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Prediction of muscle fiber composition using multiple repetition testing

Direct determination of muscle fiber composition is invasive and expensive, with indirect methods also requiring specialist resources and expertise. Performing resistance exercises at 80% 1RM is suggested as a means of indirectly estimating muscle fiber composition, though this hypothesis has never been validated against a direct method. The aim of the study was to investigate the relationship between the number of completed repetitions at 80% 1RM of back squat exercise and muscle fiber composition. Thirty recreationally active participants’ (10 females, 20 males) 1RM back squat load was determined, before the number of consecutive repetitions at 80% 1RM was recorded. The relationship between the number of repetitions and the percentage of fast-twitch fibers from vastus lateralis was investigated. The number of completed repetitions ranged from 5 to 15 and was independent of sex, age, 1RM, training frequency, training type, training experience, BMI or muscle fiber cross-sectional area. The percentage of fast-twitch muscle fibers was inversely correlated with the number of repetitions completed (r = –0.38, P = 0.039). Participants achieving 5 to 8 repetitions (n = 10) had significantly more fast-twitch muscle fibers (57.5 ± 9.5 vs 44.4 ± 11.9%, P = 0.013) than those achieving 11–15 repetitions (n = 11). The remaining participants achieved 9 or 10 repetitions (n = 9) and on average had equal proportion of fast- and slow-twitch muscle fibers. In conclusion, the number of completed repetitions at 80% of 1RM is moderately correlated with muscle fiber composition.

Inter-individual differences are evident in the number of completed repetitions at different percentages of 1 repetition maximum (1RM), with heterogeneity in muscle fiber composition suggested as an underlying mechanism [34]. Accordingly, the number of repetitions achieved at specific percentages of 1RM has been proposed as a practical approach to estimate muscle fibre composition [35]. However, despite the association of exertion at specific percentages of 1RM with the variability in repetitions performed [36], there are no published studies reporting significant correlations between direct estimates of muscle fiber composition and multiple repetition testing. In one study by Terzis et al. [37], leg press machine testing was used to determine the association of 70% and 85% 1RM repetition tests with muscle fibre composition, with participants possessing greater proportions of slow-twitch muscle fibres completing more repetitions.
However, this result did not reach statistical significance, potentially due to a limited sample size (n = 12). With access to a leg press machine not always possible, back squat exercise using an Olympic barbell offers a viable alternative for exercise testing. Furthermore, there is also a need to study participants of both genders and to do so with a sample size greater than previous investigations. Therefore, the aim of this study was to determine the interrelationship between muscle fiber composition of the vastus lateralis of 30 participants 15-85% type I, 5-77% type IIa, and 0-44% type IIx [8][9][10]. The gold-standard to directly determine muscle fiber composition is the muscle biopsy, with the VL most commonly sampled due to its mixed fiber composition and accessibility [11]. Subsequently, the fiber composition of biopsies is determined using methods such as immunohistochemical analysis, histochemical staining for myosin ATPase, gel electrophoresis and mass-spectrometry [12][13][14], with transcriptomic analysis of the MYH1, MYH2 and MYH7 genes recently used to determine type IIx, type IIa and type I muscle fibers, respectively [15]. Nevertheless, collecting muscle biopsies is invasive, and subsequent analyses are expensive to perform, limiting accessibility to direct analytical methods.
Indirect methods to estimate muscle fiber composition offer alternative approaches that can be validated against direct methods.
Tensiomyography (TMG) evaluates the morphofunctional potential of a muscle according to the radial enlargement of the muscle abdomen in response to electrical stimulation [16], with time to muscle contraction well correlated with slow-twitch muscle fiber proportion [17]. With fast-twitch fibers containing twice the carnosine content of slow-twitch fibers, magnetic resonance spectroscopy (MRS) of carnosine content is also indicative of fiber type [18]. This method was recently used to predict recovery from Wingate cycling, with considerable differences in knee extensor strength recovery between participants with predominantly slow-twitch fibers and participants with predominantly fast-twitch fibers [19]. Muscle fiber composition can also be estimated according to the vibrational properties of contracting skeletal muscle by mechanomyography (MMG), which demonstrates 80% accuracy in predicting VL fiber composition [20]. In addition to morphological measures, skeletal muscle phenotypes are associated with common genetic variants, suggesting that testing and would be independent of gender.

Subjects
Thirty recreationally active participants (

Procedures Back squat testing
All participants were familiar with back squat exercise with a minimum of two years experience. Participants performed a 1RM protocol to determine the maximal load they could lift for the back squat with correct technique using a standard 20 kg Olympic barbell.
Briefly, participants performed a warm-up with a self-selected load that allowed them to complete a minimum of 8 repetitions. slow-twitch (50.8 ± 16.9%) muscle fibers.

DISCUSSION
The aim of the present study was to determine whether muscle fiber  [39], meaning this load is commonly prescribed by practitioners working with the athletes and general population, and because 80% 1RM was originally proposed as the most appropriate load to indirectly estimate muscle fiber composition [35].

Evaluation of muscle fiber composition and cross-sectional area
Percutaneous muscle biopsies were collected from the vastus late-

Statistical analyses
Statistical analyses were conducted using GraphPad InStat (Graph-Pad Software, Inc., USA) software. Differences in traits between different groups were analysed using Mann-Whitney test. Relationships between participant characteristics, exercise testing performance and muscle fiber measurements was tested using Spearman's (non-parametric) correlation. Multiple regression analysis was used to detect independent associations between the number of repetitions achieved at 80% of 1RM and different characteristics. All data are presented as mean (SD). P values < 0.05 were considered statistically significant. Prediction of muscle fiber composition individuals, which had not been previously tested using back squat exercise. It was suggested that participants completing more than 12 repetitions would possess more than 50% slow-twitch fibers, individuals completing fewer than 7 repetitions would possess more than 50% fast-twitch fibers, and that those completing between 7 and 12 repetitions would exhibit an equal proportion of each fiber type [35]. To our knowledge, the present study is the first to find a significant interrelationship between muscle fiber composition of the VL and the variability in the number of back squat repetitions completed at 80% 1RM. the present study demonstrates the relationship between directly measured VL fiber composition and a universally recognised resistance exercise, with individualised loads derived using a reliable assessment of 1RM [41]. Whilst further studies in larger cohorts are required to support our findings, the present study highlights the potential of this approach as an accessible and cost-effective alternative for estimating muscle fiber composition.

Participants
No relationship was observed between participants' training background and muscle fiber composition in the present study.
Previously, endurance athletes completed more repetitions of leg press exercise at 70% and 80% 1RM compared to strength athletes, with no difference at 90% [42], whilst non-athletes accustomed to endurance training achieved more repetitions at 80% 1RM of half-squat exercise than those accustomed to strength training [43].
These results are of importance because endurance trained individuals typically possess a greater percentage of slow-twitch fibers [3] demonstrating increased lactate buffering capacity [44], fiber capillarisation [45] and mitochondrial content [46], i.e. the muscles of these individuals have greater oxidative capacity and fatigue resistance than their strength trained counterparts [11]. The advantages of the present study include the direct determination of VL muscle fiber composition, a common and reliable method for 1RM testing [41], and the potential practical application of these findings for individuals without access to the facilities and expertise required to directly determine fiber composition.
However, this study also has limitations. Firstly, we recognise that

CONCLUSIONS
The present study is the first to demonstrate the interrelationship between back squat performance at 80% 1RM and muscle fiber composition. We suggest that completing 8 or fewer repetitions at 80% 1RM is indicative that participants possess more than 50% fast-twitch fibers in their VL, that completing 11 or more repetitions at 80% 1RM is indicative that participants possess more than 50% slow-twitch fibers in their VL, and that completing 9 or 10 repetitions is indicative of participants possessing an equal distribution of each fiber type. Though further validation is required, the present study demonstrates the association of muscle fiber type with back squat performance and highlights potential to develop a non-invasive and cost-effective approach to estimate muscle fiber composition.

Conflict of interest
The authors declare no conflict of interest. a greater sample size could improve the statistical power of the results. Secondly, our findings are limited to the muscle fiber composition of the VL, and due to the heterogeneity of fiber composition within skeletal muscles of the human body, the validity of this method is limited to the VL. We also recognise that information regarding participants' endurance exercise capacity and genetic profile was not included in the present study, and that including such data may improve the prediction of muscle fiber composition in future studies, in addition to investigating a series of external loads. Finally, we recognise that validation of this approach in untrained populations and in elite athletes is required for this approach to be used with sedentary individuals and within highlevel sport, for example in weightlifting and powerlifting where the back squat is a fundamental exercise.