Delineating the Role of Inter-Repetition Interval in the Relationship between Maximum Repetitions to Failure or Repetitions in Reserve and Movement Velocity.

Background: Maximum repetitions to failure (RTF) and repetitions in reserve (RIR) can be estimated through fastest mean velocity (MV_fastest) and mean velocity (MV), respectively. However, the impact of inter-repetition intervals (IRI) on these relationships in free-weight back squat and bench press exercises is unclear.

Hypothesis: The IRI would affect RTF-MV_fastest and RIR-MV relationships, with a higher goodness-of-fit using self-selected IRI (SSIRI) compared with 0 seconds (IRI0) and 3 seconds (IRI3).

Study design: Crossover study design.

Level of evidence: Level 3.

Methods: Eighteen male participants completed 1 session per IRI configuration, consisting of 3 single sets of RTF (65%-75%-85% of the 1-repetition maximum) during the free-weight back squat and bench press exercises.

Results: Individualized RTF-MV_fastest and RIR-MV relationships were stronger than generalized (median R² = 0.98 vs 0.65 and 0.84 vs 0.40, respectively). The goodness-of-fit of the relationships was stronger for SSIRI than for IRI0 during back squat (P < .01) and comparable between IRIs during bench press (P ≥ .28). During back squat, MV_fastest values were higher for IRI0 than for IRI3 and SSIRI (eighth-fifteenth repetitions; P ≤ .07), whereas during the bench press, they were higher for IRI0 than for IRI3 (eleventh-fifteenth repetitions; P ≥ .28). Overall, MV values associated with each RIR were higher for IRI0 than for SSIRI (10 out of 18 comparisons) during back squat, and for IRI0 than for IRI3 and SSIRI (16 and 14 out of 18 comparisons) during bench press.

Conclusion: These results highlight the importance of standardizing the IRI during set-to-failure to establish RTF-MV_fastest and RIR-MV relationships, with SSIRI recommended as a more accurate and effective procedure.

Clinical relevance: This information may provide practitioners with a valuable tool to objectively quantify the level of effort being exerted during resistance training sets by measuring movement velocity in free-weight exercises.