Sensitivity of the load-velocity relationship variables to discriminate the level of fatigue induced by multiple sets of the smith-machine squat exercise

Background The aim of the research was to examine the sensitivity of load-velocity (L-V) relationship parameters ( L ₀ , v ₀ , and A _line (area under the L–V line; Aline =  L ₀ × v ₀ / 2) ) in detecting fatigue after different fatigue protocols as well as their correlation with changes in 1-repetition maximum (1RM). Methods After a familiarization and preliminary testing session which was used for 1RM smith-machine squat (SMS) determination and performing a set of repetition to failure with 70%1RM load, 28 resistance-trained men randomly performed three fatigue protocols. All fatigue protocols were carried out between two incremental loading tests, conducted at the beginning (pre-session) and end (post-session) of the training session. The characteristics of the fatigue protocols were as follows: (i) control protocol: no training, (ii) moderate-fatigue protocol: 5 sets of the SMS exercise at 70%1RM performing half the maximum possible number of repetitions, and (iii) high-fatigue protocol: 5 sets of the SMS exercise performed to failure against the 70%1RM. Results Post-session declines in 1RM ( p  < 0.001), L ₀ ( p  = 0.001) and A _line ( p  < 0.001) were the greatest after the high fatigue protocol, followed by the moderate fatigue protocol and finally the control protocol. Changes in v ₀ did not differentiate between the fatigue protocols ( p  = 0.325). The post-session percentage change in 1RM was significantly correlated with the percentage change in A _line ( r  = 0.832) and L ₀ ( r  = 0.764), but not with the percentage change in v ₀ ( r = -0.012). Conclusions These results suggest that L-V relationship variables offer a highly sensitive and practical solution for fatigue monitoring. Trial registration: Trial registration: ClinicalTrials.gov, NCT07307963 (First posted: 19 December 2025; retrospectively registered).