Experimental approach to the problem
Two exercises with different resistance profiles were chosen for the present study: inclined bicep curls and preacher curls. The first one places the highest amount of difficulty at the end of the exercise, with the elbow in a 90º position. The resistance increases along the RoM of the exercise, that is, an ascending kind of resistance (Figure 1). The second one places the highest amount of difficulty at the beginning of the exercise, with the elbow in a 180º position (Figure 1). The resistance decreases along the RoM of the exercise, that is, a descending kind of resistance. To avoid the influence that the resistance of other exercises may have in the adaptations to the exercises included in the study, participants were not allowed to perform any other biceps brachii or pulling exercises during the intervention.
To assess muscle growth, muscle thickness (MT) measurements were performed. MT was chosen instead of cross sectional area (ACSA) as performing reliable ACSA measurements with an ultrasound apparatus is difficult and operator dependant (Hernández-Belmonte et al. 2022). MT is defined as the thickness of all the muscles in the arm (biceps brachii and brachialis) in the field-of-view of the probe, from the skin interface to the humerus. Each region’s growth was analyzed individually and compared to the growth of other regions. The intervention lasted 9 weeks.
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Subjects
39 healthy women joined this study. Participants were required to meet the following inclusion criteria 1) Women with an age ranging between 18 and 45 years; 2) lack of musculoskeletal disorders in the upper limbs at least one year prior to the beginning of the study and 3) A resistance training experience of at least 6 months prior to the beginning of the study.
A total of 32 participants finished the study. Two participants dropped out because they tested positive for SARS-COVID19 and five participants dropped out for personal reasons not related to the study. Written informed consent was obtained from each participant after a thorough explanation of the testing protocol, the possible risks involved and the right to terminate participation at will. The study was approved by the Institutional Review Board of the University of the Basque Country UPV/EHU (ref. 118/2019) and all procedures were done in accordance with declaration of Helsinki (2013). Participants were analysed by the intention-to-treat principle in order to avoid the risk of bias (McCoy 2017) even if 2 participants did not reach the 90% of attendance to the planned training sessions.
Procedures
Measurements were performed 2 days prior to the beginning of the training program and 72–96 h after the end of it. Measurements lasted ~30 min and testing sessions were carefully scheduled to ensure that the same number of hours were left since the end of the training protocol. During the day of the first measurements, participants were randomly allocated into the INC and PREA groups. Participants in the INC group completed a 9-week resistance training program consisting of 4 sets of 12 repetitions of inclined bicep curls three times per week taking each set to muscular failure. Inclined bicep curls consisted on performing bicep curls with a dumbbell while lying supine in a bench with 45º inclination. Exercise began with the forearm perpendicular to the floor, and ended when the elbow angle was less than 90º. Participants in the PREA group completed the same training protocol, but performed the preacher bicep curl instead of the inclined bicep curl. The preacher bicep curl consisted on performing bicep curls with a dumbbell in a Scott bench. A Scott bench is a special kind of bench consisting on a 50º inclined platform where the participant places the arm while performing de curling movement. The exercise begins with the forearm parallel to the floor or slightly below, and ends when the forearm forms a 75º angle with the floor, never reaching a position perpendicular to the floor. Participants in both groups were requested not to perform any other biceps brachii or pulling exercises (also known as back exercises), that would directly or indirectly affect the stimulus provided by the exercises of the study. To make it easier for the participants to organize their training sessions with the conditions of the study, they were offered different training plans according to the number of days they wanted to train. They were also encouraged not to perform strenuous pulling or bicep-demanding activities during the period of the study.
Weekly training volume consisted of 12 weekly sets (Baz-Valle et al. 2018) for the biceps brachii exercise and varied between 60 and 85 sets for the rest of the muscles (depending on the number of days that each subject trained). This amount of weekly sets follows current guidelines of weekly resistance exercise volume to maximize muscle growth (Baz-Valle et al. 2022).
Training was periodized in a flexible fashion: the first day, participants were told to adjust the weight so that they could reach muscular failure at 12 repetitions. For that purpose, participants were told to choose a weight that would make them reach but not exceed 12 repetitions on their exercise. If they were able to perform 1–3 extra repetitions with the chosen weight, they had to use the next heaviest dumbbell. If the weight difference between dumbbells was too high (in some gyms, the weight difference between one dumbbell and the next heaviest one is 2.5 kg. As most participants lifted 10 kg, that’s a 25% increase), participants were given to options: They could lift a heavier weight and perform as many repetitions as possible trying to reach 12, or perform more repetitions than 12, always reaching volitional failure. Participants were suggested to lift a heavier weight if they could perform more than 6 repetitions with it or to choose performing more repetitions if they felt they couldn’t, but to never exceed 18 repetitions. We consider that the fact that the subjects weren’t performing the same number of repetitions occasionally didn’t had any effect on the results, as muscle size improvements are not affected by the number of repetitions performed while the repetition range does not exceed 20 or go under 6 (Baz-Valle et al. 2018; Schoenfeld et al. 2021)
As the duration of each of the phases of an exercise seems to induce regional hypertrophy (Diniz et al. 2020), the eccentric phase of the exercise had to last 2 seconds, and the concentric part of the exercise had to be performed as fast as possible for participants in both groups. Participants were also requested to rest at least 48 h between training sessions. Resting between repetitions was not allowed, and participants had to rest from 3 to 5 min between sets. Training sessions took place at the gyms in which each participant usually trained. To test whether the participants performed the exercises with the correct form, and with the right intensity, each participant had to send a video performing an effective set to the main researcher after every session. Participants registered the weight lifted in each set in a mobile phone APP (Dudysolutions version 2.3, 2020 Spain) so that they could see the weight lifted and the repetitions performed in previous sessions, and try perform better.
Muscle thickness
Muscle size/thickness was measured using B-mode ultrasound imaging (GE LOGIQTM e, GE Healthcare, WI, USA) with a linear-array transducer (code 12 L-RS, variable frequency band 4.2–13.0 Mhz, field of view 3.7 mm). Measurements were performed by an experienced technician with participants in a supine position, with arms and legs extended and relaxed. Prior to testing, participants remained in this position for 10 minutes to allow for stabilization of normal body fluids. The technician then applied a water-soluble transmission gel (Aquasonic 100 Ultrasound Transmission gel; Parker Laboratories Inc., Fairfield, NJ, USA) to each measurement site and a 9 MHz ultrasound probe was placed parallel to the tissue interface without depressing the skin. When the quality of the image was deemed as satisfactory, the technician saved the image to the hard drive. Scan acquisition sites were determined by measuring the 50%, 60% and 70% of the distance between the posterior crest of acromion and the olecranon as performed in previous articles (Matta et al. 2011). When determined, a circle was drown around the circumference of the arm in those three spots. After that, the center of the biceps brachii muscle was calculated as a line between the coracoid process and the anterocubital crest. The scan was taken in the place were both lines crossed. Thickness was measured using the line straight function of ImageJ software (Kim et al. 2020) in the center of the scan (Figure 2). Measurements were taken on the right side of the body.
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Body fat percentage and anthropometric measurements
One day before and 3–4 days after the 9-week intervention, anthropometric characteristics of the participants were measured. Participants were weighed on a calibrated digital scale whilst wearing minimal clothing. Height was measured with a stadiometer attached to the scale with participants standing shoeless and head aligned in the horizontal Frankfurt plane. Eight-site skinfold measurements (in mm) were taken from the biceps brachii, triceps, scapular, abdominal, suprailiac, thigh and medial calf sites according to standard procedures using a skinfold caliper (Harpenden1, Baty International, West Sussex, UK). All skinfolds were measured to the nearest 1 mm and the mean of three readings was recorded as the final value for each site. All body composition measurements were taken by the same investigator 24–48 h before and 72–96 h after completion of the training protocol. Body fat percentage (%BF) was estimated using the equation proposed by Faulkner (Faulkner 1966).
Dietary Adherence
To avoid the potential for dietary confounding, participants were given a document in which they were instructed to reach 2 g·kg-1 of protein intake. First, they were taught how to calculate how much protein they needed according to their bodyweight. Then, they were instructed on how to reach that amount with examples of different dishes and were requested to include at least 20 g of protein per meal divided into 3 to 5 meals with at least 3 hours between them. Participants were also trained to be in an eucaloric diet or slight energy surplus. Participants also agreed not to take any supplements that could interfere with the study outcomes (such as creatine or whey protein).
Statistical analyses
We tested all variables for normal distribution (Shapiro-Wilk test) and homogeneity of variances (Levene’s test). An independent sample t test was used to check for significant differences in MT and %BF variables prior to the beginning of the study. If Levene’s test was significant, Welch’s correction method was used. Paired sample t test were applied to compare the PRE and POST measurements of each region of each group to determine which of regions had grown during the intervention and whether fat percentage had changed or not. Wilcoxon signed rank test was used if the variable did not have a normal distribution. Cohen’s d was calculated to analyze the magnitude of the potential PRE-POST differences between participants if the variable had a normal distribution. Rank biserial correlation was used for the non-parametric variables. Effect sizes were interpreted as small (<0.3), moderate (≥0.3 and <0.5), and large ≥0.5) according to the scale proposed by Cohen (Cohen, 1988) when the variable was parametric and as and small (<0.1), medium (≥0.3 and <0.5) or large (≥0.5) when the variable was non-parametric. A multifactorial ANCOVA was performed in order to check growth differences between regions (Group x region x time). For the ANCOVAs, effect sizes were calculated as partial η2. In this case, effect sizes were considered irrelevant (<0.01); small (≥0.1 and <0.6); medium (≥0.6 and <0.14); or large (≥0.14). An intra-class correlation coefficient test was performed to measure the reliability of the measurement protocol, in a pilot study carried out with 10 participants. All the statistical analyses were performed with JASP 0.16 for Mac. The level of significance was set at p<0.05.