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 (L0, v0, and Aline (area under the L–V line; Aline = L0×v0/ 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), L0 (p = 0.001) and Aline (p < 0.001) were the greatest after the high fatigue protocol, followed by the moderate fatigue protocol and finally the control protocol. Changes in v₀ 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 Aline (r = 0.832) and L0 (r = 0.764), but not with the percentage change in v0 (r= -0.012). CONCLUSIONS: These results suggest that L-V relationship variables offer a highly sensitive and practical solution for fatigue monitoring. Trial registration: ClinicalTrials.gov, NCT07307963 (First posted: 27/11/2025; retrospectively registered).