Effect of a HIIT protocol on the lower limb muscle power, ankle dorsiflexion and dynamic balance in a sedentary type 1 diabetes mellitus population: a pilot study

Participants and research design

We recruited 19 inactive and clinically diagnosed as T1DM (10 males and 9 females) from the Valencian Diabetes Association (VDA) and social media announcement. Baseline characteristics of the sample are presented in Table 1. The following inclusion criteria were adopted: (1) aged 18–45 years, (2) duration of T1DM > 4 years, (3) HbA1C < 10% (4) no structured exercise training programs in the previous 6 months, (5) no known comorbidities not related to diabetes. In an a priori analysis of the required sample size (G*Power V.3.1.9.6), we needed 12 subjects per group, we had 11 participants in the experimental group and eight in the control group that is why we have titled the article as a pilot study.

Subjects excluded from the study include those who smoke regularly, take any medication that affects heart rate and those who had major surgery planned. Participants were informed of the purposes and risks involved in the study before giving their informed written consent to participate. Furthermore, they completed two questionnaires before the beginning of the measurement protocols: the PAR-Q to assess participants’ level of risk to safely participate and the IPAQ (short version), to ensure the previous sedentary behavior of the subjects. The study procedures were in accordance with the principles of the Declaration of Helsinki and were approved by the Institutional Review Board of the University of Valencia which granted its approval to carry out the study within its facilities, IRB code: H1421157445503.

This is a randomized experimental, parallel design, open-label trial. The eligible subjects were randomly allocated (www.randomizer.org) to the experimental (n = 11, 38 ± 5.5 years, 5 men and 6 women, height 1.68 ± 0.09 m, body mass 70.5 ± 7.4 kg and 20.5 ± 8.4 years diagnosed) or control group (n = 8, 35 ± 8.2 years, 4 men and 4 women, height 1.69 ± 0.07 m, body mass 72.05 ± 5.0 kg and 21.1 ± 6.5 years diagnosed), and stratified/classified by gender to ensure a balanced number of men and women in each group. They were instructed not to change their nutritional habits and not to perform any regular exercise program outside of the study, which were not supervised. Initially, the control group had 10 participants but there were two drops out, a man and a woman, because of illness and pregnancy, respectively.

Testing sessions

Firstly, all the participants performed an incremental test on a cycle ergometer (Excite Unity 3.0; Technogym S.p.A, Cesena, Italia) to determine peak power output (PPO) and peak oxygen consumption (VO_2peak) using a gas collection system (PNOE, Athens, Greece) that was calibrated in each test by means of ambient air. The PNOE system has proven its validity and reliability (Tsekouras et al., 2019). Before starting the test, capillary blood glucose concentrations were checked by their own blood glucose monitoring devices. They were told to arrive at the institutional gym with a glycemic level >100 mg/dl and less than 300 mg/dl in absence of ketones. If the glycemia was correct, the participant began the test normally. If not, the intake of 15–30 g of fast-acting carbohydrates (CHO) we had available was compulsory when glycemia was <100 mg/dl and a small corrective insulin dose was used if hyperglycemia occurred without ketones. In the presence of ketones the exercise was canceled. Glycemia was checked again until the level of blood glucose was optimum to start the test. In the same way, it was recommended that patients not exercise at the peak of insulin action (Scott et al., 2019).

The test consisted in a warm-up of 5 min at 40 Watts (W). After that, the workload was increased by 20 W every minute until exhaustion. Participants were verbally encouraged to give their maximum effort during the exercises. The test ends with a cool down of 5 min at 40 W. Heart rate was continuously monitored by a Polar H10 (Polar Electro, Kempele, Finland). VO_2peak was taken as the highest mean achieved within the last 15 s prior to exhaustion. Peak power output was registered to individualize the workloads in the experimental period training.

The hour of the day that each subject completed the test was recorded, as well as the menstrual phase of each female participant with the aim of repeating the same conditions in the second measurement to prevent their influence on the outcomes.

A total of 48 h after incremental testing, all the participants performed familiarization sessions to learn the correct technique to squat, as needed. This session consisted of the proper execution of the squat in a Smith machine and a reproduced experimental condition was taught, all monitored by the main researcher who is a strength and conditioning specialist. A week after the first test, the participants returned to the laboratory to perform three functional tests to measure lower limb strength, ankle dorsiflexion and dynamic balance: execution velocity performing squats with three different overloads; Weight Bearing Lunge Test (WBLT) and Y-Balance Test (YBT). The order of measurements was randomly counterbalanced to avoid any influence between them. Ten minutes of rest was set between tests to ensure the absence of all carry over effects.

Lower limb muscle power was measured by the execution velocity in the squat movement conducted in a Smith machine with no counterweight mechanism (Technogym S.p.A, Cesena, Italia) with overloads of 50%, 60% and 70% of the own body mass of each participant. The device selected to automatically calculate kinematic parameters of every repetition was an Encoder Speed4lift (Speed4lift S.L, Madrid, Spain). The encoder has proven its validity and reliability (Pérez-Castilla et al., 2019). After watching a standard video demonstration all the participants began a standard warm-up consisting of joint mobility, dynamic flexibility squats and lunges. After that, participants were required to perform three squats repetitions with each overload, conducting the concentric phase at maximal intended velocity and eccentric phase at controlled velocity. Subjects were instructed to perform parallel squats: descend until the inguinal crease was in projection with the top of the knee. Verbal and visual feedback was provided real-time to ensure the correct execution of the movement (Pallarés et al., 2020). The three sets were separated by 5-min rests to avoid the possible fatigue effect. Participants completed the test barefoot to avoid footwear influencing in the squat velocity. The data recorded and subsequently statistically analyzed were those corresponding to the highest execution velocity of the three repetitions in the concentric phase of the squat of each overload measured.

The WBLT was performed to determine the ankle joint dorsiflexion range of motion. A tape line was placed on the floor perpendicular to the wall, where a vertical line was taped. Participants placed both hands on the wall in front of them and then aligned the center of the heel and the second toe of the foot that was being tested over the tape line. WBLT was conducted with the subjects barefoot to eliminate any influence of the footwear. Participants were asked to lunge forward trying to touch the vertical line on the wall with their knee without lifting the heel of the tested foot off the ground, but no encouragement was provided during the testing. Touching the vertical line perpendicular to the floor with the knee among the line formed by the heel and the second toe, helped to control subtalar joint movement and standardized the test between participants. The contralateral limb was positioned behind the testing limb in a comfortable position. The participants were allowed to perform three practice trials before the three test trials to achieve the farthest distance from the wall to the first toe, with 30-s rests between tests. The measurement was taken between the big toe and the wall, to the nearest 0.1 cm using a standard tape measure secured to the floor. The average of the three trials scores was documented as the test result. Both feet were measured (Hall & Docherty, 2017; Langarika-Rocafort et al., 2017; Powden, Hoch & Hoch, 2015; Searle, Spink & Chuter, 2018).

Finally, the YBT was conducted using a reliable standardized protocol. A “Y” was marked with tape on the lab floor to measure the dynamic balance in the Anterior (A), Posterolateral (PL) and Posteromedial (PM) directions. The posterior lines were marked 135° from A line, with 90° between them. Before testing, the participants performed 10 min of standardized warm-up, with 5 min of submaximal cycling followed by a dynamic stretch routine consisting of functional exercises: front-to back leg swing, side-to-side leg swing, lateral lunge, and sumo squat to stand (Benis, Bonato & La Torre, 2016), the participants were allowed to practice six times with each leg in each direction to minimize the learning effect. Afterwards, participants were asked to stand barefoot on one leg with the midfoot positioned over the central point and to reach with the contralateral leg as far as possible while hands were placed on the wing of the ilium. The reach distance was measured by marking the tape measure with erasable ink at the point where the most distant part of the foot reached. The trial was discarded and repeated if the subject (1) made a heavy touch, (2) rested the foot on the ground, (3) lost balance, or (4) failed to return to the starting position in a controlled manner. The process was repeated while standing on the other leg. The testing order was three trials standing on the right foot while reaching with the left foot in the A direction, followed by three trials standing on the left foot and reaching with the right foot in the A direction. The procedure was repeated for the PM and then the PL-reach directions. This order was proposed to avoid fatigue. The YBT scores were analyzed using the average of the last three trials for each reach direction for each lower extremity. Those values were normalized to the height of each participant which was measured with tallimeter (Seca™ 709, Hamburg, Germany) (Benis, Bonato & La Torre, 2016; Gribble & Hertel, 2003; Linek et al., 2017; Plisky et al., 2009; Shah et al., 2017).

All tests were performed under similar environmental conditions (22 ± 1 °C, 40–60% humidity). The same test protocols were performed in exactly the same way after the experimental period by both control and HIIT groups.

Training protocol

Training started the following week after completion of the pre-experimental procedures. Participants of the experimental group trained three times per week for 6 weeks under researcher supervision on a cycle ergometer (Excite Unity 3.0; Technogym S.p.A, Cesena, Italy). Heart rate while exercising was monitored with a Polar H10 (Polar Electro, Kempele, Finland) that was preconfigured with their heart rate zones. HIIT was a 1:2 protocol, which means that the high-intensity intervals lasted exactly half the time that the rest intervals did. The saddle height was always adjusted to the height of the subject’s iliac crest. The training began with a 5-min warm-up at 50 W. Then, they performed repeated 30-s bouts of high-intensity cycling at a workload selected to elicit 85% of their individual PPO interspersed with 1 min of recovery at 40% PPO. The number of high-intensity intervals increased from twelve reps in weeks 1 and 2, to 16 reps in weeks 3 and 4, to 20 reps in weeks 5 and 6. Training ended with a 5-min cool down performed at 50 W. All sessions were supervised by, at least, a researcher. After the session, participants were told to check their glycemia level frequently and notify the investigators if a glycemia drop below 70 mg/dl occurred during the 24 h following the exercise (Riddell et al., 2017).

All sessions were supervised by the investigators and in order to reflect a real-world settings, researchers did not give advice about decreasing fast-acting insulin dosage or increasing carbohydrate consumption prior to each exercise session. Volunteers were only asked to arrive with glycemia <100 mg/dl (Farinha et al., 2019). Glucose levels were checked at least before and immediately after each exercise session, it was re-checked when glucose was not in the safety range. Fast-acting carbohydrates (15–30 g) were ingested when glycemia fell to ≤100 mg/dl. Hyperglycemia (250–300 mg/dl) was not set as a reason for postponing exercise if the patient felt well and ketones were negative (Farinha et al., 2018; Scott et al., 2019).

Statistical analysis

All variables were expressed as a mean and standard deviation (M ± SD) and were analyzed using a statistical package (SPSS Inc., Chicago, IL, USA). Normality assumption by Shapiro–Wilks was identified for each variable. A mixed factorial ANOVA (2 × 2) was performed to assess the influence of “condition” (i.e., control group vs. experimental group) and “time moment” variable (i.e., pre-intervention, post-intervention) over Lower limb muscle power (LLS), Y-balance test (YBT), and Weight bearing lunge test (WBLT). In the event that Sphericity assumption was not met, freedom degrees were corrected using Greenhouse-Geisser estimation. Post Hoc analysis was corrected using Bonferroni adjustment. Hedges’ G and the associated CI were used to assess the magnitude of mean differences between control vs. experimental conditions. Significant differences were established at p < 0.05.

Lower limbs muscle power

Lower limb Muscle Power experienced significant changes after the HIIT training period. With 50% of their body mass as additional load, the participants in the experimental group increased their speed of execution in squatting by 10.1%, the result of the mixed factorial ANOVA was F(1,17) = 13.63, p = 0.02. For 60% of the mass, by 9.4%, the result of the mixed factorial ANOVA was F(1,17) = 13.56, p = 0.02 and for 70%, there was an improvement of 10.1%, the result of the mixed factorial ANOVA was F(1,17) = 20.21, p = 0.00. In the control group, there were no significant changes between pre and post intervention assessment.

Y-balance test

In the dynamic equilibrium, measured by the YBT, there were changes between both assessments in the experimental group (pre-post intervention). For the right leg, there was a significant increase (p < 0.05) of 4.2%, the result of the mixed factorial ANOVA was F(1,17) = 7.1, p = 0.02, and 3.4%, the result of the mixed factorial ANOVA was F(1,17) = 4.73, p = 0.04, in the anterior and posterolateral direction respectively. In contrast, in the posteromedial direction there was no significant change for this leg (1.9%). On the other hand, in the left leg, in the anterior direction there was an improvement of 6.1%, the result of the mixed factorial ANOVA was F(1,17) = 13.6, p = 0.02, in the posterolateral one there was an improvement of 4.5%, the result of the mixed factorial ANOVA was F(1,17) = 9.1, p = 0.01, and in the posteromedial one of 5.2%, the result of the mixed factorial ANOVA was F(1,17) = 16.17, p = 0.01. The control group experienced no change between the two assessments (p > 0.05). Data are presented in Table 2.

Weight bearing lunge test

Ankle dorsiflexion improved by 15.4% in the left foot of the participants, the result of the mixed factorial ANOVA was F(1,17) = 11.33, p = 0.04, and by 11.3% in the right foot in the experimental group (pre-post intervention), the result of the mixed factorial ANOVA was F(1,17) = 19.67, p = 0.00. The control group experienced no change between the two assessments. Data are presented in Table 3.