Feasibility of velocity loss-based volume prescription during constant-load and chain-based Smith machine back squats

This study evaluated the feasibility of percentage of velocity loss (%VL)-based approaches for prescribing training volume during constant-load and chain-based Smith machine back squats. Twenty-six resistance-trained men performed repetitions-to-failure tests at 85%, 75%, and 65% of their constant-load one-repetition maximum (1RM) under variable (VRT; 40% of the total external load provided by chains) and constant resistance training (CRT; 0% chains). The percentage of completed repetitions with respect to the maximum number of repetitions (%Rep) attained before reaching low (10% VL), moderate (20% VL), and high (30% VL) levels of effort were compared between VRT and CRT across various loads and sessions. Furthermore, we evaluated the influence of regression type (individual vs. general), load specificity (load-specific vs. multiple-load), and loading configuration (CRT vs. VRT) on the predictive accuracy of the %Rep – %VL relationships established in session 1 for estimating %Rep in session 2. No condition achieved acceptable predictive accuracy (absolute errors in %Rep ≤ 10%) at 65% 1RM (11.6%–24.6%) or 75% 1RM (10.7%–16.3%). At 85% 1RM, nine cases showed acceptable errors (7.5%–9.6%), but their 95% confidence interval upper bounds exceeded the 10% threshold (10.2%–13.3%). Agreement in %Rep attained at given %VL thresholds between VRT and CRT was poor to moderate (intraclass correlation coefficient ≤ 0.54). These results indicate that, although VRT does not further compromise %VL-based regulation of effort in the Smith machine back squat, %VL should not be used interchangeably with %Rep, particularly at light-to-moderate loads, even when individualized and load-specific %Rep–%VL relationships are established.