(8) THE EFFECTS OF TRAINING NEAR VOLITIONAL FAILURE ON STRENGTH AND MOTOR UNIT FIRING RATES IN RESISTANCE-TRAINED ADULTS

Friday, July 14, 2023

10:30 AM – 12:00 PM East Coast USA Time

Location: Milano III-IV

Poster Presenter(s)

Jonathan P. Beausejour, MS, CSCS

Research Coordinator / Graduate Teaching Associate
University of Central Florida
Oviedo, Florida, United States

Author(s)

jP

jason Pagan

Research Assistant
University of Central Flordia
Orlando, Florida, United States
JR

Juan Rodriguez

Research Assistant / SPT
University of Central Flordia
Orlando, Florida, United States
DS

Daniel Sheldon

Research Assistant / SPT
University of Central Flordia
Orlando, Florida, United States
BR

Bradley Ruple

Research Coordinator / Graduate Teaching Associate
Auburn University
Auburn, Alabama, United States
DP

Daniel Plotkin

Research Coordinator / Graduate Teaching Associate
Auburn University
Auburn, Alabama, United States
KY

Kaelin Young

Associate Professor
Pacific Northwest University
Yakima, Washington, United States
MR

Michael Roberts

Professor
Auburn University
Auburn, Alabama, United States
Matt S. Stock, PhD

Associate Professor
University of Central Florida
Orlando, Florida, United States

Abstract Details: Whether resistance training sets should be performed until volitional failure versus leaving repetitions in reserve (RIR) has recently received significant attention in the literature. However, little is known about the underlying motor unit adaptations following different training paradigms.

Purpose: We sought to investigate differences in strength and motor unit firing rates following six weeks of low- versus high-RIR powerlifting training.

Methods: Nineteen resistance-trained adults (11 males and 8 females; mean ± SD age = 24 ± 3 years, one-repetition maximum [1RM] squat/mass kg ratio = 1.60 ± 0.33) participated in this study. Before (PRE) and after (POST) the training intervention, isometric knee extensor peak torque and vastus lateralis motor unit firing rates during an 80% maximal voluntary contraction (MVC) were measured. We also measured 1RM barbell back squat, bench press, and deadlift strength. Participants were randomly assigned (based off Wilks score) to low-RIR and high-RIR groups, and completed six weeks of a powerlifting-based resistance training (3 days/week). The low-RIR group performed the squat, bench press, and deadlift exercises, and completed each set close to failure (RIR of 0-1). The high-RIR group performed the same program, but completed each set within several repetitions from failure (RIR of 4-6). All dependent variables were analyzed using two-way mixed factorial analyses of variance.

Results: 1RM improved for the squat (+10.0 kg, 8.7%, p< 0.001), bench (+3.8 kg, 7.7%, p=0.002), and deadlift (+6.9 kg, 5.5%, p=0.001), but with no group × time interactions (p ≥ 0.159). A moderate effect size (partial ή² = 0.113) suggested slightly better improvements in squat 1RM with low-RIR. A significant main effect for time was also observed for isometric MVC peak torque (p=0.013), with a medium/large effect size observed in favor of the low-RIR group (η²=0.126). Significant group × time interactions were observed for both the slope (p=0.021, η²= 0.306) and y-intercept (p=0.026, η²= 0.289) of the motor unit mean firing rate versus recruitment threshold relationship (Figure 1). Post-hoc analyses demonstrated that the slopes and y-intercepts decreased and increased, respectively, for the low-RIR group, suggesting that motor unit firing rates increased, particularly for low-threshold motor units. Changes in motor unit data were not observed following high-RIR training.

Conclusion: Low-RIR and high-RIR training elicited similar increases in 1RM strength following a six week powerlifting training protocol. In contrast, low-RIR training resulted in greater improvements in unilateral, isometric MVC peak torque and motor unit firing rates. PRACTICAL APPLICATIONS: In resistance trained adults, both low- and high-RIR training can be used to improve 1RM back squat, bench press, and deadlift strength. Training close to volitional failure influenced motor unit firing rates, but these changes did not translate to better powerlifting performance.

Acknowledgements: None