Proximity-to-failure has only been investigated dichotomously, with previous research comparing training to “set failure” versus non-failure.
As such, previous research does not describe the relationship between reps in reserve (RIR) and muscle hypertrophy, and is therefore practically limited, as set termination does not have to be binary (i.e., set failure or non-failure).
Moreover, coaches often program RIR rather than failure.
However a new study looking at failure versus leaving 1 to 2 reps in reserve was just published and is worth reviewing.
Is training to failure better for hypertrophy than leaving 1-2 reps in reserve?
What did the researchers do?
Training Program
- Resistance-trained men and women completed an 8-week unilateral training program consisting of leg press and leg extension.
- 1 of their legs trained to failure while 1 leg trained to a RIR cutoff.
- The RIR leg trained to perceived 2-RIR for leg press and 1-RIR for leg extension.
Variables Measured
- Reps in reserve prediction accuracy (participants had a high absolute RIR accuracy; on average less than one repetition from the 1- and 3-RIR targets on both exercises).
- Volume accumulation measured as repetition volume (sets x repetitions) and volume load (sets x repetitions x load).
- Change in lifting velocity from first to final set (mean concentric velocity)
- Muscle thickness (ultrasound scanning to determine left and right RF and VL thickness).
What were the results?
Hypertrophy
- Authors reported a similar increase in quadriceps thickness (i.e., average of RF and VL) after eight weeks of RT performed to either FAIL (+6.96%) or RIR (+6.98%) in resistance-trained males and females.
Muscle Group Specificity
- Authors also found an 81% probability (pd > TE = 42%) of slightly greater RF thickness when RT was performed to RIR (+7.38%) versus FAIL (+5.98%), but a 79% probability (pd > TE = 22%) of slightly greater VL thickness when RT was performed to FAIL (+7.95%) versus RIR (+6.59%).
Neuromuscular Fatigue (Lifting Velocity & Repetition Loss)
- Authors observed greater decreases in lifting velocity from the first to final set for FAIL versus RIR in weeks one, four, and eight, indicating acute neuromuscular fatigue is higher when terminating sets at momentary muscular failure versus 1- to 2-RIR.
- For example, FAIL experienced decreases in lifting velocity on the leg press that ranged from −9.6% to −12.6%, with lower decreases in lifting velocity in RIR from −4.4% to −6.4%.
- Authors observed larger differences in repetition loss on the leg press between FAIL and RIR in the earlier stages of the RT intervention versus the latter (FAIL > RIR); repetition loss increased further for RIR overtime versus FAIL, suggesting that changes in intra-set fatiguability or tolerance to the training stimulus (i.e., fatigue resistance) across the intervention differed between protocols.
What does this mean?
- These findings demonstrate that in resistance-trained males and females, terminating sets at 1- to 2-RIR promotes similar overall quadriceps hypertrophy to reaching momentary muscular failure.
- Despite similar quadriceps hypertrophy observed between protocols, slightly greater VL hypertrophy occurred in FAIL versus RIR while slightly greater RF hypertrophy occurred in RIR versus FAIL.
- Acute neuromuscular fatigue is consistently greater over eight weeks when momentary muscular failure is reached versus when sets are terminated at 1- to 2-RIR.
- Acute neuromuscular fatigue can decrease across weeks of a RT intervention but this may depend on the exercises performed and the RT stimulus.
Coach's Takeaway
Regarding Muscle Size
- Both training modalities showed improvements in hypertrophy.
- An adequate set volume coupled with close proximities-to-failure, rather than reaching momentary muscular failure per se, are key stimulators of muscle hypertrophy in resistance-trained individuals.
- Results are based on performing 10 to 17 sets for a given muscle group per week, indicating the relationship between proximity-to-failure and muscle hypertrophy may be stable across this range of set volumes, on average.
Regarding Fatigue
- Performing RT to momentary muscular failure consistently induces higher levels of acute neuromuscular fatigue versus RT performed with 1- to 2-RIR.
- Improved fatigue resistance overtime may attenuate acute neuromuscular fatigue and subsequent repetition loss (but may depend on the exercise performed).
- Optimizing stimulus versus fatigue can have implications for training across longer time frames of months and years.
Regarding Programming
- Performing RT with 1- to 2-RIR allows for similar volume load and repetition volume accumulation as reaching momentary muscular failure, possibly influencing the overall RT stimulus achieved.
- The figure below does a nice job summarizing the applications based on data.
- Specifically, it shows individuality, and which populations may benefit from the different RT stimuli (e.g., FAIL for bodybuilders etc.).
This graphic from the authors is a great summary of the study 👇🏼
