Event-Specific Adaptations to CrossFit and Running-Based HIIT in Elite Distance Runners at Moderate Altitude

NCT07675577 | Completed

• 1 other identifier: IR.USB.REC.1400.021
• Study Type: interventional
• Target: 36
• Locations: 1 country
• Sites: 1

Brief Summary

The goal of this clinical trial is to compare the effects of two high-intensity training programs on physical performance and physiological adaptations in elite male middle- and long-distance runners aged 25 to 35 years who live and train at moderate altitude (approximately 1,400 meters above sea level). The main questions it aims to answer are: Does running-based high-intensity interval training improve aerobic fitness and race performance in elite distance runners? Does CrossFit training improve muscle strength, power, and physiological markers in elite distance runners? Researchers will compare running-based high-intensity interval training (HIIT) with CrossFit training to determine how each program affects aerobic capacity, strength, running performance, and selected blood markers. Participants will: Be randomly assigned to either a running-based HIIT program or a CrossFit training program Train five days per week for six weeks under supervision Complete exercise testing before and after the training period, including treadmill fitness testing and timed running trials Provide blood samples before and after the intervention to measure selected physiological markers

Conditions

Altitude Training Adaptation; Aerobic Capacity; Adaptation Reaction; Athletic Performance; Physical Endurance; Neuromuscular Performance; Exercise-Induced Inflammation

Keywords

CrossFit; High-Intensity Interval Training; HIIT; Distance Running; Elite Runners; Moderate Altitude; VO₂max; Lactate Threshold; Peak Power Output; Erythropoietin; Hemoglobin; Creatine Kinase; Interleukin-6; TNF-alpha; Middle-Distance Running; Long-Distance Running; Altitude Training; Endurance Performance; Neuromuscular Adaptation; Exercise Physiology

Outcome Measures

Primary Outcomes (4)

• Lactate Threshold (LT)

  Lactate threshold determined from a graded treadmill test using capillary blood lactate sampling and the modified Dmax method.

  Baseline and immediately after 6 weeks of intervention

• Maximal Oxygen Uptake (VO₂max)

  VO₂max assessed by automated breath-by-breath cardiopulmonary exercise testing during an incremental running protocol to volitional exhaustion.

  Baseline and immediately after 6 weeks of intervention

• 1,500-Meter Time Trial Performance

  Time to complete a 1,500-meter maximal effort run on a standardized 400-meter synthetic track, recorded using an infrared timing system.

  Baseline and immediately after 6 weeks of intervention

• 5,000-Meter Time Trial Performance

  Time to complete a 5,000-meter maximal effort run on a standardized 400-meter synthetic track, recorded using an infrared timing system.

  Baseline and immediately after 6 weeks of intervention

Secondary Outcomes (7)

• Peak Power Output (PPO)

  Baseline and immediately after 6 weeks of intervention

• Deadlift One-Repetition Maximum (1RM)

  Baseline and immediately after 6 weeks of intervention

• Hemoglobin Concentration

  Baseline and immediately after 6 weeks of intervention

• Erythropoietin (EPO) Concentration

  Baseline and immediately after 6 weeks of intervention

• Creatine Kinase (CK) Activity

  Baseline and immediately after 6 weeks of intervention

Other Outcomes (1)

• Back Squat One-Repetition Maximum (1RM)

  Baseline and immediately after 6 weeks of intervention

Study Arms (4)

Endurance-HIIT

EXPERIMENTAL

Elite male runners specializing in long-distance events (≥5,000 m) who performed running-based high-intensity interval training (HIIT) five days per week for six weeks at moderate altitude (approximately 1,400 m). Each session consisted of 4-6 intervals of 2-4 minutes at 90-95% HRmax, interspersed with 2-3 minutes of active recovery at approximately 50% HRmax.

Behavioral: Running-Based High-Intensity Interval Training (HIIT)

Endurance-CF

EXPERIMENTAL

Elite male runners specializing in long-distance events (≥5,000 m) who performed multimodal CrossFit-based high-intensity training five days per week for six weeks at moderate altitude (approximately 1,400 m). Each session included a dynamic warm-up, strength and skill work, a high-intensity Workout of the Day (WOD) at 85-95% HRmax, and a cool-down period.

Behavioral: CrossFit-Based High-Intensity Training (CF)

SemiEnd-HIIT

EXPERIMENTAL

Elite male runners specializing in middle-distance events (800-3,000 m) who performed running-based high-intensity interval training (HIIT) five days per week for six weeks at moderate altitude (approximately 1,400 m). Each session consisted of 4-6 intervals of 2-4 minutes at 90-95% HRmax, interspersed with 2-3 minutes of active recovery at approximately 50% HRmax.

Behavioral: Running-Based High-Intensity Interval Training (HIIT)

SemiEnd-CF

EXPERIMENTAL

Elite male runners specializing in middle-distance events (800-3,000 m) who performed multimodal CrossFit-based high-intensity training five days per week for six weeks at moderate altitude (approximately 1,400 m). Each session included a dynamic warm-up, strength and skill work, a high-intensity Workout of the Day (WOD) at 85-95% HRmax, and a cool-down period.

Behavioral: CrossFit-Based High-Intensity Training (CF)

Interventions

Running-Based High-Intensity Interval Training (HIIT) BEHAVIORAL

Participants performed five supervised HIIT sessions per week for six weeks at moderate altitude (approximately 1,400 m) in Zahedan, Iran. Each session lasted 60-75 minutes and was conducted on a standard 400-m synthetic track. Sessions consisted of 4-6 intervals of 2-4 minutes duration performed at 90-95% of maximum heart rate (HRmax), interspersed with 2-3 minutes of active recovery at approximately 50% HRmax. Participants self-selected their interval pace to maintain heart rate within the prescribed target zone throughout each work bout. Exercise intensity was continuously monitored using Polar H10 chest-strap heart rate sensors. Progressive overload was applied throughout the intervention period.

Also known as: Endurance-HIIT

Endurance-HIIT; SemiEnd-HIIT

CrossFit-Based High-Intensity Training (CF) BEHAVIORAL

Participants performed five supervised CrossFit sessions per week for six weeks at moderate altitude (approximately 1,400 m) in Zahedan, Iran. Each session lasted 60-75 minutes and followed a structured four-part format: (i) Dynamic warm-up incorporating mobility exercises and movement preparation (10-15 minutes); (ii) Strength and skill work targeting foundational lifting patterns and gymnastic movements (15-20 minutes); (iii) High-intensity Workout of the Day (WOD) at 85-95% HRmax (10-20 minutes); (iv) Cool-down consisting of static stretching and controlled breathing exercises (5-10 minutes). WODs were selected from the official CrossFit programming archive (2021-2023) and individually scaled by a Level 2 CrossFit-certified coach. Loading for barbell movements was prescribed at 30-85% of baseline 1RM values. Bodyweight and kettlebell movements were regulated by repetition targets and RPE scores of 14-18 on the Borg 6-20 scale. Progressive overload was applied biweekly through increa

Also known as: Endurance-CF

Endurance-CF; SemiEnd-CF

Eligibility Criteria

• Age: 25 Years - 35 Years
• Sex: male
• Healthy Volunteers: Yes
• Age Groups: Adult (18-64)

You may qualify if:

• Nationally ranked elite male middle- or long-distance runner
• Ranked within the national top 10 in their respective primary competitive event
• Aged between 25 and 35 years
• Minimum of six months of structured high-intensity training prior to enrollment, defined as more than five supervised sessions per week
• Living and training year-round at moderate altitude (approximately 1,400 m) in Zahedan, Iran
• Competing in one of the following event categories: middle-distance (800-3,000 m) or long-distance (≥5,000 m)
• Willing and able to provide written informed consent prior to participation
• Able to commit to five supervised training sessions per week for the full six-week intervention period

You may not qualify if:

• Musculoskeletal injury within the three months preceding enrollment
• Current or recent use of performance-enhancing substances
• Use of new nutritional supplements during the study period
• Any medical condition or physical limitation that could interfere with safe participation in high-intensity exercise training
• Inability to complete the required number of training sessions due to scheduling conflicts or other commitments

Sponsors & Collaborators

• University of Sistan and Baluchestan: lead

Study Sites (1)

Sport Sciences Department, University of Sistan and Baluchestan

Zahedan, Sistan and Baluchestan, 98135-986, Iran

Location

Study Officials

• Mohammadreza Rezaeipour

  University of Sistan and Baluchestan

  PRINCIPAL INVESTIGATOR

Study Design

• Study Type: interventional
• Phase: not applicable
• Allocation: RANDOMIZED
• Masking: SINGLE
• Who Masked: OUTCOMES ASSESSOR
• Masking Details: Although participants could not be blinded to their assigned training modality due to the inherently observable nature of the interventions, all data analysts and outcome assessors were masked to group assignment throughout the study to ensure objective evaluation of all physiological, performance, and biochemical outcomes.
• Purpose: BASIC SCIENCE
• Intervention Model: FACTORIAL
• Sponsor Type: OTHER
• Responsible Party: PRINCIPAL INVESTIGATOR
• PI Title: MD, PhD, Associate Professor at the Sport Sciences Department

Model Details: A 2×2 factorial design was used to evaluate the independent and combined effects of training modality (running-based HIIT vs. CrossFit) and event specialization (middle-distance: 800-3,000 m vs. long-distance: ≥5,000 m) on physiological, neuromuscular, performance, and biomarker outcomes in elite male runners training at moderate altitude (approximately 1,400 m) over six weeks.

Study Record Dates

• First Submitted: June 22, 2026
• First Posted: June 30, 2026
• Study Start: September 1, 2021
• Primary Completion: October 15, 2021
• Study Completion: October 30, 2021
• Last Updated: June 30, 2026

Record last verified: 2026-06

Data Sharing

• IPD Sharing: Will share
• Time Frame: Data will become available immediately following publication and will be accessible for a period of five years.
• Access Criteria: Data will be shared with researchers who provide a statistically sound proposal and sign a data access agreement. Requests should be directed to the corresponding author at rezaeipour@ped.usb.ac.ir.

Locations

IR (1)