Seasonal Training Adaptations in Adolescent Swimmers: Effects of Equal Load 12-Week Programs Across Two Annual Cycles

Background: This study aimed to examine changes in force, impulse, and kinematic variables in relation to swimming performance during two equal load twelve-week training programs across two annual cycles. Methods: Eight national-level adolescent swimmers participated in two twelve-week training cycles across Year 1 and Year 2. The swimmers underwent performance tests (50 m, 200 m, 400 m), and stroke rate (SR), stroke length (SL), and stroke index (SI) were assessed. A 30 s tethered swimming test was applied to evaluate tethered force (TF) and impulse (IMP) before and after each training cycle. The training load was assessed during the twelve weeks across Year 1 and Year 2. Results: The training load was similar in Year 1 and Year 2 (d = 0.51–0.52, p = 0.09). The performance in 50, 200, and 400 m front crawl tests was improved following the 12-week training period in both years (d = 0.29–0.90, p = 0.01–0.04). The mean TF and maximal TF increased in Year 1 (d = 0.64, p = 0.01), while IMP was decreased in Year 2 (Year 1: +4.3 ± 6.8% vs. Year 2: −6.8 ± 4.5%, d = 1.28, p = 0.01). SR remained unchanged, but SL was increased in the 200 m (d = 0.19–0.41, p = 0.01), and SI improved in both 200 (d = 0.27–0.59, p = 0.01) and 400 m tests (d = 0.23–0.24, p = 0.01). Significant correlations were found in Year 1 between IMP and SR (r = −0.74, 95%CI: −0.08, −0.95, p = 0.01), and between IMP and both SL (r = 0.94, 95%CI: 0.98, 0.68, p = 0.01) and SI in the 400 m test (r = 0.87, 95%CI: 0.97, 0.44, p = 0.01). Conclusions: An equal load twelve-week swimming training period improved performance and kinematic parameters across Year 1 and Year 2. Changes in the tethered swimming force and impulse were observed only in the first year, highlighting the importance of targeted strength interventions for enhancing swimming performance.