Inuence of Body Mass Index on Mechanical Properties in People With Obesity

Background: The study was to determine inuence of body mass index on muscular mechanical properties in people with obesity. Methods: A total of 300 individuals (mean age: 27.31±7.21 years) were participated. The participants were assigned in groups base on BMI classication (Group 1 (BMI=18.50-24.99 kg/m 2 ), Group 2 (BMI=25.00-29.99 kg/m 2 ), and Group 3 (BMI ≥ 30 kg/m 2 )). The biceps brachii (BB), biceps femoris (BF) were measured bilaterally using the "MyotonPRO" device. Results: All mechanical properties of the right and left BB muscle, left BF tone and stiffness were found signicantly difference between groups (p<0.05). The bilateral BB tone in Group 3 was lower than the other two groups. The right BB stiffness of Group 2 was found to be higher compared to the other two groups (p<0.05). While the right and left BB elasticity was similar in Groups 2 and 3, it was lower compared to Group 1 (p<0.05). The left BF tone and stiffness of Group 3 were found to be signicantly higher than Groups 1 and 2 (p<0.05). The right BB tone showed a weak negative correlation with BMI in females, and for left side in males. A weak positive correlation was found between the right and left BB elasticity and BMI in males and females. The left-right BF tone and left BF stiffness showed a weak positive correlation in males. Conclusions: The bilateral BB tone and elasticity decreased, and the left BF stiffness increased as BMI increased. Different mechanical properties were observed in sex comparasion base on BMI clasication. The BB and BF mechanical properties were affected more in males than females. than females. The muscular actions and eciency can adopt to inactivity. Therefore, our study demonstrated the obesity or higher BMI lead to the human body for developing different muscular adaption. The upper and lower extremity muscles acting differently for daily living activities or functions. The increased mechanical load in lower leg cause to rising stiffness of muscles. The upper extremity muscles, which lacks mechanical loading, may adversely affected. (UT) myotonometric there between the elasticity and and a moderate between the SCM and stiffness and BMI study investigating study shown found between studies Previous studies investigated 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.


Introduction
Obesity is a signi cant health problem that is gradually increasing. It can be de ned as excessive fat accumulation in a way that can disrupt health, and it predisposes to chronic diseases (1). The calculation of body mass index (BMI) is the simplest indicator of an increase in adipose tissue in the body and the frequently used method (2). According to BMI, individuals are evaluated as underweight (< 18.5 kg/m 2 ), normal (18.5-24.9 kg/m 2 ), overweight (25-29.9 kg/m 2 ), rst-degree obese (30-34.9 kg/m 2 ), second-degree obese (35-39.9 kg/m 2 ), and third-degree morbidly obese (≥ 40 kg/m 2 ) (3). A decrease or increase in BMI may be a factor in the formation of chronic diseases (4).
Obesity is closely related to adipose tissue, and it may have direct or indirect effects on physical activity and the musculoskeletal system (5). While the relationship of obesity with cardiovascular diseases and type 2 diabetes draws attention, its effects on the musculoskeletal system are less questioned.
Disorders that can affect bone health, such as osteoarthritis or osteoporosis, which threaten joint health due to overload, are the most well-known. There are few studies on the effects of increased BMI on the mechanical properties of muscles or the methods evaluated these properties (6). and movements were excluded from the study. It was stated that they should not consume alcohol for at least 24 hours and not engage in strenuous physical activity for at least 48 hours before the test (17). Individuals were divided into three subcategories according to sexes and BMI range: Group 1 (BMI = 18.50-24.99 kg/m 2 ) (n = 100), Group 2 (BMI = 25.00-29.99 kg/m 2 ) (n = 100), and Group 3 (BMI ≥ 30 kg/m 2 ) (n = 100).

Procedures
The ethics committee approval numbered 2020/101 and dated 16.12.2020 was obtained from the non-invasive research ethics committee of Hasan Kalyoncu University, Faculty of Health Sciences. All participants voluntarily involved and they were informed about the content and purpose of the study and signed the consent form The physical characteristics and demographic information of the individuals were recorded prior to the test. Weight was evaluated using an electronic scale GSE 450 (GSE Scale Systems, Novi, Michigan), and height was evaluated using a standard stadiometer. BMI was calculated by dividing the weight in kilograms by the square of height in meters.
The tone and viscoelastic properties of the biceps brachii (BB) and biceps femoris (BF) muscles were evaluated bilaterally using a Myoton Pro (Müomeetria Ltd., Tallinn, Estonia) device. This device is known to have good to excellent reliability in healthy individuals (18,19). It can be used for objective diagnosis and monitoring in soft tissues in terms of validity and inter-user reliability (20,21).
The BB mechanical properties were evaluated by palpating the lateral end of the acromion and the cubital fossa in the middle from the ¾ of the distance between them with the individual in the resting supine position (22). Concerning the BF, the individual lay in the prone position and was asked to contract the hamstring muscle after placing a pillow under the ankle. The muscle was palpated while the individual was contracting it. Along with the contraction, the most prominent part of the muscle was marked and measured in muscle contraction, as suggested by Gavronski et al. (23). These muscles were preferred since they had been studied previously in many studies (24)(25). For each measurement, mean deviation, median and 95% con dence interval were given, and mean values obtained from three consecutive measurements from the reference points were used in statistical analysis.

Statistical Analysis
Descriptive statistics were presented as mean ± standard deviation. The Shapiro-Wilk test was used to check whether the data were normally distributed. The Mann-Whitney U test was used to compare differences between males and females (sex), and the Kruskal-Wallis test was used to compare differences in three groups (according to BMI range) for non-normally distributed data. Post-hoc binary comparisons (after Dunn's correction) were used to determine the source of the difference. The relationship between numerical variables was evaluated by Spearman correlation. As Spearman's rank correlation coe cient, 0.00-0.10 was interpreted as very weak correlation or no correlation, 0.10-0.39 as weak correlation, 0.40-0.69 as moderate correlation, 0.70-0.89 as high correlation, and 0.90-1.00 was interpreted as very strong correlation (27).
Statistical analysis was conducted using Windows version 24.0 for SPSS (IBM Corp. Armonk, NY IBM Corp.), and the value p < 0.05 was considered statistically signi cant. The minimum total number of participants required for each group was determined to be 44 (α = 0.01) in order to determine the expectation that there would be a signi cant difference between three different BMI groups at the large effect level (f = 0.75) obtained by referring to the published article with a power of 0.90. G-power program version 3.9.1.7 was used in power analysis (28).
Correlation between the BB and BF mechanical properties and BMI All individuals A weak negative correlation was found between the right and left BB tone and BMI (r= -0.177-p = 0.002, r= -0.157-p = 0.006, respectively). A weak positive correlation was revealed between the right and left BB elasticity and BMI (r = 0.258-p = 0.000, r = 0.211-p = 0.000, respectively). No correlation was determined in the bilateral BB stiffness (p > 0.05). A weak positive correlation was found between the left BF stiffness and tone and BMI (r = 0.164-p = 0.004, r = 0.143-p = 0.013, respectively). No correlation was detected in other mechanical parameters of the BF (p > 0.05)( Table 1).   Table 1).

Males
A weak positive correlation was revealed between the right and left BB elasticity and BMI (r = 0.285-p = 0.000, r = 0.199-p = 0.015). No correlation was found in the other parameters of the BB (p > 0.05).
A weak positive correlation was observed with the left BF stiffness and tone (r = 0.284-p = 0.000, r = 0.301-p = 0.000, respectively). No correlation was found between the bilateral BF elasticity and BMI (p > 0.05) ( Table 1).

All individuals
A statistical difference was found in all mechanical properties of the right and left BB muscle, left BF tone and stiffness (p < 0.05) ( Table 2). When advanced statistical methods were used, it was observed that the bilateral BB tone in Group 3 was lower than the other two groups (p < 0.05). The right BB stiffness of Group 2 was found to be higher compared to the other two groups (p < 0.05). While the right and left BB elasticity was similar in Groups 2 and 3, and it was higher than Group 1 (p < 0.05). The left BF tone and stiffness of Group 3 were found to be signi cantly higher than Groups 1 and 2 (p < 0.05) ( Table 3).

Males
The statistical differences were found in the right BB all mechanical properties (stiffness, tone, elasticity) and the right BF stiffness, left BB stiffness and elasticity, left BF tone and stiffness base on BMI group comparasion (p < 0.05) ( Table 2). The right BB tone of Group 3 was found to be lower than the other two groups (p < 0.05). The right and left BB stiffness of Group 2 was determined to be higher than that of Groups 1 and 3 (p < 0.05). The left BF tone of Group 3 was higher than that of Group 1 and Group 2 (p < 0.05). The right and left BF stiffness increased as BMI increased, and it was found to be higher in Group 2 than Group 1 (p < 0.05). The right and left BB elasticity in Group 1 was higher then other two groups (p < 0.05) ( Table 3).

Females
The statistical differences were found in the right and left BB elasticity (p < 0.05) ( Table 2). The right BB elasticity was higher in Group 1 compared to the other groups (p < 0.05), the BB elasticity in Group 2 was better than in Group 3, but it was not statistically signi cant in Groups 2 and 3 (p > 0.05). Although the left BB elasticity was higher in Group 1 compared to the other groups (p < 0.05), no statistical difference was found between Group 1-2 and Group 2-3 (p > 0.05) ( Table 3).

Discussion
This study was conducted to determine in uence 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 clasi cation. The BB and BF mechanical properties were affected more in males than females.
Resting muscle tone is classi ed 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 ndings, 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 in ltration into skeletal muscles in people with obesity can create higher muscle stiffness and reduce exibility 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 exibility 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.