Investigation of the effect of critical speed on maximal endurance running and achilles tendon stiffness in triathletes

Objective: This study aims to examine the effect of critical speed on 5000-meter running performance, changes in Achilles tendon stiffness, and the relationship between step parameters and running performance in triathletes. Method: The study included 20 male triathletes with an average age of 25.4 years and at least four years of experience in the triathlon discipline. Participants performed five separate maximal running tests with 48-hour rest intervals between each test. Critical speed values were calculated based on the results of the first three tests (1000 m, 1800 m, and 2600 m). In the fourth test, participants were asked to run 5000 meters at a maximal effort suited to their performance level, while in the fifth test, they were required to run at a speed close to their calculated critical speed. During the runs, the participants' speed, step length, step frequency, and ground contact times were measured using a smartwatch. Heart rate was monitored with a heart rate strap, and power data were recorded with a power meter. Additionally, Achilles tendon stiffness was analyzed using a Myoton device. Statistical analysis of the collected data was conducted using paired t-tests, Bland-Altman analysis, and regression models, all performed using Jamovi software. Results: The 5000-meter performance at the critical speed pace was found to provide greater energy efficiency and endurance compared to running at a self-selected pace. Significant increases were observed in step length and frequency (p<0.05), while ground contact time decreased. There was no significant difference in Achilles tendon stiffness between tests run at the critical speed pace and those at a self-selected pace (p>0.05). Furthermore, critical speed calculations strongly predicted 5000-meter performance with high accuracy (r=0.89, p<0.01). Conclusion: This study demonstrates that critical speed is an effective tool for enhancing performance and supporting Achilles tendon health in triathletes. Incorporating critical speed into training programs allows for improved endurance and optimized energy economy.