The volume of adult female lungs is typically 10–12% smaller than that of males who have the same height and age. In this study, we investigated how this volume difference is distributed between the rib cage and the diaphragm abdomen compartments. Internal rib cage dimensions, diaphragm position relative to spine, and diaphragm length were compared in 21 normal male and 19 normal female subjects at three different lung volumes using anterior–posterior and lateral chest radiographs. At all lung volumes examined, females had smaller radial rib cage dimensions in relationship to height than males, a greater inclination of ribs, a comparable diaphragm dome position relative to the spine, and a shorter diaphragm length. Female subjects exhibited a greater inspiratory rib cage muscle contribution during resting breathing than males, presumably reflecting an improved mechanical advantage conferred to these muscles by the greater inclination of ribs. Because of a greater inclination of ribs, female rib cages could accommodate a greater volume expansion. The results suggest a disproportionate growth of the rib cage in females relative to the lung, which would be well suited to accommodate large abdominal volume displacements as in pregnancy.
All recommended normal prediction equations for lung volumes predict smaller values in females than in males having the same height and age (1). The smaller lung volume of females appears to be established in the first few years of life and is attributable to a lower rate of alveolar multiplication in girls than boys (2). The reason for the lower rate of alveolar multiplication is unknown. Within each sex, there is also a substantial variation in the volume of lungs among subjects having the same age and stature (3). Both findings suggest that lung growth is not very tightly coupled to longitudinal growth. A disproportionate growth of the lower limbs has been suggested as a factor contributing to the difference in lung size between males and females who have the same stature (4). However, the inclusion of sitting height in regression models for lung size (4) or referencing lung volume to thoracic spinal height instead of body height (5) does not abolish the difference between males and females.
In a recent study, we have found that the smaller lung volume of females could be entirely accounted for by smaller radial rib cage dimensions, the axial dimension, which is determined by the position of the diaphragm, being if anything greater in females than in males of the same height (5). These findings suggested important sex differences not only in the volume of lungs but also in thoracoabdominal configuration. Differences in thoracoabdominal configuration could impact the function of the respiratory muscles (6, 7). The primary objectives of this study were to compare thoracic configuration in normal male and female subjects of various ages and to assess the impact that this may have on the function of the respiratory muscles. Some of the results of this study have been the object of a previous report in the form of an abstract (8).
After approval by the institutional Human Ethics Committee, 40 normal subjects (21 males and 19 females) were recruited from advertisement in a local newspaper and within the hospital. They had to be nonsmokers, without a history of lung, cardiovascular, systemic, or neuromuscular diseases and free of medication and at least 3 months away from any form of acute lung infection. Body weight had to be less than 120% ideal body weight. Eligible subjects were invited for a physical examination, a resting supine electrocardiogram, and a complete pulmonary function testing, all of which had to be within normal limits. All signed a written informed consent in which the risk of ionizing radiation for lung cancer estimated to be equivalent to the risk of smoking several cigarettes was specified.
Chest radiographs served to measure thoracic dimensions and diaphragm length (5, 9–13). Details of the techniques and measurements can be found in two recent publications (11, 12). Briefly, subjects stood with their heel, calf, buttocks, back, and head fixed with a strap against a vertical backboard. Lateral and anterior–posterior films were taken in this position at the end of a normal expiration, that is, FRC, after a full inspiration to total lung capacity (TLC) and at the end of a full expiration to residual volume (RV).
Internal anterior–posterior and lateral diameters of the rib cage were measured at the level of the third, fifth, seventh, and ninth vertebrae and ribs, respectively, and were averaged (5) (Figure 1)

Figure 1. Schematic diagrams of the measurements made on lateral (A) and anterior–posterior (B) projections. (a) Internal anterior–posterior rib cage diameters. (b) the angle formed by the sixth rib with the vertical. (c) The length of the contours of the right hemidiaphragm (thicker lines). (d) Internal lateral diameters of the rib cage. (e) The height of the dome of the left and right hemidiaphragms.
[More] [Minimize]Diaphragm length (9–12) (Figure 1) was measured from diaphragmatic contours on the right side using anatomic landmarks to identify diaphragm insertions on the chest wall and divided into the length of visible contours and the length of rib cage–apposed zone. A shape factor for the dome was calculated as the ratio between the length of visible contours and the length of chords intersecting contours end points (14).
Magnification factors were calculated based on each subject's geometry and applied to all dimensions measured (15). All dimensions were normalized to standing height (in meters).
In a separate experiment carried the same day, esophageal, gastric, and transdiaphragmatic pressures were recorded conventionally with balloon-tipped catheters during relaxation at FRC, during 1 minute of resting breathing and during inspiratory and expiratory capacity maneuvers each repeated at least three times. Lung volume changes were measured with a mouth piece–pneumotachograph assembly.
Descriptive statistics (16) and a general linear model of analysis of covariance were used for between-group comparisons, with age as covariate; p values of less than 0.05 were considered statistically significant. Values in text and tables are group means ± 1 SD and in figures group means ± 1 SEM. All computations were performed with commercially available statistical software (SPSS version 10 for Windows; SPSS, Chicago, IL).
The physical and pulmonary function data in the two groups are summarized in Table 1
Males | Females | |
---|---|---|
n | 21 | 19 |
Age, yr | 44.1 ± 18.9 | 48.2 ± 19.5 |
Weight, kg | 70.7 ± 10.9 | 57.3 ± 6.1 |
Height, cm | 171 ± 4.9 | 159.9 ± 7 |
Percentage ideal body weight | 103.8 ± 7.8 | 104.2 ± 8.3 |
FVC, % predicted | 109.4 ± 11 | 114.3 ± 13.1 |
FEV1, % predicted | 107.1 ± 13.1 | 111.6 ± 13.7 |
FEV1/FVC, % | 80.5 ± 7.2 | 81.2 ± 4.1 |
FRC, % predicted | 109 ± 18.1 | 104.8 ± 17.2 |
RV, % predicted | 109.5 ± 25.6 | 93.6 ± 27.3 |
TLC, % predicted | 107.5 ± 9.8 | 107.5 ± 12.2 |
Raw, % predicted | 99.1 ± 31.4 | 102 ± 35.2 |
DLCO/VA, % predicted | 118.6 ± 30.3 | 109.2 ± 16.2 |
In Table 2
Variable | Sex | RV | FRC | TLC |
---|---|---|---|---|
Volume, % predicted male TLC | M | 30.44 ± 10.31 | 53.71 ± 8.9 | 106.98 ± 11.29 |
F | 28.6 ± 9.37 | 51.39 ± 8.83 | 94.58 ± 11.25* | |
M + F | 29.56 ± 9.68 | 52.61 ± 8.83 | 101.1 ± 12.77 | |
Average anterior–posterior rib cage diameter, cm/m† | M | 8.61 ± 0.9 | 9.06 ± 0.83 | 10.46 ± 0.81 |
F | 8.05 ± 0.7* | 8.47 ± 0.7* | 10.23 ± 0.83 | |
M + F | 8.34 ± 0.85 | 8.78 ± 0.82 | 10.35 ± 0.82 | |
Average lateral rib cage diameter, cm/m† | M | 14.16 ± 0.76 | 14.45 ± 0.73 | 15.26 ± 0.75 |
F | 13.68 ± 0.62* | 14.06 ± 0.77 | 14.76 ± 0.67* | |
M + F | 13.93 ± 0.73 | 14.26 ± 0.76 | 15.02 ± 0.75 | |
Average rib cage cross-section area, cm2/m2† | M | 95.87 ± 12.47 | 102.97 ± 11.82 | 125.4 ± 12.44 |
F | 86.65 ± 9.88* | 93.64 ± 11.15* | 118.68 ± 11.17* | |
M + F | 91.49 ± 12.11 | 98.54 ± 12.3 | 122.21 ± 12.19 | |
Thoracic index | M | 0.61 ± 0.06 | 0.63 ± 0.06 | 0.69 ± 0.06 |
F | 0.59 ± 0.05 | 0.6 ± 0.05 | 0.69 ± 0.06 | |
M + F | 0.6 ± 0.06 | 0.62 ± 0.05 | 0.69 ± 0.06 | |
Rib angle, degree | M | 51.78 ± 8.26 | 55.67 ± 6.5 | 63 ± 6.13 |
F | 47.16 ± 4.22* | 51 ± 4.57* | 60.53 ± 6.59 | |
M + F | 49.41 ± 6.83 | 53.27 ± 6 | 61.73 ± 6.4 | |
Average diaphragm height, cm/m† | M | 10.45 ± 1.21 | 12.14 ± 1.12 | 13.26 ± 1.24 |
F | 10.97 ± 1.1 | 12.4 ± 1.26 | 13.26 ± 1.24 | |
M + F | 10.7 ± 1.17 | 12.27 ± 1.18 | 13.26 ± 1.23 |
The relationships between rib cage anterior–posterior and lateral diameters and between rib cage cross-section area and diaphragm dome height are shown in Figures 2A and 2B

Figure 2. Thoracic dimensions and shape in males and females. (A) Relationship between anterior–posterior and lateral rib cage diameters in males (closed squares) and females (open squares). Data points are group means ± 1 SEM at three different lung volumes that are connected by solid lines. (B) Relationship between estimated rib cage cross-section area and the height of the diaphragm domes below the first thoracic vertebra. Diagonal dotted lines represent lung volume isopleths with a slope of −20 as determined by linear regression of lung volume on rib cage cross-section area and diaphragm height. Other features are as in (A). The relationship is shifted downward in females, showing smaller radial rib cage dimensions and greater axial dimension of the thorax in females than males at any given lung volume.
[More] [Minimize]Diaphragm length and shape indices are listed in Table 3
Variable | Sex | RV | FRC | TLC |
---|---|---|---|---|
Diaphragm length, cm/m† | M | 36.83 ± 4.26 | 29.44 ± 2.78 | 22.96 ± 2.21 |
F | 33.76 ± 2.97* | 27.29 ± 2.29* | 21.01 ± 1.4* | |
M + F | 35.38 ± 3.97 | 28.42 ± 2.75 | 22.04 ± 2.09 | |
Length of the visible contours, cm/m† | M | 21.24 ± 1.76 | 20.67 ± 1.31 | — |
F | 19.82 ± 1.53* | 19.39 ± 1.53* | — | |
M + F | 20.57 ± 1.79 | 20.06 ± 1.54 | — | |
Length of the zone of apposition, cm/m† | M | 15.59 ± 3.72 | 8.77 ± 2.05 | — |
F | 13.94 ± 2.83 | 7.89 ± 1.72 | — | |
M + F | 14.81 ± 3.39 | 8.35 ± 1.93 | — | |
Dome shape factor on anterior–posterior projections | M | 1.17 ± 0.1 | 1.1 ± 0.1 | 1.1 ± 0.09 |
F | 1.14 ± 0.12 | 1.11 ± 0.11 | 1.1 ± 0.1 | |
M + F | 1.16 ± 0.11 | 1.11 ± 0.11 | 2 ± 0.09 | |
Dome shape factors on lateral projections | M | 1.2 ± 0.06 | 1.15 ± 0.06 | 1.21 ± 0.06 |
F | 1.23 ± 0.04* | 1.18 ± 0.05* | 1.21 ± 0.07 | |
M + F | 1.21 ± 0.06 | 1.16 ± 0.06 | 1.21 ± 0.07 |

Figure 3. Diaphragm length in males and females. (A) Comparison of the length of the visible contour of the diaphragm (open triangles) and of the length of the diaphragm in the zone of apposition with the rib cage (closed squares) between males (abscissa) and females (ordinate) at two different lung volumes (functional residual capacity and residual volume). Data points are group mean ± 1 SEM. Error bars for visible contours were too small to be reproduced satisfactorily and were therefore omitted. The solid diagonal is the line of identity and has a slope of 1. The dotted line has a slope of 0.9. (B) Comparison of diaphragm length (open diamonds) between males and females at three different lung volumes (same volumes as in [A] plus total lung capacity). Other features are as in (A). In both (A) and (B), all measures of length are normalized to standing height. These show that at any given lung volume, the diaphragm is shorter in females than in males having the same height.
[More] [Minimize]Mean values of esophageal, gastric, and transdiaphragmatic pressures obtained in the various conditions of this study are listed in Table 4
Maneuver | Sex | Volume (L) | Pes (cm H2O) | Pga (cm H2O) | Pdi (cm H2O) |
---|---|---|---|---|---|
End expiration | M | — | −3.92 ± 3.03 | 8.76 ± 3.25 | 12.69 ± 3.96 |
F | — | −2.22 ± 2.95 | 10.37 ± 4.33 | 12.59 ± 3.69 | |
M + F | — | −3.12 ± 3.08 | 9.52 ± 3.83 | 12.64 ± 3.79 | |
Tidal breathing† | M | 0.79 ± 0.31 | −3.83 ± 2.16 | 6.81 ± 2.19 | 10.67 ± 3.84 |
F | 0.60 ± 0.2* | −4.14 ± 1.72 | 4.82 ± 2.42* | 9.06 ± 2.94 | |
M + F | 0.70 ± 0.28 | −3.98 ± 1.94 | 5.84 ± 2.49 | 9.89 ± 3.49 | |
Inspiratory capacity† | M | 3.06 ± 0.52 | −26.0 ± 12.0 | 13.79 ± 11.85 | 39.85 ± 20.32 |
F | 2.11 ± 0.28* | −21.93 ± 8.99 | 8.44 ± 12.35 | 29.92 ± 15.38 | |
M + F | 2.60 ± 0.64 | −24.02 ± 10.7 | 11.18 ± 12.24 | 35.02 ± 18.54 | |
Expiratory capacity† | M | −1.24 ± 0.55 | 65.7 ± 41.32 | 79.02 ± 38.95 | 13.25 ± 25.69 |
F | −9 ± 0.4* | 50.41 ± 29.89 | 57.46 ± 26.95* | 7.04 ± 19.46 | |
M + F | −1.07 ± 0.51 | 55.67 ± 33.15 | 66.3 ± 32.53 | 10.63 ± 22.95 |
During inspiratory capacity maneuvers, inspiratory reserve volume was significantly smaller in females than males, but esophageal, gastric, and trandiaphragmatic pressure changes were not significantly different. The ratio of esophageal to transdiaphragmatic pressure changes tended to be greater (i.e., more negative) (−0.93 ± 0.59 vs. −0.68 ± 0.18, p = 0.092) and of gastric to transdiaphragmatic pressure changes lower (0.17 ± 0.31 vs. 0.32 ± 0.17, p = 0.07) in females than males, although not significantly so.
During expiratory capacity maneuvers, expired volume was significantly lower in females than males, but esophageal pressure and transdiaphragmatic pressure changes were not. Gastric pressure changes were significantly lower in females than males, but pressure ratios, including the ratio of esophageal to gastric pressure changes (0.83 ± 0.40 vs. 0.90 ± 0.39, p = 0.491), were not significantly different.
During resting breathing, tidal volume was significantly smaller in females than males, but esophageal pressure swings were not significantly different, thus reflecting the smaller lung compliance and lung size in females. Gastric pressure swings, however, were significantly lower in females (p < 0.005). Transdiaphragmatic pressure swings also tended to be smaller in females than males though not significantly (p = 0.096). The ratios of esophageal to transdiaphragmatic pressure swings were significantly greater (i.e., more negative) (−0.48 ± 0.17 vs. −0.34 ± 0.11, p < 0.009), and ratios of gastric to transdiaphragmatic pressure swings were significantly smaller (0.52 ± 0.16 vs. 0.65 ± 0.1, p < 0.005) in females than males.
Age was not significantly different in males and females, and in all comparisons examined, the interaction terms between age and sex were not significant. Age, therefore, was not a significant factor in the previously mentioned comparisons between males and females. Furthermore, age was entered as a covariate, and all comparisons between males and females were examined at an age of 46.
This study has shown that under all conditions examined, females have a smaller rib cage size and a shorter diaphragm than males having the same height. This smaller rib cage size was associated with a greater anterior–posterior inclination of ribs and, during resting breathing, with lower gastric pressure swings in relationship to esophageal or transdiaphragmatic pressure changes, suggesting a greater contribution of inspiratory rib cage muscles in females than males.
Several imaging techniques can be used to measure rib cage dimensions and diaphragm length other than chest radiographs, including computed tomography (20) or spiral computed tomography (21, 22) and nuclear magnetic resonance (23, 24). Although a claim of superiority of these techniques over the radiographic technique can be made, particularly concerning shape information, the radiographic technique provides highly accurate estimates of linear dimensions (25). Furthermore, the computed tomography and magnetic resonance imaging techniques are limited to the supine posture. Because the shape of the chest wall changes markedly between the supine and the upright posture, the information obtained with these techniques cannot be extended to the upright posture. Findings obtained in the upright posture with the radiographic technique should thus be viewed as being complementary.
Nevertheless, the validity of the radiographic technique for the measurement of diaphragm length has been questioned on the premise that points along the contours of the diaphragm as seen on chest radiographs may not lie in the same plane and that the plane on which they lie may change with changes in lung volume (26, 27). In the supine posture, this question could be resolved by comparing diaphragm length measured by the X-ray technique with that measured by the computed tomography or magnetic resonance imaging techniques. In the upright posture, however, this question can be resolved only by comparing diaphragm length measured in situ by the radiographic technique with diaphragm length measured directly at necropsy. Because the diaphragm is not passively tensed at FRC in the upright posture (28), a comparison is possible at that volume. In normal subjects, diaphragm length measured with the radiographic technique at FRC (27.1 ± 2.6 cm/m) (11) was found to be almost identical to the length of the excised diaphragms measured at necropsy (26.7 ± 2.4 cm/m) (9). This kind of validation cannot be extended to other lung volumes or to the other imaging techniques mentioned previously here because under those conditions, diaphragm length predictably differs from resting length. Nevertheless, this comparison suggests that the radiographic technique provides reliable estimates of diaphragm length, at least at FRC in the upright posture.
Because ribs lie in a three-dimensional plane, rib angles measured on two-dimensional images must be interpreted with caution. Furthermore, the angles measured this way cannot be compared with those calculated for rib planes using the method of Wilson and colleagues (29) or Dansereau and Stokes (30). Nevertheless, the angles measured here on lateral films changed in the expected way with changes in rib cage dimensions and lung volume (Table 2). Furthermore, the values we measured are within the range of those reported by Sharp and colleagues (13) using the same technique in normal subjects.
For practical reasons, the radiographs and the respiratory pressures could not be obtained simultaneously. The possibility should thus be considered that the pattern of respiratory muscle recruitment could have been different in these two circumstances. However, less than 3 hours separated the two evaluations. Furthermore, the maneuvers were the same in both circumstances and required only simple instructions. We believe that it is very unlikely that females and males would respond differently to these instructions in one of these circumstances but not in the other.
Our method of assessing the relative contribution of the diaphragm and inspiratory rib cage muscles is based on the analysis of Macklem and colleagues (31). Although the relative contribution of different muscles cannot be quantified by this method, qualitative differences can be assessed. The analysis is based on the fact that the diaphragm is the only respiratory muscle that can increase gastric pressure while at the same time reducing esophageal pressure. In contrast, the contraction of inspiratory rib cage muscles, while also reducing esophageal pressure, can only reduce abdominal pressure. Thus, when the inspiratory rib cage muscles and the diaphragm contract simultaneously, the gastric pressure changes for a given change in esophageal pressure are smaller than if the diaphragm was the only muscle contracting. Although there are several limitations to this approach that have been well described (31), the method has proved useful in several circumstances (32–34). The relative contribution of inspiratory muscle is usually assessed as the ratio between esophageal and gastric pressure changes taken at the beginning and end of inspiration. However, this ratio can vary widely when gastric pressure changes are very small and oscillate around zero, which can obscure the comparisons between subjects. In this study, we used the ratios of esophageal to transdiaphragmatic pressure changes and of gastric to transdiaphragmatic pressure changes. These ratios carry the same information as the esophageal to gastric pressure ratio, but because transdiaphragmatic pressure is always positive at least in normal subjects, they are less variable and thus provide a more robust comparison between subjects.
In all comparisons examined, thoracic dimensions were normalized for standing height. With this procedure, we assume that the relative proportions of the trunk and legs are the same in males and females. Schwartz and colleagues (4) have shown that the inclusion of sitting height in the regression models for lung size, as measured by FVC, abolishes the interaction term between sex and standing height. However, they did not specify whether the inclusion of sitting height reduced or not the difference in lung size between males and females. The fact that differences in lung size between males and females persisted despite the inclusion of sitting height in their regression models suggests there was no major difference in the proportions of trunk and legs between males and females. We have shown before that referencing lung volume to thoracic spinal height instead of standing height does not reduce the difference in lung size between males and females (5). If the trunk grew less rapidly than the legs in females, then the ratio of thoracic spinal height, as measured on chest X-rays, to standing height should be smaller. We find no difference in this ratio between males (0.153 ± 0.006) and females (0.154 ± 0.007, p = 0.64). Thus, there appears to be no difference in the proportions of trunk and legs between males and females. The normalization for height should thus be valid.
We have shown that under all conditions examined, females have disproportionately smaller rib cage dimensions than males. A comparable difference was found whether the comparison was made at full active lung inflation or deflation or during relaxation at FRC. These findings confirm and extend a previous finding in our laboratory showing smaller radial rib cage dimensions in females at TLC (5). Because similar findings were obtained under widely different conditions of respiratory muscle recruitment and activation, our findings suggest that the observed differences in thoracic dimensions and configuration do not result from thoracic deformations caused by a different pattern of respiratory muscle recruitment or force distribution between males and females. Nevertheless, the kind of shift of the relationship between rib cage cross-section area and diaphragm height depicted in Figure 2B could conceivably be caused by a different level of tonic respiratory muscle activity between males and females. In particular, a lower level of tonic abdominal muscle activity in females could shift this relationship to the right and thus account for our findings. We are not aware of any study comparing the level of tonic respiratory muscle activity in males and females. However, a different level of tonic activity should be reflected in corresponding differences in intrathoracic and/or intra-abdominal pressures, particularly when measured at normal end expiration. In this study, we found no significant differences in end-expiratory esophageal, gastric, or transdiaphragmatic pressures between males and females that would be indicative of a different level of tonic respiratory muscle activity. Thus, a different level of tonic activity between males and females unlikely accounts for our findings. With the exception of gastric pressure during expiratory capacity maneuvers, respiratory pressure swings or their ratios during forced maneuvers were also not significantly different between males and females. Thus, the differences in thoracic configuration at the volume extremes between males and females are also unlikely to be explained by a different pattern of respiratory muscle recruitment. In contrast, the trends observed during inspiratory capacity maneuvers are opposite to those that would be anticipated if the observed differences in thoracic configuration were to be explained by differences in the pattern of respiratory muscle recruitment. We do not imply that the thorax, particularly at volume extremes, was not deformed by respiratory muscle forces. In fact, it very likely that the thorax was deformed under those conditions of maximum inspiratory and expiratory muscle contraction. The rounder rib cage in females at TLC (Figure 2A) may represent a deformation caused by a different distribution of inspiratory muscle forces in females than males. We imply, rather, that these deformations are unlikely to explain the smaller rib cage size found under all conditions examined and hence the observed difference in thoracic configuration between males and females depicted in Figure 2B.
Structural factors are more likely to be responsible for our findings. Possible structural factors include a greater inclination of ribs, shorter ribs, or a different rib geometry. We are not aware of any study comparing rib length or geometry in males and females. It is also unknown whether the growth of ribs is coupled to that of the lung or to that of the axial skeleton. However, the sixth rib, as viewed on lateral films, appeared to be more inclined in females than males both at RV and FRC, and a similar tendency was found at TLC. Everything else being constant, anterior–posterior diameters of the rib cage would be expected to change as the sine of this angle. Using the data of Table 2, the sine of the acute angle formed by the sixth rib at RV, FRC, and TLC can be calculated to be, respectively, 7.1%, 6.4%, and 2.4% greater in males than females. The corresponding differences in rib cage anterior–posterior diameters were in fact very close to these values, that is, 7%, 7%, and 2.2% greater, respectively, in males than females. Thus, the observed difference in the inclination of the sixth rib fully accounts for the observed difference in anterior–posterior rib cage diameters that we observed between males and females. Whether a greater lateral inclination of ribs can also account for the smaller lateral rib cage diameters found in females cannot be determined from our study. Thus, although a greater inclination of ribs must contribute to the smaller rib cage size in females, the contribution of other structural factors cannot be excluded. The mechanism causing the ribs to be more inclined in females than males is also unknown, although a compression of the rib cage by the weight of the breast could be envisaged as a possible contributing factor. Indeed, external loading of the rib cage in the anterior–posterior direction in normal subjects causes a rightward shift of the relationship between rib cage volume and abdominal volume as measured at the body surface (35) that resembles the kind of shift depicted in Figure 2B. However, other mechanisms are equally plausible that would require further investigations.
Everything else being constant, the smaller lung volume in females would predict a longer diaphragm length in relationship to height (9, 10). We found the opposite. At all lung volumes investigated, diaphragm length for height was shorter in females than males. Our findings are in keeping with those of Loring and colleagues, who showed that diaphragm length is not only dependent on lung volume but also on thoracoabdominal configuration (14). They emphasized the effect of rib cage displacements on diaphragm length. In their analysis, a decrease in rib cage cross-section area at constant abdominal cross-section area increased the length of the diaphragm by virtue of its insertions on the ribs that are displaced relative to the position of the dome of the diaphragm within the thorax. Our findings differ from this prediction in that for a given position of the dome within the thorax, the smaller rib cage size in females was associated with a shorter not a longer diaphragm. This is particularly evident at TLC, where the position of the dome was almost identical in males and females. Thus, in this comparison between males and females, the effect of radial rib cage dimensions on the length of the dome of the diaphragm outweighed the effect of rib inclination on diaphragm ribs insertions. A similar result was seen at RV and FRC. However, at these lower volumes, the dome of the diaphragm tended to assume a lower axial position in females than males and thus contributed to shorten the diaphragm. We found minimal differences in the shape of the dome between males and females and only in the lateral projections. Although these differences in the shape of the dome tend to preserve diaphragm length in females, they were clearly insufficient to compensate for the smaller radial rib cage dimensions and lower diaphragm dome position. The end result was that in females the diaphragm was on average 9% shorter than in males at all lung volumes investigated. Although a 9% shortening may seem to be small, it must be considered that a shortening of that magnitude occurs when FRC increases by as much as 150% (10). A shortening of that magnitude is likely to have a significant impact on diaphragm function that will require further investigation. Indeed, because the diaphragm is shorter and the rib cage cross-section area is smaller in females, the capacity of the diaphragm to produce volume displacements and inspiratory flow rates should be reduced. In contrast, the smaller rib cage size and greater inclination of ribs should increase the length of the inspiratory rib cage muscles, and this, in turn, should increase their capacity to produce volume displacements and flow rates. Whether the improved rib cage muscle function could compensate for the reduced diaphragm function is questionable, and this too will require further investigation.
As our results show, gastric pressure changes and the ratio of gastric to transdiaphragmatic pressure changes during resting breathing were smaller in females than males. Both findings are suggestive of a stronger inspiratory rib cage muscles contribution to inspiratory pressure swings in females than males. A similar tendency was found during inspiratory capacity maneuvers, although the differences between males and females did not reach the level of statistical significance. Presumably this could be explained by a greater variability in the pattern of respiratory muscle recruitment during volitional efforts than during resting breathing.
A greater inspiratory rib cage muscle contribution could be explained either by a greater neural activation or a greater mechanical advantage of these muscles relative to the diaphragm. The distribution of neural drive to the diaphragm and inspiratory rib cage muscles was not investigated. However, if we assume neural drive to be distributed similarly in males and females, then some inferences can be made from these measurements regarding the mechanical advantage of these muscles. According to Wilson and colleagues (36–38), the mechanical advantage of the inspiratory muscles can be defined as their fractional shortening over inspiratory capacity per unit change in lung volume. Because of a smaller lung size, mechanical advantage defined this way should be greater in females than males, but fractional shortening over inspiratory capacity should be the same. Fractional shortening can be estimated from our data. The fractional shortening of the diaphragm over inspiratory capacity was estimated using data shown in Table 3. From the data of Braun and colleagues (9), 25% of unstressed diaphragm length should be tendon, and 75% should be muscle. We assumed this to be the case in our subjects at FRC. After subtracting tendon length estimated this way, fractional shortening of the muscular portion of the diaphragm over inspiratory capacity was calculated to be 28.7 ± 6.6% in males and 29.6 ± 6.7% in females, that is, a 3.4% difference (p = 0.616). When expressed per unit change in lung volume, the fractional shortening of the diaphragm was on average 49% greater in females than males (i.e., 12.7 ± 3.6 vs. 8.5 ± 2.8% shortening per liter, p < 0.000). To the extent that fractional shortening would be the same during passive inflation, these values would also be equal in the model of Wilson and colleagues (36–38) to the mechanical advantage of the diaphragm. The fractional shortening of the inspiratory rib cage muscles was estimated geometrically using the rib angles shown in Table 2 and a simple two-dimensional model as described by Sharp and colleagues (13) and which is illustrated in Figure 4

Figure 4. Schematic diagram illustrating model used to compute intercostal muscle shortening. (A) Angle between rib and vertical. (B) Angle between intercostal muscle and inferior border of upper rib.
[More] [Minimize]To the extent that the smaller rib cage size in females can be solely accounted for by a greater inclination of ribs, as was shown here for the anterior–posterior dimension, our results would imply that despite a smaller lung growth, the rib cage in females grows to the same extent as in males, having the same final height. Put another way, in females, there would be a disproportionate growth of the rib cage in relationship to the lung. It is not immediately clear what purpose such a differential growth could serve. We believe it may have to do with the dual role of the rib cage in accommodating both lung volume and abdominal volume displacements. Indeed, only part of the rib cage faces the lung; the other part faces the diaphragm and the peritoneal cavity. The latter part, also called abdominal rib cage, actually forms part of the abdominal wall and is thus subjected to a pressure close to abdominal pressure (39). Mead and coworkers emphasized the role of the rib cage in accommodating abdominal volume displacements, which they considered to be its major contribution to breathing (40). The rib cage accommodates abdominal volume displacements not only during breathing but perhaps even more importantly when the abdomen is distended (41, 42). The expansion of the rib cage that occurs under those conditions minimizes the effect of the distending pressure on the lungs and thus minimizes lung volume changes. In addition, depending on the action of the inspiratory rib cage muscles, rib cage expansion may directly contribute to reduce abdominal pressure and thus limit its rise when the abdomen is distended (40). As a result, the changes in lung volume and abdominal pressure are less than they would otherwise be without the abdominal rib cage. If there is a condition that distinguishes females from males it is abdominal distension caused by pregnancy. The large volume capacity of the rib cage of females in relationship to the size of their lungs should thus be well suited to accommodate the large abdominal distension caused by pregnancy, thereby minimizing its effects on lung function and abdominal pressure (43, 44).
In summary, this study has shown systematic differences in thoracic dimensions and configuration between males and females consisting in disproportionately smaller radial rib cage dimensions and shorter diaphragm. These differences could not be attributed to a different pattern of respiratory muscles recruitment. Rather, the smaller rib cage size could be accounted for at least in part by a greater inclination of ribs. The pattern of respiratory pressure changes during resting breathing and to a lesser extent during inspiratory capacity maneuvers was indicative of a stronger inspiratory rib cage muscle contribution in females than males. Model predictions suggest that an improved mechanical advantage conferred to the inspiratory rib cage muscles by the greater inclination of ribs may be responsible for this. The findings are consistent with a disproportionate growth of the rib cage in relationship to that of the lungs in females as compared with males, which would be well suited to accommodate abdominal distension during pregnancy.
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