Forced spirometry is the principal test for evaluating lung function in individuals. The main factors determining spirometric results are sex, age, height, and weight, and these variables are used to produce equations predicting normal reference values. Several authors1,2 believe that there must be other factors that determine spirometry results, which would explain the variability in test values obtained in healthy individuals with similar characteristics. The premise of this study is that one of the factors that might cause this variability could be muscle mass. Although some previous studies3–6 have already evaluated this possibility, we believe that the influence of an individual's muscle mass on spirometric results has not been clearly established.

The technique of choice for evaluating muscle mass is dual energy X-ray absorptiometry (DEXA). This is an imaging technique used to quantify lean body mass (fat-free mass) in both the whole body and in different body regions. Lean body mass is an estimation of muscle mass. This test offers reliable measurements that are comparable over time. It is, moreover, a technique widely used in respiratory clinics,7–9 mainly for assessing muscle mass in patients with chronic obstructive pulmonary disease (COPD), who often present muscle dysfunction of the ventilatory system and the lower limbs.10,11 Muscle training improves exercise tolerance, muscle strength, dyspnea, fatigue, and quality of life in these patients. The impact of muscle mass on the symptoms and prognosis of asthmatics has also been studied.12,13 Increasing numbers of studies are using DEXA to evaluate the outcome of muscle training in respiratory rehabilitation programs in COPD patients and to determine their prognosis.14–17 For this reason, we identified a need to establish the association between spirometric values and muscle mass parameters determined by DEXA in healthy individuals.

The aim of this study was to quantify the association between spirometric variables and muscle mass estimated by DEXA, to establish whether muscle mass is an important determinant in the spirometric results of healthy individuals, and to help establish the normality of this association.

Descriptive statistics of the muscle mass and lung function variables in men and women are shown in Table 1.

All variables analyzed showed significant differences in mean values of both sexes, except age and sex. Means were higher in men, as shown in Table 1, with statistically significant p values.

In the principal components analysis, positive correlations between all spirometric variables and all lean mass variables were found in both sexes, with the greatest correlation being observed for LLM (Figs. 1 and 2).

Fig. 1.

Principal component analysis in men with an overall explanation of 76.07% (47.1% explains the first component and 28.97% the second). FEV1: forced expiratory volume in 1s; FVC: forced vital capacity; BMI: body mass index; PEF: peak expiratory flow; LLM: leg lean mass; TLM: total lean body mass; TrLM: trunk lean mass.

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Fig. 2.

Principal component analysis in women with an overall explanation of 73.03% (46.97% explains the first component and 26.05% the second). FEV1: Forced expiratory volume in 1s; FVC: forced vital capacity; BMI: body mass index; PEF: peak expiratory flow; LLM: leg lean mass; TLM: total lean body mass; TrLM: trunk lean mass.

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Partial correlations between lean mass and spirometric variables, adjusting for age and BMI, were positive and highly statistically significant in both women and men, with higher coefficients in the latter (Tables 2 and 3).

Variables showing higher correlation coefficients with FVC and with FEV1 were LLM in men and TrLM in women.

In the multiple linear regression analysis, equations to predict spirometric variables according to age, BMI and lean mass variables were obtained, along with adjusted coefficients of determination (R2) (Table 4).

R2 between lean mass and spirometric variables are greater in men. The spirometric variable with the highest values was FEV1, followed by FVC and PEF, in both men and women. In men, LLM is the variable that showed the highest R2 with spirometric variables, while in women, it was TrLM.

All estimated regression coefficients in lean mass were positive and statistically significant (p<0.05), suggesting that the greater the amount of lean body mass, the greater the absolute values obtained on spirometry. The highest coefficients were for LLM in men and TrLM in women.

The aim of this study was to establish the association between spirometric results and lean mass determined by DEXA in healthy individuals. Variability in the spirometry values obtained in healthy individuals of similar characteristics have led some authors1,2 to believe that factors other than sex, age, height, and weight might determine results. Determinants such as inherited genetic factors and the presence of certain vitamins in an individual's diet have been discussed. The premise of this study is that one of the factors that might cause this variability could be muscle mass. Other studies have attempted to establish the influence of the muscle component of the body on spirometric results in healthy individuals:

Cotes et al.3 found a significant association between the Fat-Free Mass Index (FFMI), in which lean mass is adjusted for subject height obtained from anthropometric measurements, and the main spirometric parameters. They used linear regression models to obtain reference equations that included FFMI, and found that these equations were more accurate than those which included only sex, age, height, and weight.

Mohamed et al.4 also measured a muscle mass parameter, bone-free lean body mass (BF-LBM), using DEXA, and, like us, found a significant positive association between that parameter and the main spirometric variables in a group of healthy Italian subjects. The age range is similar, but the series size is rather smaller than ours (58 men and 60 women), and the influence of regional lean mass variables was not studied.

More recently, Lim et al.,5 in a study of elderly Korean individuals with no chronic respiratory diseases, found a significant association between trunk and leg muscle measured by DEXA and the main spirometric parameters, in both women and men. Their results are similar to our findings, although only elderly subjects of non-Caucasian origin were included.

Park et al.6 also conducted a study in Korean subjects, but with a mean age of 46 years. They found a significant correlation between lean body mass – measured in this case by bioelectric impedance – and spirometric parameters, in both men and women. However, they did not obtain a sufficiently high R2 to conclude that muscle mass was a factor that influenced breathing in the study individuals.

Mean values of all respiratory parameters are significantly higher among men than women. The larger amount of total lean body mass in men might explain the difference in the spirometric results between both sexes, taking into account that respiratory mechanical properties are basically the same in both.20 Mean amounts of lean body mass in the different body regions are also higher in men. These results are in line with those of Aguado et al.,21 who determined total and regional lean mass by DEXA in a series of healthy individuals of both sexes, grouped according to age. These authors found consistently larger amounts of total and regional lean body mass in men.

One of the principal results of the study is the significant association of lean mass in the legs with the main spirometric parameters. In men, leg lean mass affected spirometric results even more than total lean mass or trunk lean mass, where the major ventilatory muscles are located. In the clinical setting, Hillman et al.22 believe that muscle strength and muscle mass in the legs are a possible determinant of the efficacy of training programs for pulmonary rehabilitation. They also observed that approximately 33% of patients with COPD presented muscle dysfunction in the lower limbs, even in the early stages of the disease.

While leg lean mass in women is also significantly associated with spirometric parameters, this is not the variable that shows the greatest association. Rather, the amount of muscle mass in the trunk has most impact on spirometric results. This difference between the sexes does not appear to be explained by differences in the regional distribution of lean mass, since no significant differences in this distribution were revealed in the study. In both sexes, the amount of lean mass is greater in the trunk than in the legs, and in similar proportions. The explanation for this difference might be the predominantly thoracic breathing pattern in women.

The possible direct relationship between muscle mass and strength may explain the association between lean mass parameters and spirometric values. This hypothesis is also supported by the fact that a significant association is also found between PEF values, the spirometric parameter most dependent on expiratory muscle strength, and lean mass values, in both men and women. In women, PEF is most associated with trunk mean mass. However, some authors23,24 found that strength does not depend exclusively on muscle mass, and the relationship between these parameters is not linear. The most widely used functional tests for measuring respiratory muscle strength are maximum inspiratory pressure (MIP) and maximum expiratory pressure (MEP). A further study, evaluating the association between lean mass determined by DEXA and MIP and MEP values would help clarify this issue. In this respect, Costes et al.25 found in a study of elderly individuals of both sexes that trunk lean mass determined by DEXA predicts MIP values, but only in women. Hillman et al.22 also found an association between lean mass in the legs and muscle strength determined by dynamometry in COPD patients.

Another result from our study is that all lean mass variables have more effect on FEV1 than on the other spirometric parameters, in both men and women. The greater capacity of the expiratory muscles to generate strength may lead to a greater volume of air being expired in the first second of the forced spirometry maneuver. A positive association is also seen between the amount of lean mass and FEF25%–75%, altered values of which suggest small airway obstruction, and may point to the presence of early-stage obstructive disease. This might mean that a lower mean muscle mass might be a factor in the appearance of obstructive spirometric patterns. In this respect, Minas et al.13 compared muscle mass measured by bioelectric impedance in smokers without COPD and in patients with COPD, and found it to be significantly lower in the latter, even in patients in mild stages of the disease. The same authors studied a group of patients with asthma, another of the most prevalent obstructive diseases. They found that patients with severe treatment-resistant asthma had lower muscle mass measured by bioelectric impedance than those with milder forms of asthma, suggesting that muscle mass may be used as a prognostic factor in these patients.

Another publication reports that up to one third of COPD patients had deteriorated muscle function in the lower limbs, even in the early stages of the disease.26 However, in our study, we did not find a significant association between lean mass parameters and FEV1/FVC index in either sex. We did not find, then, that lower lean mass was a factor for a lower FEV1/FVC index, which would belie the idea that lower muscle mass might be associated with a greater tendency to develop obstructive spirometric patterns.

Spirometric values in healthy individuals are known to peak in the third decade of life, in both men and women. These values then gradually decline with age.27 The course of muscle mass is similar: after reaching a peak, it too declines progressively over the years.28,29 The association between lean mass and spirometric parameters might suggest that loss of muscle mass is one of the factors that might explain the age-related progressive decline in spirometric parameters, as well as deterioration of the mechanical properties of the ribcage and loss of elasticity and distensibility of the alveoli and the airway.

In conclusion, muscle mass has a significant influence on lung function in health individuals of both sexes, but this influence is greater in men.

The amount of lean mass in the legs is the variable that shows closest association with all spirometric parameters in men, while in women, it is trunk lean mass.

In both sexes, lean mass is mainly associated with FEV1 followed by FEV and PEF. However, it is not significantly related with the FEV1/FVC index.

The authors state that they have no conflict of interests.