The effects of resistance training to near failure on strength, hypertrophy, and motor unit adaptations in previously trained adults

Abstract

Limited research exists examining how resistance training to failure affects applied outcomes and single motor unit characteristics in previously trained individuals. Herein, resistance-trained adults (24 ± 3 years old, self-reported resistance training experience was 6 ± 4 years, 11 men and 8 women) were randomly assigned to either a low-repetitions-in-reserve (RIR; i.e., training near failure, n = 10) or high-RIR (i.e., not training near failure, n = 9) group. All participants implemented progressive overload during 5 weeks where low-RIR performed squat, bench press, and deadlift twice weekly and were instructed to end each training set with 0-1 RIR. high-RIR performed identical training except for being instructed to maintain 4-6 RIR after each set. During week 6, participants performed a reduced volume-load. The following were assessed prior to and following the intervention: (i) vastus lateralis (VL) muscle cross-sectional area (mCSA) at multiple sites; (ii) squat, bench press, and deadlift one-repetition maximums (1RMs); and (iii) maximal isometric knee extensor torque and VL motor unit firing rates during an 80% maximal voluntary contraction. Although RIR was lower in the low- versus high-RIR group during the intervention (p < 0.001), total training volume did not significantly differ between groups (p = 0.222). There were main effects of time for squat, bench press, and deadlift 1RMs (all p-values < 0.05), but no significant condition × time interactions existed for these or proximal/middle/distal VL mCSA data. There were significant interactions for the slope and y-intercept of the motor unit mean firing rate versus recruitment threshold relationship. Post hoc analyses indicated low-RIR group slope values decreased and y-intercept values increased after training suggesting low-RIR training increased lower-threshold motor unit firing rates. This study provides insight into how resistance training in proximity to failure affects strength, hypertrophy, and single motor unit characteristics, and may inform those who aim to program for resistance-trained individuals.

Keywords: failure; motor unit; resistance training; strength.

FIGURE 1

Training volume and RIR. Data in panel (a) illustrate week‐by‐week training volume (for squat, bench press, and deadlift only) between training conditions. Data in panel (b) illustrate total training volume over the 6‐week period (for squat, bench press, and deadlift only) between training conditions. Data in panel (c) illustrate week‐by‐week RIR reported by participants (for squat, bench press, and deadlift only) between training conditions p < 0.001). Data in panel (d) illustrate total averaged RIR over the 6‐week period (for squat, bench press, and deadlift only) between training conditions. n = 10 low‐RIR and n = 9 high‐RIR participants for panels (a, b), and n = 10 low‐RIR and n = 8 high‐RIR participants for (c) and (d). high‐RIR, group training further from failure per set for exercises indicated in Table 1 (4–6 RIR); low‐RIR, group training closer to failure per set for exercises indicated in Table 1 (0–1 RIR); RIR, repetitions in reserve.

FIGURE 2

Muscle strength adaptations. All figures show pre and post values for respective lifts, divided by the individuals body mass (kg). (a) Back squat, (b) bench press, and (c) deadlift. Values are presented as mean ± SD. Individual responses also illustrated, with open circles indicating females, and open squares indicating males. Pre, 7 days before the 6‐week training intervention. Post, 48 h following the last training bout. n = 10 low‐RIR and n = 9 high‐RIR participants in all panels. 1RM, one‐repetition maximum; C × T, condition by time interaction; high‐RIR, group training further from failure per set for exercises indicated in Table 1 (4–6 RIR); low‐RIR, group training closer to failure per set for exercises indicated in Table 1 (0–1 RIR); RIR, repetitions in reserve.

FIGURE 3

Changes in VL mCSA. All figures show pre and post values for mCSA values of the VL from the (a) proximal (33%), (b) middle (50%), and (c) distal (67%) portion of the femur. As indicated in the Figure 2 legend, white bars are the low‐RIR group and gray bars are the high‐RIR group. Values are presented as mean ± SD. Individual responses also illustrated, with open circles indicating females, and open squares indicating males. Pre, 7 days before the 6‐week training intervention. Post, 48 h following the last training bout. n = 9 low‐RIR and n = 9 high‐RIR participants in panels (a, c), and n = 10 low‐RIR and n = 9 high‐RIR participants in panel (b). C × T, condition by time interaction; high‐RIR, group training further from failure per set for exercises indicated in Table 1 (4–6 RIR); low‐RIR, group training closer to failure per set for exercises indicated in Table 1 (0–1 RIR); mCSA, muscle cross‐sectional area; RIR, repetitions in reserve; VL, vastus lateralis.

FIGURE 4

Motor unit data. All figures show pre and post values. Data in panel (a) illustrate isometric MVC torque. Data in panel (b) illustrate the 80% MVC slope. Data in panel (c) illustrate 80% MVC y‐intercept. Values are presented as mean ± SD. (*p < 0.05). As indicated in the Figure 2 legend, white bars are the low‐RIR group and gray bars are the high‐RIR group. Individual responses also illustrated, with open circles indicating females, and open squares indicating males. Pre, 7 days before the 6‐week training intervention. Post, 48 h following the last training bout. Data are for n = 10 low‐RIR and n = 9 high‐RIR participants for panel (a), and n = 8 low‐RIR and n = 9 high‐RIR participants for all other panels. C × T, condition by time interaction; high‐RIR, group training further from failure per set for exercises indicated in Table 1 (4–6 RIR); low‐RIR, group training closer to failure per set for exercises indicated in Table 1 (0–1 RIR); MVC, maximal voluntary contraction of the knee extensors; RIR, repetitions in reserve.

FIGURE 5

VL motor unit mean firing rate versus recruitment threshold relationships. Both figures show pre and post VL motor unit data, displaying the linear relationships between mean firing rates (y‐axes) and recruitment thresholds (x‐axes). The linear regression lines represent the mean slopes and y‐intercepts derived from each time point. Whereas the data appears stable for high‐RIR, note the upward shift of the linear regression line for low‐RIR. high‐RIR, group training further from failure per set for exercises indicated in Table 1 (4–6 RIR); low‐RIR, group training closer to failure per set for exercises indicated in Table 1 (0–1 RIR); MVC, maximal voluntary contraction of the knee extensors; RIR, repetitions in reserve; VL, vastus lateralis.