The aim of this study was to examine the association between muscle quantity and quality and muscle strength in adults with obesity. The main finding showed that muscle mass quantity and quality can be used to predict muscle strength in the adult population with obesity. This finding is important as there is considerable heterogeneity between subjects regarding strength and muscle mass ratios, and it reveals the involvement of the muscle’s intrinsic aspects in the ability to generate force, in addition to the quantity.
The association between the amount of muscle mass and muscle strength found in the results was expected, even in an obese population. Although the relationship between muscle quantity and muscle strength cannot be ignored, there is a consensus that this relationship is not quite clear and that losses or gains in strength are only partially explained by decreases or increases in muscle mass. Producing force depends on neural, neuromuscular, and muscle tissue characteristics , while muscle tissue modulates the functional capacity of skeletal muscle through muscle architecture, composition, metabolism, fat infiltration, and fibrosis . When maximum muscle strength is normalized to body mass in individuals with obesity, for example, they seem to be weaker, and this relative weakness can be caused by different determinants, including changes in muscle morphology . Thus, it is reasonable to assume that it is more difficult to establish a direct relationship between the amount of muscle mass and muscle strength as body fat increases.
Although the results of the study indicate an association between both muscle quantity and quality and muscle strength, intracellular water (quality indicator) demonstrated the best predictive power for muscle strength. This information reiterates the importance of assessing muscle quality, as it aggregates different components of the functional structure and better represents the variability of muscle performance. In the specific case of ICW, it is usually proportional to the total amount and size of muscle fibers in the segment , and it has a good correlation with muscle function and strength . Obesity and age tend to increase the extracellular space and reduce the concentration of intracellular water, which directly affects the muscle’s functional properties. Thus, the excess fat mass that normally coexists with low muscle mass and function tends to reduce muscle quality and muscle strength as a consequence , thereby increasing the risk for health problems such as metabolic syndrome, hospitalization, and higher mortality . Furthermore, excess fat mass favors the infiltration of ectopic fat into the muscle (myosteatosis), which can facilitate the transition process of muscle fibers from type II to type I and reduce contractile function, power, and mobility .
In addition to assessing muscle quality using ICW, the ratio of intracellular water to lean mass was also investigated. An association between this muscle quality indicator and muscle strength was also observed, but to a lesser extent. Serra-Prat et al.  examined a sample of older adults with obesity and also found a positive association between ICW/LM and muscle strength . The ICW/LM ratio is based on the hypothesis of reduced muscle cells or reduced hydration, which is plausible in the obese population. Regardless of the reason, the altered cell volume determined by ICW seems to be an important metabolic signal that regulates cell and muscle functions . However, little is known about muscle quality indicators, including the ICW/LM ratio in the adult population with obesity. The few studies that exist were conducted with older adults and even considered the presence of predisposing factors in this population (cellular dehydration and MM reduction). Therefore, it is still not possible to clearly determine the impact of excess body fat on muscle quality through ICW/LM.
In this context, the literature reports that uniformity and clarity are still needed to define muscle quality and the measurement method . Despite these gaps, the findings of this study contribute to amplifying the view of the muscle’s functional qualities in the adult population with obesity. For reasons that have been widely explained, decreases in muscle strength tend to occur faster than decreases in muscle mass. However, this condition may be even more evident in the obese population when intrinsic aspects of muscle function are considered. The recommendation in this case is to consider muscle quality in the preliminary assessments and in prescribing physical exercises for subjects with obesity, which should focus on improving body composition and reducing body weight qualitatively. High muscle quality, even in subjects with low muscle mass, can reduce the risk of functional deficiencies  associated with age and excess body fat.
Finally, assessing muscle quality in younger populations is also relevant given the possibility to alleviate aging complications. Many determinants of muscle function, such as obesity, physical activity, eating habits, cardiovascular and metabolic diseases, play an early role and have significant impacts on muscle strength and quality [31, 22]. It is therefore necessary to consider reduced muscle strength and poor muscle quality as population problems that precede senescence. There have been reports of a high prevalence (84%) of poor muscle quality in the obese population (18–75 years), which was mainly attributed to low specific strength, and age, sex, and BMI were independent predictors .
This study broadens the perspective for analyzing muscle strength predictors by adopting muscle quantity and different muscle quality indicators (ICW and ICW/ML ratio). This study is unique in that it examines the adult population with obesity, whose relation to this particular topic has not been thoroughly examined, and in its analyses adjusted for important determinants such as physical activity and protein consumption. Moreover, the use of BIA deserves attention since it is portable, accessible, and considered to be appropriate for determining the amount of body water. It can be used daily in the clinical environment to assess both muscle quality and quantity.
Muscle quantity and quality demonstrated good predictive capacity for muscle strength in obese adults. In this population, muscle mass and muscle strength are extremely important for good functional and metabolic capacity and for preventing health problems. Since it represents aspects of the muscle’s functional structure, assessing muscle quality can enhance quantitative analyses of muscle mass and provide further clarification on the muscle’s capacity to produce strength.