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

Background and Rationale:

Altitude training is widely employed by elite middle- and long-distance runners to stimulate key physiological adaptations, including erythropoietic upregulation, expansion of oxygen transport capacity, mitochondrial biogenesis, and skeletal muscle capillarization. These adaptations are considered central mechanisms through which hypoxic exposure enhances endurance performance. Despite the growing adoption of both running-based high-intensity interval training (HIIT) and multimodal CrossFit (CF) as primary high-intensity training modalities in elite endurance sport, no study has directly compared their respective physiological, neuromuscular, performance, and biomarker effects in runners residing and training at moderate altitude. Moreover, the extent to which event specialization (middle-distance: 800-3,000 m vs. long-distance: ≥5,000 m) moderates modality-specific training adaptations remains unexamined. This study is designed to address these gaps by providing the first direct empirical comparison of these two high-intensity training modalities in elite altitude-resident runners, stratified by competitive event specialization.

Study Design:

This is a six-week stratified quasi-experimental study employing a 2×2 factorial design with pre- and post-intervention assessments, conducted during the general preparatory phase of the competitive season at moderate altitude (approximately 1,400 m above sea level) in Zahedan, Iran. The two independent factors are training modality (HIIT vs. CF) and event specialization (middle-distance vs. long-distance). The study protocol has received ethical approval from the Ethics Committee of the University of Sistan and Baluchestan (Approval No. IR.USB.REC.1400.021) and will be conducted in full accordance with the Declaration of Helsinki and its subsequent amendments. Written informed consent will be obtained from all participants prior to any study-related procedures.

Randomization and Allocation Concealment:

Eligible participants will be stratified by primary event specialization and randomly allocated to one of two training modalities using computer-generated variable block randomization with block sizes of four and eight, conducted by an independent investigator using R software (version 4.2.1; R Foundation for Statistical Computing, Vienna, Austria). Allocation concealment will be maintained through the use of sequentially numbered, opaque, sealed envelopes, which will be opened only after the completion of all baseline assessments. Baseline equivalence across groups will be confirmed using propensity score matching prior to the commencement of the intervention.

Training Interventions:

All participants will complete five fully supervised training sessions per week for six weeks, with each session lasting 60-75 minutes. All sessions will be conducted at moderate altitude (approximately 1,400 m) on a standard 400-m synthetic track. Exercise intensity will be continuously monitored throughout all sessions using Polar H10 chest-strap heart rate sensors (Polar Electro Oy, Kempele, Finland), with real-time data transmitted to the coaching staff via the Polar Team application.

Intervention 1 - Running-Based High-Intensity Interval Training (HIIT):

Each HIIT session will consist of 4-6 work 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 will self-select their running pace to maintain heart rate within the prescribed target zone throughout each work bout.

Intervention 2 - Multimodal CrossFit Training (CF):

Each CF session will follow a structured four-part format:

Dynamic warm-up incorporating mobility exercises and movement preparation (10-15 minutes) Strength and skill development targeting foundational lifting patterns and gymnastic movements (15-20 minutes) High-intensity Workout of the Day (WOD) maintained at 85-95% HRmax (10-20 minutes) Cool-down consisting of static stretching and controlled breathing exercises (5-10 minutes) WODs will be selected from the official CrossFit programming archive (2021-2023) and individually scaled by a Level 2 CrossFit-certified coach to ensure appropriate and safe loading for each participant. Loading for barbell movements will be prescribed at 30-85% of each participant's baseline one-repetition maximum (1RM) values. Bodyweight and kettlebell movements will be regulated by repetition targets and ratings of perceived exertion (RPE) of 14-18 on the Borg 6-20 scale. Progressive overload will be systematically applied biweekly through incremental increases of 5-10% in load, repetitions, and movement complexity.

Blinding:

Given the inherently observable nature of the training interventions, participant blinding is not feasible in this study. However, all outcome assessors and data analysts will remain masked to group assignment throughout the study period to ensure objective and unbiased evaluation of all outcome measures.

Dietary and Environmental Controls:

All participants will adhere to a standardized nutritional plan of approximately 2,800 ± 150 kcal·day-¹ with a macronutrient distribution of approximately 55% carbohydrates, 25% protein, and 20% fat, supervised and monitored by a registered sports nutritionist. Dietary intake will be assessed using validated three-day food logs at baseline, week 3, and week 6, and records will be analyzed using Nutritionist Pro software (version 7.0; Axxya Systems LLC, Redmond, WA, USA). The introduction of new nutritional supplements will be prohibited throughout the study period, and dietary compliance will be verified weekly via structured interviews. Caffeine consumption will be withheld for a minimum of 24 hours prior to all testing sessions.

All training and testing sessions will be conducted under standardized and monitored environmental conditions (ambient temperature: 25-30°C; relative humidity: 40-50%; wind speed: <2 m·s-¹), measured using a handheld anemometer (Kestrel 3000; Nielsen-Kellerman Co., Boothwyn, PA, USA) prior to each outdoor session. Sleep duration will be maintained at a minimum of seven hours per night, and subjective recovery status will be monitored daily using a validated five-point Likert scale (1 = very poor; 5 = very good).

Statistical Analysis Plan:

Linear mixed-effects models (LMMs) will serve as the primary analytical framework, implemented using the lme4 package in R (version 4.2.1). Models will incorporate fixed effects for group, time, and group × time interaction, with random intercepts specified for individual participants to account for within-subject correlation. Model selection will be guided by the Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC), with emphasis on model parsimony. Residual diagnostics will be performed to validate the assumptions of normality, homoscedasticity, and independence.

Between-group differences in change scores will be further examined using analysis of covariance (ANCOVA), adjusting for age, baseline values, and training history. Effect sizes will be quantified using Cohen's d and interpreted as small (0.2), moderate (0.5), or large (0.8). All observed changes will be evaluated against predefined minimal clinically important difference (MCID) thresholds to facilitate meaningful clinical interpretation. The significance level will be set at α = 0.05 for all statistical comparisons.

Sample size was determined a priori using G*Power (version 3.1; Heinrich-Heine-Universität Düsseldorf, Germany). Calculations indicated that a minimum of nine participants per group would provide greater than 80% statistical power to detect a moderate-to-large effect size in the primary outcome of maximal oxygen uptake (VO₂max; α = 0.05; Cohen's d = 0.6; intraclass correlation coefficient r = 0.70).