Physical exercise is one of the main recommendations included in clinical guidelines for people with type 1 diabetes (T1D) because it is associated with substantially lower cardiovascular and overall mortality risks. Adherence to regular physical activity is low in the T1D population due to factors like time limitations, no access to appropriate equipment at home, or economic costs. High-intensity interval training (HIIT) has been proposed as a time-efficient methodology, consisting of sessions of short duration and high intensity exercises. An easier and cheaper training alternative for HIIT is exercising with elastic bands, either in a clinic or at home. Even so, training with elastic bands in T1D in combination with the HIIT method has not been compared to aerobic training. Besides, it is necessary to understand the glycemic impact of both kinds of exercise in order to build better artificial pancreas systems able to deal with exercise automatically. The aim of the study will be to determine the impact of aerobic exercise versus high-intensity interval training (HIIT) exercise on glycemic control.

This HIIT modality presents differences with respect to other studies since it consists of strength exercises with elastic bands. This novelty represents an advantage because it can be an easier training alternative to perform in a clinical or home setting, compared to previous studies where cycle ergometer is used. Thus, one session of each type of exercise will be performed.

Prior to the exercise sessions, an incremental exercise test will be performed to determine the working power in the cycle ergometer. At the beginning of the test, subjects will be sit quietly on the cycle ergometer for 3 min (0 W) before they will start the warm-up period of 3 min with cycling at a workload of 60 W. Then, the workload will increase by 40 W every 3 min until volitional exhaustion. Finally, 3 min active recovery at 60 W followed by 3 min passive recovery (0 W) will be conducted on the cycle ergometer. Lactate turn point 1 (LTP1) and maximum power (Pmax) will be determined in order to prescribe the exercise intensity. Additionally, patients will conduct a familiarization session with the strength exercises in order to adapt the participants to exercises with elastic bands. Thus, the subjects will perform two sets of twelve repetitions of the exercises.

The aerobic exercise session will begin with a 3 min resting period (sitting quietly on the cycle ergometer (0 W), followed by a 3 min warm-up at 60 W and a stepwise intensity increase by 20 W / min (to control for day-to-day variations in exercise response) until the 5% of Pmax from IET above PLTP1 is achieved (i.e. moderate steady state walking or low-intensity running or cycling). This target workload will be maintained for 30 min. Active and passive recovery periods of 3 min each will be the same as the incremental test.

The elastic band protocol is a modification of the HIIT protocols used by previous authors changing body weight exercises by exercises with TheraBand CLX. The exercise program is chosen to involve large muscle groups simulating conventional bodybuilding exercises by applying the external load to the elastic band's resistance. Four exercises of upper limb (bench press, seated dumbbell, biceps curl, and seated row) and four of lower limb (squats, stiff-legged deadlift, hamstring curl exercise, and quadriceps curl exercise) are chosen, which are intercalated in the program without doing two consecutive exercises of the same area. All exercises are done with both arms or legs at the same time.

Prior to HIIT session, participants will carry out a warm-up consisting of 3 minutes at 60 W on the cycle ergometer and 15 no-load shoulder flexo-extensions. Session consists of two set of exercise intervals of 20s separated by 10s of rest (i.e. 4min in total) with a rest of 3 minutes between both series, making at each interval a different exercise resisted by the elastic band. The participants have to choose a width in the elastic band grip that supposed a maximum effort for the 20s of each exercise. Subjects will be encouraged to perform as many repetitions per interval as possible while maintaining correct form.

Perceived exertion will be used to determine the correct intensity of HIIT sessions. Immediately following each set of eight intervals, subjects will report ratings of perceived exertion using the Thera-Band resistance exercise scale of perceived exertion with Thera-Band® resistance bands. HIIT sessions correspond to a "Maximal". Heart rate will be measured continuously via chest belt telemetry (Polar H10) during all tests and also averaged over 5s.

A sample size of n=40 will be chosen for each study.

Inclusion Criteria:

• age between 18-40 years
• T1D with a diabetes duration for more than 2 years
• glycated hemoglobin (HbA1c) < 8.5% (<69 mmol mol-1)
• stable insulin regimen in the past 6 months with less than 20% change in total insulin daily dose
• multiple daily injections
• weekly physical activity of 90 min or more, but no practicing any sport as amateur or professional

Exclusion Criteria:

• clinical conditions or use of medications (other than insulin) known to affect glycemic control (e.g., oral/parenteral steroids or metformin, among others).