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Original Research

Accuracy in Estimating Repetitions to Failure During Resistance Exercise

Hackett, Daniel A.; Cobley, Stephen P.; Davies, Timothy B.; Michael, Scott W.; Halaki, Mark

Author Information
Journal of Strength and Conditioning Research: August 2017 - Volume 31 - Issue 8 - p 2162-2168
doi: 10.1519/JSC.0000000000001683
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Abstract

Introduction

Resistance training performed to momentary failure is thought to be a necessary practice for maximizing muscular strength and hypertrophy (24). However, data from a recent meta-analysis conducted by Davies et al. (7) revealed no difference in muscular strength gains achieved when performing sets to failure compared with nonfailure. Also, there is some evidence of a similar relationship when targeting muscular hypertrophy (22). Several issues have been raised concerning the implementation of failure training into resistance training programs and are largely centered on the extra fatigue experienced. Some of the concerns include the possible risk of overtraining and overuse injuries (30,34) and being unable to replicate sets with the desired loads and repetition range (31–33). To avoid these potentially detrimental effects while allowing a sufficient resistance training stimulus, the majority of sets should be performed close to but without reaching failure. If sets are performed to failure, it is advised that this practice should be used sparingly (e.g., final set of an exercise) to minimize possible risks (7). Therefore, it seems imperative that an individual has the ability to accurately estimate the number repetitions away from muscular failure so that training may be optimized and sufficiently monitored.

The use of a rating of perceived exertion (RPE) scale is a convenient and practical method to monitor resistance training performance (8,19,26). Several exertional signals (e.g., muscle activation, afferent signals) have been shown to influence the reporting of RPE during resistance training (6,15,18). However, the ability to successfully use these cues is shown to be dependent on training experience (27). Perceived effort has also been found to be influenced by the type of resistance exercise performed, with a trend toward higher RPEs for lower-body compared with upper-body exercises (23). Despite the popular use of RPE, as a monitoring tool for resistance exercise, the information gathered can be difficult to quantify and practically use for training purposes (i.e., prescription of repetitions and loads to use). Furthermore, there is evidence that during sets of resistance exercise performed to failure, less than maximal RPE can be reported (20,23). This issue was partially addressed with the creation of the repetitions in reserve system (28), where an RPE value corresponds to a certain amount of repetitions that could still be performed. However, the repetitions in reserve system use a scale where the value reported and its corresponding meaning has an inverse relationship, which may reduce the accuracy of feedback provided by the trainer. For example, higher values correspond with a lower number of repetitions that could be performed. Thus, it would seem more logically appropriate, valid, and easier if repetitions to failure were monitored solely on their own without associating with RPE and then requiring translation.

Hackett et al. (11) previously validated a subjective scale created around this idea known as the estimated-repetitions-to-failure (ERF) Scale. Results from this previous study showed there were very small differences (<1 repetition) between the ERF and actual repetitions to failure (ARF), for the majority of sets for the bench press and squat, within a small group of male bodybuilders. However, the validity of the ERF scale is unknown in less-experienced resistance trainers, between sexes and when performing resistance exercises with machines.

Therefore, the aim of this study was to assess the accuracy in ERF during resistance exercise. Furthermore, this study aimed to investigate whether the accuracy in ERF was affected by training status, sex, or exercise type. It was hypothesized that the ERF accuracy would be highly dependent on the fatigue state of an individual with greater accuracy when resistance exercise was performed under conditions of increased fatigue (i.e., closer to concentric failure). Furthermore, it was hypothesized that more experienced resistance trainers would have greater accuracy in ERF. It was also expected that there would be no difference in accuracy in ERF between sexes, or between upper-body and lower-body exercise.

Methods

Experimental Approach to the Problem

The accuracy in ERF during resistance exercises and factors that affect ERF accuracy are largely unknown. This study was therefore exploratory but also designed to investigate the error between ERF and ARF during sets of resistance exercise. Male and female resistance trainers of various levels of experience performed sets of 10 repetitions for resistance exercises at a fixed percentage of 1 repetition maximum (%1RM). Subjects briefly paused when the prescribed number of repetitions for each set was reached while they reported their ERF, and then continued to concentric failure. The accuracy (amount of error) of ERF was determined over an ARF range of 0–10.

Subjects

Eighty-one adults (men, n = 53, women, n = 28) aged 18–60 years participated in this study. To be eligible for inclusion, potential subjects needed to be apparently healthy and free from injury or illness. There was no requirement that subjects needed to have previous resistance training experience. Sixteen subjects had ≤6 months of resistance training experience, with 14 and 51 subjects with 1–2 years and ≥3 years resistance training experience, respectively. Subject characteristics are summarized in Table 1. Before data collection, subjects were informed of the purpose of the study, the experimental procedures involved, and all the potential risks involved before obtaining written consent. Informed consent documents were signed for all subjects prior to participation, which was approved by the University of Sydney Human Research Ethics Committee.

Table 1.
Table 1.:
Characteristics of subjects.*†

Procedures

Subjects were invited to attend 2 testing sessions with >48 hours in-between these sessions. During the first session, subjects performed 1RM testing and were familiarized with the ERF scale before commencing the experiment (i.e., assessing the accuracy in ERF for the exercises). The second session involved subjects repeating only the experiment (i.e., no 1RM testing). Exercises were performed with the pin loaded vertical chest press machine (Maxim, Kidman Park, South Australia) and pin loaded horizontal leg press machine (Kolossal, Sydney, Australia). All subjects attended the first session whereas 70 of 81 subjects attended the second session because of various reasons unrelated to the actual study. Subjects were instructed to maintain their normal diet during the days preceding the visits, to consume their last meal at least 2 hours before exercise, and to avoid using preworkout supplements because of their potential influence on perceptual responses (3). In addition, subjects were instructed to refrain from resistance training or any other strenuous type of exercise 48 hours before actual testing.

One Repetition Maximum

Subjects performed 2–3 sets of 4–8 repetitions with light-moderate loads to warm-up before maximal efforts for the chest press and leg press. After completing the warm-up, loads were adjusted to enable subjects to perform ≤10RM to allow 1RM to be accurately estimated (21). If it was perceived that >10 repetitions could be performed or failure was not reached before 10 repetitions, the load was increased and 5-minute recovery was provided before the next RM attempt. The Brzycki 1RM prediction equation (5) was used to estimate the 1RM based on the load and repetitions performed. Therefore, 1RM was not directly assessed but predicted. The 1RM for the chest press was determined in 5.0 ± 2.3 repetitions and for the leg press in 6.0 ± 3.8 repetitions. The test-retest intraclass correlation coefficient for the above testing protocol in our laboratory is ≥0.90.

Familiarization of ERF Scale

After the 1RM testing, subjects were introduced to the ERF scale and were given instructions about using this scale during resistance exercise (Table 2). The subjects were informed that the scale would be shown at the conclusion of a set and they will be asked “how many additional repetitions can you perform?” An ERF score of “10 or greater” indicated that the subject estimated that 10 or more repetitions could be completed, whereas a “0” is where the subject estimated no additional repetitions could be completed (concentric failure reached). To help subjects link their full exercise stimulus range with their ERF response range, a memory anchoring procedure was used. This involved asking each subject to think of their previous efforts for the 1RM testing when they reached a level of exertion that was equal to the verbal cues at the bottom and top of the ERF scale.

Table 2.
Table 2.:
Estimated-repetitions-to-failure scale.*

Accuracy in ERF

Subjects were required to perform multiple sets of 10 repetitions (or to muscular failure if 10 repetitions was not possible) at 70% 1RM and 80% 1RM for the chest press and leg press, respectively. The rationale for different %1RM used for the exercises was to have subjects perform a similar number of repetitions to failure with the loads used. Based on results from pilot testing, ≤20 repetitions to failure could be performed with 70% 1RM and 80% 1RM for the chest press and leg press, respectively. Subjects aimed to perform 5 sets per exercise, but if concentric failure occurred before reaching 10 repetitions for a set, the testing for the exercise ceased. At the completion of a set of 10 repetitions, subjects were required to pause briefly (approximately 5 seconds) at the end of the concentric phase during which time the ERF scale was shown and an ERF was reported. After the pause, subjects proceeded to ARF. During all sets, repetitions were performed in a controlled manner during both the concentric (∼2 seconds) and the eccentric (∼2 seconds) phases. There was 1–2 minutes of recovery between sets. It was expected that subjects would perform a greater number of ARF for the initial sets with less ARF in the final sets. Therefore, extra recovery time was permitted (up to 5 minutes) on the latter sets if concentric failure was reached at 10 repetitions before reporting ERF. During all sets, subjects were encouraged to complete each repetition through a full range of motion without deviating from the proper technique while keeping the lifting speed constant. Verbal encouragement (i.e., shouting positive words) was provided throughout sessions to ensure that “true” concentric failure was achieved.

Statistical Analyses

A linear mixed model analysis with an autoregressive (order 1) covariance structure with subjects as a random factor, sets as the repeated variable and ARF, sex, and exercise as fixed categorical factors with resistance training experience as a fixed continuous factor was used to investigate the error of ERF (ERF-ARF). Follow-up analyses were run as above for each exercise separately. A linear mixed model was used to allow for differences in the number of sets performed per subject (i.e., missing data) (29). When significant effects were found, Bonferroni adjusted pairwise comparisons were conducted to investigate where the significant differences were found. All analyses were performed using SPSS version 22.0 for Windows (IBM Corp., Armonk, NY, USA). Data are presented as mean ± SD, and level of significance was set at p ≤ 0.05.

Results

Summary of Performance

Actual repetitions to failure for the chest press and leg press ranged from 0 to 10 for both sexes. There were a greater number of sets performed with ARF ranging from 0 to 6 and 2 to 6 for the chest press and leg press, respectively. On the last set of each testing session for the chest press and leg press, 25 and 14% of subjects, respectively, were unable to complete 10 repetitions before reporting ERF.

Overall Chest and Leg Press Results

The overall mixed model analysis results indicated that the accuracy of ERF (i.e., error of ERF) differed over the ARF range of 0–10, with greater accuracy with lower ARFs (p < 0.001). Greater accuracy of ERF was found for the chest press compared with the leg press (p = 0.012), and for men compared with women (p = 0.008). Accuracy of ERF was not affected by resistance training experience (p = 0.134). The interaction between sex and exercise was significant (p = 0.002), whereas no other interaction was significant (p > 0.05).

Chest Press Results

The overall mixed model results for the chest press indicated that the accuracy of ERF (i.e., error of ERF) differed over the ARF range of 0–10 (p < 0.001). The accuracy of ERF was within 1 repetition for ARF 0–5, with a systematic decrease in accuracy (i.e., increase in underestimation in ERF) as ARF increased (Figure 1A). Post hoc analyses revealed that the accuracy of ERF was similar for the following ARF groupings 0–3 (p = 1.0), 3–4 (p = 0.191), 4–5 (p = 0.226), 5–6 (p = 0.578), 7–8 (p = 1.0), 8–9 (p = 1.0), and 9–10 (p = 1.0). Accuracy of ERF was not affected by sex (p = 0.60) or resistance training experience (p = 0.70).

Figure 1.
Figure 1.:
Accuracy in estimation of repetitions to failure for the chest press and leg press. Accuracy determined by error of ERF (i.e., ERF minus ARF). Positive numbers (repetitions) indicate overestimation, whereas negative numbers indicate underestimation. A, B) ERFs within groups of ARFs that were not significantly different (p > 0.05). C) Differences in error of ERF for ARFs between men (n = 53) and women (n = 28). *Significant difference in error of ERF p ≤ 0.05.

Leg Press Results

The overall mixed model results for the leg press indicated that the accuracy of ERF (i.e., error of ERF) differed over the ARF range of 0–10 (p < 0.001). The accuracy of ERF was less than 1 repetition for ARF 0–3, with a systematic decrease in accuracy (i.e., increase in underestimation in ERF) as ARF increased (Figure 1B). Post hoc analyses revealed that the accuracy of ERF was similar for the following ARF groupings 0–1 (p = 1.0), 1–2 (p = 1.0), 3–4 (p = 0.247), 3–4 (p = 0.535), 4–5 (p = 0.234), 5–6 (p = 1.0), 6–7 (p = 0.277), 7–9 (p = 1.0), and 8–10 (p ≥ 0.65). Accuracy of ERF over the ARF range of 0–10 was affected by sex (p = 0.006), but not by resistance training experience (p = 0.35). The post hoc analysis revealed that men were generally more accurate than women when ≥4 repetitions (specifically ARF 4, 7, 9, and 10) to failure could be performed (p ≤ 0.05; Figure 1C).

Discussion

The purpose of this study was to assess the accuracy in estimation of repetitions to failure (ERF) during resistance exercise. This experiment involved assessing the error of ERF (ERF-ARF) over an ARF range of 0–10 repetitions for the chest and leg press resistance exercises. The overall results showed that the accuracy of ERF differed over the ARF range of 0–10 for both exercises, with accuracy in ERF decreasing as ARF increased (∼1 repetition at ARF 0–5 compared with >2 repetitions at ARF 7–10). Therefore, the hypothesis that the accuracy in ERF would be dependent on the fatigue state was supported. A significant difference for the accuracy in ERF between exercises was found, with error of ERF <1 repetition at ARF 0–5 for the chest press compared with ARF 0–3 for the leg press. This did not support the hypothesis of no difference for the accuracy in ERF between upper- and lower-body exercises. A significant interaction was found for sex and exercise, with men being more accurate in ERF than women at specific ARFs for the leg press, whereas no interaction was evident for the chest press. Resistance training experience did not affect the accuracy of ERF, and therefore, the hypothesis of increased accuracy in ERF with greater resistance training experience was not supported.

A widely accepted method at present for assessing resistance exercise effort is the RPE scale, and studies have demonstrated that active muscle RPE ratings increase with lifting of heavier loads and as one approaches failure (9,10,17). However, the information gathered from RPE ratings can be difficult to interpret, quantify, and use for training purposes because of the subjective nature of this method. Furthermore, RPE ratings during exercise can be affected by psychological and social factors (e.g., personality, mood, and self-efficacy) (12); therefore, it would seem logical that a resistance exercise monitoring tool to objectively quantify performance be developed. Findings from the present study suggest that the ability to accurately measure ERF for resistance exercises is highly dependent on whether the resistance trainer is close to concentric failure. The accuracy in ERF was quite good for lower ARFs (<1 repetition at ARF 0–3) compared with higher ARFs (>2 repetitions at ARF 7–10). With less physically demanding sets, indicated by a greater number of ARF, subjects were found to underestimate ERF. The greatest error of ERF occurred at ARF 9–10 and was equal to ∼4 repetitions. These results are in agreement with the findings from Hackett et al. (11). In their study, nonsignificant differences between ERF and ARF were found in the latter sets of the testing protocol when the subjects were closer to concentric failure, thus probably more fatigued.

The accuracy in ERF differed between the types of exercises performed in the present study, with greater accuracy found for the chest press compared with the leg press. Based on the finding that error of ERF decreases with lower ARF, it seems likely that the subjects used exertional sensations (e.g., muscle activation, afferent signals from Golgi tendon organs, muscle spindles, and mechanoreceptors) to assist with their ERF. Previous research has shown that for resistance exercise performed at a specific %1RM, the perceived effort tends to be higher for exercises that use greater absolute loads (e.g., higher RPE for squats compared with bench press) (23). Therefore, it would have been reasonable to hypothesize that greater accuracy in ERF would be observed for the leg press compared with the chest press because of the anticipated greater exertional sensations. However, it seems that the difference between the accuracy in ERF for upper-body compared with lower-body exercises may be related to the interplay of afferent and efferent feedback. The upper limbs perform more precise movements than the lower limbs; therefore, they have a higher sensory organ density than the lower limbs (13). Accuracy in ERF was also found to be greater for men than women for the leg press generally for ARFs ≥4, which may be partly related to differences in sensory organ densities of the lower limbs between sexes (13). Further research is needed to assess whether sex difference affects the accuracy in ERF during lower limb resistance exercises.

The ability to accurately assess the number of repetitions from concentric failure during a resistance exercise is important for prescription and monitoring. To allow for optimal muscular strength and hypertrophy adaptations, there is evidence that repetitions should be performed close to failure with moderate-to-heavy loads (7,22,25). However, if the majority of resistance training is performed to failure, there is a possible risk of overtraining and injury (30,34). To date, the repetitions in reserve system is a common method that attempts to objectively quantify resistance exercise performance following sets. The repetitions in reserve system links RPE values using the category-ratio scale 0–10 (CR10) to the number of repetitions performed to failure (e.g., rating of 9 corresponds to 1 repetition remaining). However, responses using the CR10 grow in a nonlinear and positively accelerating manner during exercise (4) and an increase in CR10 by 1 point is unlikely to correspond with 1 less repetition being performed. Furthermore, the inverse relationship between the CR10 rating and number of repetitions before failure (i.e., higher CR10 corresponding to less repetitions in reserve) may affect the accuracy of feedback reported, especially by a novice resistance trainer. The present study provides evidence that the ERF is accurate for ARF 0–5; therefore, it seems redundant and possibly erroneous that this measure be associated with RPE.

Previous studies have found differences between experienced and novice resistance trainers for reporting of effort or repetitions in reserve (27,35). Zourdos et al. (35) showed that experienced compared with less-experienced squatters (>1 year vs. <1 year, respectively) were more accurate with reporting repetitions in reserve. However, Zourdos et al. (35) did not assess ARF and relied on the %1RM—RM continuum devised by Baechle and Earle (2) to assess the accuracy of reporting. Hackett et al. (11) validated the ERF scale in a group of bodybuilders; however, the present study showed that the ability to accurately measure ERF was not influenced by training experience. A possible explanation for this finding might be because of the type of exercises that were used to assess ERF. It is well-known that machine compared with free-weight exercises require less skill to perform (16). Therefore, subjects in the present study only needed to concentrate on lifting the load in one direction and were not challenged by having to stabilize joints to enable the movement to be performed. For an inexperienced trainer, the extra effort required to control a movement would likely interfere with the exertional sensations used to ERF. However, some caution is required when interpreting the ERF and training experience results from the present study because of the low number of novice resistance trainers. Only 16 subjects had ≤6 months of resistance training experience compared with 65 subjects who had ≥1 year of experience.

Studies have shown interindividual variation in the number of repetitions to failure at fixed %1RM for specific exercises (1,14) and variation between exercises targeting different muscle groups (14). For example, if resistance trainer A and B were to perform 1 set of 10 repetitions at 75% 1RM for the bench press, it is possible that the training stimulus could differ between the trainers. This would be evident if resistance trainer A had the ability to perform an additional 2 repetitions to failure, whereas resistance trainer B could perform another 8 repetitions to failure. Therefore, it may be prudent to prescribe resistance exercise based on a specific number of repetitions to failure or an RM. Although this would solve the issue of variations in effort required to perform sets of resistance exercise, the physiological and psychological stress of prolonged training to failure has its inherent risks. Additionally, reductions in training load might be required to enable a trainer to stay within the selected repetition range, thus leading to a reduction in training volume (load × repetitions) (31–33). Prescribing resistance training based on ERF could be an effective strategy to match resistance training performance between trainers and achievement of targeted training volumes at specific %1RM.

It is possible that the accuracy in ERF was influenced by goals set by individual subjects. During the sets of resistance exercises, subjects could have used their ERF as a goal and terminated the set once this was achieved. This may have been more of an issue for subjects with less resistance training experience because of being less familiar with performing sets to failure. However, after subjects reported their ERF, they received equal encouragement in all sets to perform as many repetitions as they could to concentric failure. Furthermore, the similar accuracy in ERF between resistance exercisers of various experience suggests that it is unlikely that less-experienced resistance trainers ceased sets before reaching failure. Another potential limitation was that no descriptive measure was used as criteria for resistance training experience (e.g., relative strength). Furthermore, subjects were stratified by years of resistance training experience without knowledge of their training frequencies (i.e., days per week) and experience with the exercises performed in this study. Therefore, the generalizability of these results for trained and untrained populations may be impaired. However, more than likely, the participants were less trained than the subjects involved in the study by Hackett et al. (11), based on the 1RM values. Therefore, the ERF scale should be considered a valid tool to assess or monitor resistance exercise performance in novice to advanced resistance trainers (<6 to >12 months experience).

In conclusion, results suggest that resistance trainers, independent of resistance training experience, can accurately measure ERF (<1 repetition) for the chest press and leg press when close to failure (≤5 and ≤3 ARF, respectively). However, accuracy in ERF progressively decreases with increases in ARF. The accuracy in ERF over ARF 0–10 was found to be greater for the chest press compared with leg press, whereas the accuracy in ERF for the leg press was greater for men compared with women generally at ARF ≥4. These differences in accuracy in ERF for sex and exercise type may be linked to sensory organ density of lower versus upper limbs. Based on study findings, it seems that resistance trainers have the ability to ERF with fairly good accuracy (∼1 repetition) up to ARF 5.

Practical Applications

For coaches, athletes, and trainers interested in programming for resistance training, ERF could be used to equate performances between individuals. This could be done by selecting loads that allow resistance trainers to be within a specific ERF range following sets (e.g., 2–3 ERF). This approach would help to alleviate discrepancies in individual exertion or fatigue responses following sets of resistance exercise when performed using a specific %1RM for selected repetition ranges (e.g., 70% 1RM for 8–10 repetitions). Also, ERF could be used to monitor individual responses throughout or between training sessions. This would inform coaches and trainers on how well an athlete is progressing and could be used to identify overtraining or overreaching states. For example, extra recovery time may be required for an athlete if they report an ERF of 0 following the first set of 10 repetitions for bench press, while the previous week an ERF of 4 was reported for the same exercise prescription.

Although significant increases in muscular strength and hypertrophy are achieved with nonfailure resistance training, it has been advised that the majority of sets should be performed close to failure, with failure sets used sparingly (e.g., final set) (7). Using the ERF could assist with selecting and monitoring resistance loads that would result in failure (or close to) during the final set without training volume being negatively affected. As an example, if 3 sets of 10 repetitions with 1.5–2 minutes of recovery between sets were prescribed, selecting an initial load corresponding to an ERF of 3–4 would conceivably result in muscular failure being achieved at the third set. However, it is important to note that for ERF to be a reliable measure to use for resistance exercise requires practice. It is recommended that resistance trainers have their accuracy in ERF at various ARFs for specific exercises assessed periodically throughout a training program.

Acknowledgments

No funding was received for the study. No conflicts of interest are declared by the authors.

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Keywords:

resistance training; RPE; training intensity; weight lifting

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