Measuring the strength of hip muscles can be a difficult task. Several limitations have been reported regarding this task, including limited access to accurate equipment, difficulties in positioning the patient properly, variations in the support area on which the device is placed, the possibility of patient movement, and inconsistency in the verbal stimulus intensity [27]. Manual muscle testing is the most commonly used method for this purpose since it is easy and quick to perform, is free of charge, and does not require equipment [32]. Nevertheless, this test is subjective and descriptive, so it leads to low reliability and frequently overestimates the actual strength. The gold standard device is an isokinetic dynamometer for measuring strength [33]. Because of the cost of an isokinetic testing device and difficulties in routine clinical testing, there is evidence that supports the clinical use of the hand-held dynamometer in routine medical examinations [34]. This device measures strength in an objective, precise, and sensitive way [35]. The hand-held dynamometer is a valid method when it is stabilized by a belt, and although these devices do not yield the same measures as isokinetic dynamometers, the values for hip muscle groups are correlated [36]. Thus, we chose to use a digital dynamometer stabilized by a brace to measure of the strength of the gluteus medius muscle.
The determination of the force required by the abductor muscles to balance the body in a standing position depends on two variables: pelvic anatomy and body weight [37]. In the present study, since individuals were paired in relation to sex, age, and height, it is assumed that there was a similarity in the pelvic anatomy between the pairs. Since the examination was performed by a single examiner, variability in the measurements due to differences in the technique used were not observed. Weight was the only relevant variable that could interfere with the strength data.
In the present study, obese individuals did not present a statistically significant difference in gluteus medius strength compared to nonobese individuals (p > 0.05). The absolute strength values were 292.0 N for the RGM and 290.7 N for the LGM in the control group. In the obese group, the values were 256.2 N and 261.1 N, respectively.
In a literature review, Benfica et al. [38] reported the hip abductor muscle strength values in individuals aged between 50 and 59 years old to be 208.12 N for the dominant limb and 203.27 N for the nondominant limb in women and 305.97 N for the dominant limb and 298.49 N for non-dominant limbs in men. In the present study, this variation in the measurements can be explained by the age differences among the individuals included in the analysis, differences in sex, and differences related to the measurement technique.
The age range of the participants (from 20 to 60 years old) was chosen since it corresponds to an economically active group in whom movement disorders can greatly impact function and work. Additionally, people over 60 years of age may have reduced muscle mass and function [39].
It is worth noting that the study population in the present study consisted predominantly of women (92%) for reasons of convenience and that abductor muscle strength varies between sexes. Women have lower abductor muscle strength, which corresponds to a higher risk of developing musculoskeletal pathologies [40].
In contrast to these findings in our study, some authors suggest that the antigravity muscles of obese individuals generate higher absolute forces [41-44]. Increased muscle strength is described as a beneficial adaptation to obesity, with excess body weight acting as a chronic training stimulus for daily activities [44].
Several studies have reported increased knee extension strength in obese individuals, with values varying from 10 to 30% higher than those of normal-weight individuals [45]. However, gait analyses in obese individuals have shown a shorter stride length with a strategy involving quadriceps overloading and decreased hamstring activation [6,46]. Due to gait changes, obesity can cause mechanical adaptations that favour the use of the strongest muscles and minimize the use of the weakest ones.
Regarding the gluteus medius, Lerner et al. [11] reported that obese individuals have higher absolute strength during gait and correlated this change with an increased BMI, reflecting the same theory of overload adaptation. These data were not confirmed by the gluteus medius strength analysis performed in the present study since there was no difference in the isometric strength values between the groups tested (p > 0.05). An analysis using nuclear magnetic resonance suggested that the gluteal musculature presents an increase in fat infiltrate as the BMI increases [47]. Although obesity increases muscle mass in the short term in young individuals, lipid infiltration in skeletal muscle can reduce the incorporation of amino acids into muscle proteins over time, with a decrease in total muscle mass [17]. It is possible that the long-term effect of obesity on muscle tissue overlaps with this weight stimulus on antigravity muscles and culminates in muscle loss over time [15]. This possibility may justify the findings of our study.
When the gluteus medius strength values were normalized to body weight, there was a significant difference (p < 0.05) between groups, which indicates that obese individuals have relative gluteus medius weakness compared to normal-weight individuals. Obesity results in larger and lower quality muscles, which have the same absolute strength and power as smaller muscles in thin individuals [48]. However, obese individuals generally struggle to move their body mass. A lack of strength can culminate in functional adaptations and imbalance, predisposing these individuals to injuries [10,49].
Although some studies have suggested that obese individuals have higher absolute strength, they have less relative strength in some muscles, such as the quadriceps [50,51]. Lafortuna et al. [52] also corroborated these data when they evaluated lower limb muscle strength through a leg-press exercise. Compared with normal-weight individuals, the obese individuals were stronger, but when the values were normalized by muscle mass, this difference disappeared.
When Lerner et al. [11] normalized the strength of the gluteal muscles by weight, there was no relevant difference between the obese and normal-weight groups. Regarding muscle mass, the authors also reported that obese individuals required greater gluteal muscle strength for normal gait. This evidence is relevant since it suggests that obese individuals need stronger gluteal muscles, causing them to be more susceptible to fatigue. Thus, it was expected that overweight individuals have higher muscle strength to maintain balance while standing or walking. This fact was not proven by the results in the present study. When strength was normalized to body weight, the obese individuals had relative weakness in the gluteus medius muscle (p < 0.05).
The gluteal strength of obese individuals is a relevant factor since these two variables, obesity and weakness, are independently associated with musculoskeletal system changes [20-26,53]. Moreover, according to new scientific evidence, muscle strength is inversely and independently associated with all-cause mortality [54].
It can be concluded that strength alone does not seem to be an adequate parameter for assessing the abductor musculature since more than half of the world’s population is overweight and these strength values can be overestimated [55,56].
Some authors even recommend the use of an algorithm to remove the dependence on body size and to more appropriately compare the strength of the hip muscles across individuals since it cannot be concluded that the force is directly proportional to body weight [57,58].
When the statistical analysis of the factor loads was performed, it was possible to differentiate the two distinct groups for all gluteus medius force variables, regardless of whether they were normalized to body weight. This finding indicates that both the absolute strength values and those related to weight were different, constituting two distinct groups: the obese group and the normal-weight or control group.
The present study has some limitations. First, the sample size was small. Therefore, additional studies are needed to confirm and increase the generalizability of the results found. Second, the study population was predominantly composed of women (92%). Although this limitation did not interfere with the conclusions since the individuals were paired between groups, it can represent a bias because men are stronger than women and may not present similar results. Additional studies are needed to prove whether there are morphological and functional changes in obese gluteal muscles that may justify gait imbalances and associations with musculoskeletal disorders.