The present study aims to compare the acute and training responses of (1) eccentric high intensity interval training (EI), (2)work-matched continuous eccentric training (EC), and (3) concentric high intensity interval training (CI), all performed on cycle ergometers. The variables of interest include ratings of perceived exertion (RPE), cognitive demand (Fat), heart rate (HR), maximal oxygen consumption (VO₂max), maximal aerobic power (MAP), and various functional and health-related parameters.

It is hypothesized that eccentric interval training will produce comparable or superior improvements in functional outcomes relative to concentric interval training, but at a lower metabolic and perceptual cost. Furthermore, eccentric interval training is expected to yield greater physiological benefits than continuous eccentric training for a similar perceived and metabolic load.

Forty-three sedentary healthy adults (23 men and 20 women) were recruited for this study based on predefined inclusion and exclusion criteria. Participants were randomly (stratified randomization) allocated into one of three training groups.

Participants attended the laboratory on 28 occasions over a 14-week period, with two sessions per week.

Participants in the EI and EC groups trained using an eccentric ergometer, while the CI group trained on a concentric ergometer. Baseline and post-intervention assessments were conducted during weeks 1 and 14, respectively. These included a maximal incremental cycling test to determine VO₂ peak and concentric MAP, followed by six functional performance assessments.

The present study aims to compare the acute and training responses of (1) eccentric HIIT, (2)work-matched continuous eccentric training, and (3) concentric HIIT, all performed on cycle ergometers. The variables of interest include ratings of perceived exertion (RPE), cognitive demand (Fat), heart rate (HR), maximal oxygen consumption (VO₂max), maximal aerobic power (MAP), and various functional and health-related parameters.

It is hypothesized that eccentric HIIT will produce comparable or superior improvements in functional outcomes relative to concentric HIIT, but at a lower metabolic and perceptual cost. Furthermore, eccentric HIIT is expected to yield greater physiological benefits than continuous eccentric training for a similar perceived and metabolic load.

Forty-three healthy adults (23 men and 20 women) were recruited for this study based on predefined inclusion and exclusion criteria. Participants were randomly (stratified randomization) allocated into one of three training groups:

• Eccentric Interval Group (EI): n = 15 (8 men, 7 women); age = 62.6 ± 3.9 years old (yo)
• Eccentric Continuous Group (EC): n = 15 (8 men, 7 women); age = 61.6 ± 6.3 yo
• Concentric Interval Group (CI): n = 13 (7 men, 6 women); age = 60.5 ± 5.1 yo All participants were unaccustomed to eccentric cycling and engaged in no more than 2 hours of recreational physical activity per week. Individuals with recent lower limb injuries or chronic medical conditions were excluded. Prior to participation, all individuals were fully informed of the study procedures, potential risks, and provided written informed consent. The study protocol received ethical approval from the Ethics Committee of the University of Liège (Ref. 2023/72), and all procedures conformed to the Declaration of Helsinki.

Participants in the EI and EC groups trained using an eccentric ergometer, while the CI group trained on a concentric ergometer. Baseline and post-intervention assessments were conducted during weeks 1 and 14, respectively. These included a maximal incremental cycling test to determine VO₂peak and concentric MAP (cMAP), followed by six functional performance assessments.

The training protocol consisted of three consecutive 4-week phases:

1. Familiarization Phase (Weeks 2-5)
2. Initial Training Phase (Weeks 6-9)
3. Progressive Training Phase (Weeks 10-13) with increased training intensity.

At the first and final visits, participants underwent a concentric incremental cycling test to determine VO₂peak and cMAP. The test began with a standardized 2-minute warm-up at 30 watts (W) and a cadence of 60 revolutions per minute (rpm). The workload was then increased every minute by 15 W for women and 20 W for men until volitional exhaustion.

Participants were instructed to maintain a cadence above 60 rpm and were verbally encouraged throughout the test.

The 12-week training intervention was structured into three consecutive phases: (1) familiarization, (2) initial training, and (3) progressive training. All participants completed two supervised sessions per week, either on a recumbent eccentric ergometer (Cyclus 2, RBM Elektronik-Automation GmbH, Germany) or a seated concentric cycle ergometer (Technogym, Italy), depending on group allocation.

1. Phase 1 (T1) - Familiarization (Weeks 2-5) During the familiarization phase, all groups completed work-matched sessions adapted to their respective contraction modality (eccentric or concentric cycling). Based on the findings of Lipski et al., a conversion ratio of 1.5 was used to estimate eccentric MAP (eMAP) from cMAP, such that 100% eMAP was considered equivalent to 150% of cMAP.

   Training intensity and duration were progressively increased across the four weeks. Participants began with 5 minutes of cycling at 30% of their cMAP and progressed to 30 minutes by the end of the phase.

2. Phase 2 (T2) - Initial Training (Weeks 6-9)

   In the second phase, all groups trained at an intensity equivalent to 80% of their modality-specific MAP for 30 minutes per session:

   • EI Group: 10 x 2' training/1' rest at 120% of cMAP (~80% of eMAP)
   • EC Group: 30' at 80% of cMAP
   • CI Group: 10 x 2' training/1' rest at 80% of cMAP

3. Phase 3 (T3) - Progressive Training (Weeks 10-13)

Training intensity was further increased to 90% of each group's modality-specific MAP:

• EI Group: 10 x 2' training/1' rest at 135% of cMAP (~90% of eMAP)
• EC Group: 30' at 90% of cMAP
• CI Group: 10 x 2' training/1' rest at 90% of cMAP This progressive overload aimed to enhance aerobic capacity and muscular adaptations in a contraction-specific manner. All sessions remained isocaloric and time-matched across groups to allow for valid comparisons.

Heart rate, rating of perceived exertion, cognitive demand / fatigue and muscle soreness were monitored during each training session.

A battery of six functional tests was performed pre- and post-intervention to evaluate intra-group improvements and inter-group differences. All tests were administered by the same trained evaluator to minimize inter-rater variability. Standardized instructions and verbal encouragement were provided to ensure consistency across participants. These tests include : maximal isometric force, handgrip strength, balance, ten times sit-to-stand time, timed up and go and six-minute walking test.

Inclusion Criteria:

• Sedentary people (less than 3 hours of physical activity a week)
• No smoker

Exclusion Criteria:

• Pain or injury in the 6 previous months in the lower limbes
• Presenting an uncontrolled chronic condition
• Presenting cardiovascular issues