The purpose of this study was to examine the effects of 2 different resistance training regimens (L-S and L-S-BHT) on the muscle thickness of the gluteus maximum in untrained young women. The main finding of this study was that exercise selection may influence the magnitude of increases in muscle thickness of the gluteus maximus. Specifically, both training groups increased muscle thickness of the gluteus maximus in response to 10 weeks of regimented resistance training. Importantly, the L-S-BHT (i.e., performing 45º leg press, stiff-leg deadlift plus barbell hip thrust) elicited greater gains on the gluteus maximus size compared to the L-S (i.e., performing exclusively 45º leg press and stiff-leg deadlift). Therefore, our hypotheses that a) performing exclusively 45º leg press and stiff-leg deadlift effectively induces gluteus maximus hypertrophy and b) adding barbell hip thrust would elicit greater gains were confirmed. Potential mechanisms and explanations for our results are presented following.
The first finding of the present study was that exclusively performing the 45º leg press and stiff-leg deadlift was effective in inducing muscle hypertrophy of the gluteus maximus. Regarding the 45º leg press, our results are in line with a previous study that observed hypertrophy of the gluteus maximus after 8 weeks of leg press training (Popov et al., 2006). As for the stiff-leg deadlift, the activation of the gluteus maximus during this exercise is similar, for example, to the glutes activation during the squat (McCurdy et al., 2018). Moreover, the glutes are primarily hip extensors, thus, it is probably one of the main muscles to contribute during the stiff-leg deadlift (McCurdy et al., 2018). Thus, it is plausible to suggest that 45º leg press and stiff-leg deadlift acted synergistically to elicit gluteus maximus hypertrophy. Interestingly, based on the greater perpendicular distance between the axis of rotation and the line of force, both exercises likely impose peak forces in the position where the gluteus maximus is at longer lengths (i.e., at the start of the concentric phase). Considering the results observed in the present investigation—as well as on previous studies with leg press (Popov et al., 2006) and squat (Kubo et al., 2019)—it is possible that the hypertrophy resulting from these exercises may occur due to the high peak forces imposed while the gluteus maximus is more elongated.
Indeed, growing evidence suggests that training specific muscles at longer muscle lengths optimizes hypertrophy (Kassiano, Costa, Kunevaliki, et al., 2023; Kassiano, Costa, Nunes, et al., 2023; Maeo et al., 2022). Among the potential explanations for this finding, one that stands out is based on the length-tension relationship (Kassiano, Costa, Nunes, et al., 2023; Ward et al., 2010). Specifically, muscles that have fibers that work on the descending limb of the length-tension curve may experience greater overall mechanical tension (due to the sum of forces from the active and passive elements) when trained at longer muscle lengths (Lieber & Ward, 2011) and, as consequence, experience stretch-mediated hypertrophy (Kassiano, Costa, Nunes, et al., 2023; Nishikawa, 2020). This conceivably helps to explain, at least in part, the muscle hypertrophy of the gluteus maximus observed in the group that performed exclusively 45º leg press and stiff-leg deadlift. However, it is important to note that, to the best of our knowledge, the working sarcomere length ranges of the gluteus maximus have not yet been measured (Ottinger et al., 2022). Therefore, this explanation needs investigations to verify its support with empirical data.
The second finding of the present study was that adding the barbell hip thrust enhanced muscle growth of the gluteus maximus. Notably, EMG studies have reported heightened activation of the gluteus maximus during the barbell hip thrust, comparable to or greater than other exercises such as squats (Contreras et al., 2015; Delgado et al., 2019). Importantly, concerns have arisen regarding the effectiveness of sEMG amplitude as a valid indicator of the hypertrophic potential of an exercise (A. D. Vigotsky et al., 2022); denoting the importance of long-term investigations. In this sense, our study adds an important finding to the literature when presenting the hypertrophic effects of performing the barbell hip thrust. Regarding the potential explanations for the increased hypertrophy with the addition of the barbell hip thrust, we highlight exercise biomechanics. The barbell hip thrust imposes high forces throughout the entire lifting phase (Brazil et al., 2021; Contreras et al., 2016); thus, there are substantial peak forces in the full hip extension. Interestingly, the gluteus maximus has a greater internal moment arm length with increasing proximity to full hip extension (Ward et al., 2010). Therefore, the resistance profile of the barbell hip thrust fits with the ability of the gluteus maximus to produce force and contribute to hip extension torque. This may have enhanced the stimulus for the gluteus maximus.
This fit between the resistance profile of the barbell hip thrust and the contribution capacity of the gluteus maximus is important because other muscles (such as hamstrings, and adductor magnus) play an important role during the task of hip extension (Neumann, 2010; Ward et al., 2010). Notably, the contribution of the hamstrings and adductor magnus is greater with increased hip flexion and lower with increased hip extension (Neumann, 2010; Ward et al., 2010). Therefore, it is likely that during the performance of the barbell hip thrust, the gluteus maximus plays a greater role than during other exercises involving peak forces with increased hip flexion (e.g., leg press, stiff-leg deadlift, squat, etc.). Moreover, it is interesting to note that in the top portion of the barbell hip thrust, the gluteus maximus is at shorter muscle lengths. Theoretically, this factor conflicts with the previously raised hypothesis that the gluteus maximus fibers work on the descending limb of the length-tension curve. In this sense, it is plausible to suggest that the gluteus maximus has fibers that work on the descending limb as well as in the plateau portion (Ottinger et al., 2022). This conceivably helps to reconcile the fact that the gluteus hypertrophies in longer and shorter lengths.
Our study has limitations that need to be addressed. First, in both groups, the subjects performed a combination of exercises; for the L-S group, 45º leg press and stiff-leg deadlift, for the L-S-BHT group, 45º leg press, and stiff-leg deadlift, plus barbell hip thrust. This characteristic of the training program prevents us from inferring how much each exercise, in particular, contributed to the hypertrophy observed in the present study. Especially when trying to determine how much hypertrophy was induced by the 45º leg press and the stiff leg deadlift individually. Therefore, future studies should compare the hypertrophic effects of these exercises separately. It is important to note that hypertrophy-oriented resistance training programs are commonly comprised of a variety of exercises (Hackett, 2022; Kassiano et al., 2022). This denotes the external validity of our study. Second, we measured the changes in muscle thickness at one site of the gluteus maximus to express muscle hypertrophy; a more valid substitute would be the change in total muscle volume. Furthermore, given that the gluteus has different subdivisions, measurements in different regions could indicate whether the hypertrophy could be regional. Finally, we measured exclusively untrained young women. Thus, future studies should consider testing subjects of different ages, sex, or training status.