This study was conducted to determine influence of body mass index on muscular mechanical properties in people with obesity. There was found a weak relation between BMI and the mechanical properties of the BB and BF muscles. The bilateral BB tone and elasticity decreased as BMI increased, and the left BF stiffness increased. Different mechanical properties were observed in sex comprasion base on BMI clasification. The BB and BF mechanical properties were affected more in males than females.
Resting muscle tone is classified into two categories as neural and non-neural. If there is no neural activation, muscle tone contains passive stiffness and viscoelastic properties (22). When all individuals were examined, a weak negative correlation was observed between BMI and the bilateral BB tone. The left BF tone showed a positive correlation. While the left BF tone was positively correlated in males, the bilateral BB tone was found to be weakly negatively correlated in females. This correlation in tone suggests that it is caused by different neural and muscular adaptations that people with obesity can develop in the lower extremity and upper extremity. In a study conducted with 12 people with obesity (BMI > 27) adolescent girls and 12 healthy girls, it was reported that with increased mechanical load in people with obesity, adaptation would occur in muscles and nerves, and as a result, people with obesity might have a larger pennation angle, anatomical cross-sectional area and muscle thickness (29). While this advantage in mechanical loading is observed in the positive direction in the lower extremity depending on the increase in weight, it may explain that it is in the negative direction in the upper extremity. The upper extremity, which lacks mechanical loading, and the reduced inactivity, may bring along a disadvantage that will result in the loss of the cross-sectional area and contractile components. In studies comparing athletes and sedentary individuals, it is stated that sedentary individuals have a smaller cross-sectional area (30). This opposing relationship proves that muscular and neural structures will develop different adaptations in the upper and lower extremities.
In the evaluations we performed in males and females, the right and left BB elasticity showed a similar weak positive correlation in all three groups. In the study in which Kocur et al. evaluated the relationship between the SCM muscle stiffness and elasticity and BMI, it was indicated to be highly correlated with elasticity and moderately correlated with stiffness (31). In a study comparing mechanical properties, it was reported that males with high BMI had lower biceps brachii elasticity than females (28). Interestingly, in our findings, a weak correlation with the bilateral BB elasticity in the upper extremity in all three groups (all individuals, males and females) was not observed in the bilateral BF elasticity in the lower extremity. Furthermore, no correlation was observed in the bilateral BB stiffness. A weak positive correlation was found between the left BF stiffness and BMI only in males. Fat infiltration into skeletal muscles in people with obesity can create higher muscle stiffness and reduce flexibility compared to the people with non-obesity group due to the limitation of range of motion and stable posture (28). Moreover, the increase in adipokines, which regulate the production of metalloproteinases, prostanoids, and cytokines in adipose tissue, can affect stiffness and flexibility in overweight and people with obesity (32). The different elasticity relationship in the lower and upper extremities suggests that it may be caused by changes in adipose tissue according to sex.
When all individuals were compared in the sub-groups according to BMI, decreased bilateral BB tone and elasticity were found in individuals who were first-degree people with obesity (Group 3). The right BB and left BF stiffness of overweight (Group 2) and first-degree people with obesity (Group 3) individuals was higher compared to normal and underweight individuals. Comparisons in males were close to the characteristics we obtained from all the individuals above, while in females, mechanical properties were not affected, except for the bilateral BB elasticity. Along with excessive weight gain, adipocyte hypertrophy, intramuscular adipose tissue infiltration, an increase in fibrous components (a decrease in contractile elements), a decrease in the size and number of muscle fibers can be said to be the causes of decreased elasticity and increased stiffness (33–35). However, at this point, we think that adaptations that develop in daily life according to the mechanical loads on the lower and upper extremities will be the primary cause. Increased BMI can affect stability and may provide a biomechanical advantage by increasing the stiffness and tone of the lower extremity muscles due to excessive trunk oscillations in stance or walking.
An inverse correlation between muscle tone and subcutaneous fat was previously observed in a study on sedentary individuals (22). Therefore, it can be assumed that more thickness of subcutaneous fat may alter the response of muscles, reduce their oscillation and frequency, and thus affect tone. In a study comparing female athletes with sedentary females, it was stated that athletes had low BMI values, which was the reason for the decrease in the percentage of subcutaneous fat and the high muscle tone found (30). The calculation of BMI using height and weight in our study may have limited our study in terms of not measuring subcutaneous fat tissue thickness. At this point, we think that regional fat deposition together with BMI may be important for future studies.
From a practical point of view, the increased tone and muscle stiffness in relation to BMI may lead to a decrease in the risk of falls, injury and overall muscular performance, resulting in a limitation in the ability to perform daily activities (36). In this study, increased BMI changes the mechanical properties of the muscles. The decrease in the muscular performance of people with obesity may indicate why physical activity is reduced or vice versa.