot Above]O2max, VT, and Exercise Capacity
V[Combining Dot Above]O2max, VT, and exercise capacity variables are presented in Figures 2A and B. There was no significant training or interaction effect (p > 0.05) on absolute V[Combining Dot Above]O2max (short, pre: 1,264.3 ± 226.7 vs. post: 1,197.8 ± 293.5 ml·min−1; p = 0.396; long, pre: 1,229.3 ± 194.0 vs. post: 1,238.0 ± 307.5 ml·min−1; p = 0.904) or relative V[Combining Dot Above]O2max (short, pre: 22.3 ± 3.0 vs. post: 23.0 ± 5.5 ml·kg−1·min−1; p = 0.647; long, pre: 20.4 ± 2.7 vs. post: 21.7 ± 4.0 ml·kg−1·min−1; p = 0.304). However, there was a significant main effect of training on peak exercise time, with follow-up tests indicating significant pre-to-post increases in patients with long-standing diabetes (pre: 456 ± 105 vs. post: 542 ± 119 seconds; p = 0.001) rather than those with a shorter-duration disease (pre: 485 ± 78 vs. post: 519 ± 98 seconds; p > 0.05). A significant diabetes duration × training interaction was observed for peak exercise load because patients with long-standing diabetes required a greater workload to attain V[Combining Dot Above]O2max after training (pre: 75.7 ± 14.0 vs. post: 91.4 ± 17.0 W; p = 0.001), whereas those with a shorter-duration disease did not (pre: 85.0 ± 12.4 vs. post: 85.0 ± 15.1 W; p > 0.05).
Regarding VT, there was no significant diabetes duration × training interaction (p > 0.05), but a significant main training effect was observed. Post hoc analysis revealed significant training-induced improvements in both the short-duration (pre: 67 ± 9 vs. post: 76 ± 8%V[Combining Dot Above]O2max; p = 0.003) and long-duration (pre: 65 ± 8 vs. post: 73 ± 8%V[Combining Dot Above]O2max; p = 0.002) groups. Likewise, there was no significant diabetes duration × training interaction on time at VT (p > 0.05); however, a significant main effect of training was found. Follow-up testing indicated the effect to be the result of significant improvements (p < 0.01) by both patient cohorts (short, pre: 270 ± 63 vs. post: 390 ± 63 seconds; long, pre: 246 ± 59 vs. post: 384 ± 82 seconds). Finally, there was a significant diabetes duration × training interaction on workload at VT (p = 0.018). At baseline, the long-standing cohort exhibited significantly lower values in comparison to those with a shorter-duration disease (short: 50.0 ± 10.5 vs. long: 40.0 ± 11.1 W; p = 0.027). After training, both the groups demonstrated a significantly greater workload at VT compared with pretraining (short: pre: 50.0 ± 10.4 vs. post: 61.7 ± 15.9 W; p = 0.012; long, pre: 40.0 ± 11.1 vs. post: 65.7 ± 12.2 W; p < 0.01), whereas they were not significantly different at posttraining (p = 0.47).
Hemodynamics at Rest, During Exercise, and Postexercise
Values for each hemodynamic measurement are reported in Table 4. For the resting measurements of heart rate, systolic blood pressure, and diastolic blood pressure, no significant diabetes duration × training interactions were noted. However, there was a significant main effect of training on resting heart rate values that follow-up testing found to be the result of training-induced increases in both the groups (short, +13%; p = 0.009; long, +19%; p = 0.004). There was no significant main effect of training on resting systolic or diastolic blood pressure (p > 0.05).
Regarding peak heart rate and systolic and diastolic blood pressure obtained during the graded exercise test, no significant diabetes duration × training interactions were observed (p > 0.05). Significant main effects of training were noted, though, on peak systolic and diastolic blood pressure but not peak heart rate (p = 0.806). Post hoc tests found significant peak systolic blood pressure reductions in patients with long-standing diabetes (−5%; p = 0.048) but not in those with a shorter-duration disease (p = 0.287). This was also the case for peak diastolic blood pressure during exercise because there was a significant pre-to-post decrease in patients with long-standing diabetes (−5%, p = 0.011) rather than in those with a shorter-duration disease (p = 0.389).
In the recovery phase after graded exercise, heart rate and systolic blood pressure responses as a result of training were not different between the 2 cohorts (diabetes duration × training, p > 0.05) nor was a main training effect noted (p > 0.05). However, there was a significant interaction on postexercise diastolic blood pressure (p = 0.028), with follow-up tests revealing a significant decrease from pre to post in patients with long-standing disease (−13%; p = 0.004) but not in those with a shorter-duration diabetes (p = 0.844).
Patients with short-duration or long-standing diabetes demonstrated similar reductions in HbA1c, fasting and postprandial glucose, fasting insulin, and C-peptide. Collectively, these changes suggest that glycemic control can be improved by structured resistance band training with dietary advice irrespective of when it is initiated during the course of a patient's disease trajectory. In a recent investigation, Tan et al. (36) explored the impact of regular aerobic and resistance training (6 months) on analytes for glucose control in a group of patients with a disease duration greater than the present work (16.7 ± 6.7 vs. 10 ± 3 years). After training, patients showed significant reductions in HbA1c (−8%), fasting plasma glucose (−10%), and fasting insulin (−23%) that parallel the changes observed in our cohort of patients with long-standing diabetes. In each instance, however, these percent changes were lower than what was observed in the current work (HbA1c: −16%; fasting glucose: −27%; insulin: −35%). Variations in training protocols may partly account for these differences as could diabetes duration. Perhaps, we may have seen a divergent response between cohorts if our long-standing group had a diabetes duration similar to that of Tan et al. (e.g., approaching 20 years). With respect to lipid profile, the decrease in total cholesterol (−13%) is consistent with the work of others that explored training responses in patients with a long history of diabetes (36), and despite the lack of a significant training effect on triglycerides (−22%; p > 0.05), the percentage decrease was also similar to a previous investigation of similar scope (36).
Training with dietary counseling produced favorable anthropometric and body composition alterations in both groups of patients including lower-body mass, BMI, waist circumference, WHR, and fat mass. The magnitude of change in these variables was comparable between the 2 cohorts. Consequently, anthropometric features and body composition responses to training do not seem to depend on existing diabetes duration. As was the case with the analytes for glucose control, however, such a conclusion may not apply in reference to patients with longer disease duration. For instance, a recent examination of training in patients with a history of diabetes exceeding the present work found no significant improvement in body weight, BMI, or percent body fat (36). This suggests that at disease durations exceeding our long-standing diabetes cohort (>10 years), anthropometrics and body composition (fat mass in particular) may be less responsive to repeated exercise stimuli. Age may be a potential confounding factor in this instance because patients with a diabetes duration that approaches 20 years are also likely to be older. However, Short et al. (34) did not find exercise-induced reductions in abdominal fat to be affected by age. Consequently, it would be reasonable to suggest that in the aforementioned work, diabetes duration rather than age explained the lack of training-induced fat loss. Finally, the lack of training-induced hypertrophy suggests that our method of overload was not sufficient to stimulate muscle anabolism; therefore, a greater degree of overload may be required to achieve skeletal muscle growth.
In contrast to the changes in blood markers, anthropometrics, and body composition, several indices of physical function and work capacity responded in a diabetes duration–dependent manner. Improvements in grip strength, for example, were demonstrated by changes in patients with long-standing diabetes rather than in those with shorter-duration disease. Likewise, the greater peak exercise time and load attained during the graded exercise test was demonstrated by increases in patients with a longer history of diabetes. This points to some degree of enhanced functionality and tolerance to an exercise stressor exclusively in those with a longer disease duration. The same interpretation may also be applied to workload at VT, where significant pre-to-post increases occurred in both the groups; however, because patients with long-standing diabetes were significantly lower at baseline but not different at posttraining, the greater degree of gain occurred in those with a longer disease duration. A possible explanation is that because those with long-standing diabetes had lower (or tendencies for lower) baseline values in these measurements (excluding peak exercise time), they stood to show greater responsiveness to training.
Despite these select instances, performance in the majority of functional tests improved in both patient cohorts after resistance training combined with dietary advice, as evidenced by gains in maximum bicep curl, curl-up, and sit and stand repetitions, as well as increased sit-and-reach flexibility and faster mobility (8-foot up and go time). Also, VT and time at VT also increased as a result of training in patients with both short-duration and long-standing diabetes. Collectively, our data suggest that both patient cohorts are likely to display improved function and work capacity that, in turn, likely translates into activities of daily living being performed with greater ease. It should also be mentioned that the training-induced improvement in function occurred without significant muscle hypertrophy. This suggests that neural factors rather than muscle mass accounted for the beneficial adaptations (26).
Interestingly, both groups displayed significantly greater resting heart rate after the intervention. Such findings are surprising as combined aerobic and resistance training has been shown to reduce resting heart rate (23), whereas resistance training alone had no effect on resting heart rate (41). A possible explanation could be that the participants may have been in an overreaching state. Elevated resting heart rate is recognized as an indicator of overreaching; although we do not have additional evidence to support this assertion (e.g., cortisol), this is certainly feasible considering the high daily and weekly frequency training program implemented combined with possible energy restriction because patients were instructed to limit their consumption of carbohydrates. Fortunately, overall physical function improved as a result of training; therefore, the participants may have been in the early stages of an overreaching state where widespread performance decrements did not yet manifest. In considering the potential for overtraining with this particular intervention, strategic recovery periods should be integrated in the future.
Regarding maximal hemodynamic responses during the graded exercise test, significant main effects of training on systolic and diastolic blood pressure were observed. The training effects of lower systolic (−5%) and diastolic (−5%) blood pressure were seen in patients with long-standing diabetes, which is suggestive of less exertion experienced by this specific cohort in response to an exercise stressor. The idea of reduced exertion during the incremental exercise test is consistent with the enhancement of performance seen in the various field tests (e.g., maximum sit-and-stand and curl-up repetitions). Interestingly, there were no changes in postexercise heart rate or systolic blood pressure as a result of training in either group, although patients with long-standing diabetes exhibited significantly lower postexercise diastolic blood pressure. The lack of an effect on postexercise heart rate was unexpected because a faster return to normal cardiovascular function (i.e., recovery) is a documented training adaptation (41). Despite using a circuit-style program, our exercise stimulus may not have been sufficient to generate robust cardiovascular adaptations. The lack of improvement in absolute or relative V[Combining Dot Above]O2max in both the groups provides some degree of support for this argument.
Although different exercise protocols were not evaluated in the current study, the training program prescribed merits further discussion. Two sessions were performed daily at a high weekly frequency (5 d·wk−1) for a total of 12 weeks. With this prescription, both groups of patients showed impressive reductions in HbA1c (short: −13%; p = 0.01; long: −18%; p = 0.001). In comparison, a previous study reported that an identical duration (i.e., 3 months) of aerobic and resistance training performed once per day at a frequency of 3 d·wk−1 lowered HbA1c by 6% and 2%, respectively (35). Values for HbA1c are significant as they represent the mean plasma glucose concentration from the previous 2 to 3 months (35). Sigal et al. (35) highlight the importance of HbA1c by pointing out that even very small decreases in HbA1c (i.e., 1%) are linked with significant reductions in the incidence of cardiovascular (−15 to −20%) and microvascular (−37%) complications. In consideration of these associations, the magnitude of reduction exhibited by both of our patient groups points to the highly beneficial impact of a circuit-style training program consisting of multiple daily sessions at a high weekly frequency.
It is conceivable that diabetes durations exceeding that in the present study could result in a differential training response. For instance, patients with diabetes duration of 16.7 ± 6.7 years demonstrated smaller training-dependent improvements in glycemic control variables compared with the present investigation (36). Therefore, an approach that uses a wider range of diabetes duration along with age as a covariate may be warranted in future research. Additionally, although patients were provided with regular nutritional education and counseling, formal analysis of dietary intake was not performed. This information would be of value because it indicates whether our subjects were in an energy restricted state, which in turn provides insight on the contribution of exercise training vs. diet to outcomes such as weight loss. Previous work has shown that resistance training without energy restriction resulted in weight loss of approximately 1 kg, whereas resistance training under energy restriction led to weight loss of 10–14 kg (35,45). In comparison, patients in the present study demonstrated weight loss in the region of 4–5 kg. When considering that our patients were instructed to limit carbohydrate intake, it is possible that training may have occurred under mild energy restriction and that our findings resulted from diet in addition to exercise rather than exercise alone.
We did not observe convincing evidence pointing to differential training adaptations as a function of diabetes duration, although a few exceptions were noted (primarily physical function and exercise capacity) in which patients with a longer history of diabetes demonstrated greater responsiveness. In consideration of these outcomes, long-standing disease should not deter patients with type-2 diabetes from beginning exercise training and dietary counseling. We encourage previously inactive patients with either short-duration or long-standing type-2 diabetes and no contraindications to begin structured resistance training with appropriate dietary modifications. Assuming time permits, a training program consisting of low-resistive overload and twice daily bouts at a high weekly frequency (5 d·wk−1) could possibly produce greater improvements in glucose control compared with a single daily bout irrespective of type-2 diabetes duration.
Bong-Sup Park and Andy V. Khamoui contributed equally.
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