Resumo. BACKGROUND: The capacity to produce force should be promoted especially for the fastest swimming events. AIM: This study aimed to analyze the effects of different test periods in mean, maximal and coefficient of force variation and to correlate the coefficient of force variation and 50m front crawl time. METHOD: Twelve well trained swimmers (age: 22.33±8.08 years, 1.69±0.48 m height, body mass 56.08±7.86 kg) randomly performed three maximal 30s repetitions of full swimming and lower limbs actions in front crawl swimming technique with ~15min resting period. A load cell system permitted the continuous measurement of exerted tethered forces, and 50m time was registered as the best time obtained at 12 months before the test. RESULTS: Mean swimming force was greater at 10s compared to 20 and 30s period (26.40 [20.66-54.20], 25.33 [21.70-52.35] and 21.39 [17.91-43.98] respectively, p<0.05). Mean, maximal and force coefficient variation at tethered test with full swimming were higher than lower limbs (24.37 vs. 12.71, 77.97 vs. 39.78, 0.77 vs. 0.49 respectively, p<0.05). Strong and inverse correlations were noticed between coefficient of force variation at 10 s during full swimming, at 20s during kicking and performance (28.75s [26.20-30.68s];r=-0.75;p<0.01). Very strong correlation was noticed between coefficient of force variation during swimming at 20s, 30s and performance (r=-0.91 and -0.92;p<0.0001). CONCLUSION: Mean swimming force at full swim and lower limbs is affected by the period of the 30s maximal tethered swimming test and is associated with performance.Data reported may be used as reference for setting training strategies at short distance events.