INTRODUCTION

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THE ABDOMINAL WALL CONTAINS the four most powerful expiratory muscles in mammals: the rectus abdominus (RA), the external oblique (EO), the internal oblique (IO), and the transverse abdominis (TA). In addition to breathing, the abdominal muscles may serve other roles, especially at higher levels of chemical drive or at increased end-expiratory lung volumes. They contribute to protective reflexes, such as coughing, sneezing, and vomiting, generate the intra-abdominal pressures necessary for expiratory efforts, and are active during postural adjustments (21). Understanding how abdominal muscles function physiologically during respiration requires a better knowledge of the architectural design of their muscle fibers. It is known that the internal architectural arrangement of muscle fibers has a significant effect on length-tension relationship, shortening velocities, and force-generating capacity of the muscle (24).

In vivo, abdominal muscles experience pressure load, and, therefore, they may carry stress, not only in the longitudinal direction of the muscle fibers but also transverse to the fibers. However, unlike the diaphragm, the abdominal muscles are arranged in multiple layers with fibers in an individual muscle oriented perpendicular to fibers in the adjacent muscle. For example, the IO and EO are oriented in such a way that the direction of muscle fibers in the IO is perpendicular to the direction of muscle fibers in the EO. Therefore, unlike the diaphragm, the abdominal muscles could carry stress only in the direction of muscle fibers. We wondered whether abdominal muscles exhibit a fiber architecture that is similar to diaphragm muscle. We chose the two internal abdominal muscles, the IO and TA. These muscles are adjacent to each other and are separated by a network of connective tissue. Therefore, during respiration, myofascial force transmission is expected between the two muscle layers, the IO and TA, via the interfacial connective tissue. Furthermore, the IO has about the same muscle length as the costal diaphragm muscle, and the TA is twice as long as the diaphragm. Functionally, the TA and IO are loaded with pressure and respond more to increases in chemical or volume-related drive than do the two external abdominal muscles, the RA and EO (21).

Anatomically, the IO is a muscle that lies internal to the EO muscle in the lateral abdominal wall. The IO muscle originates from the thoracolumbar fascia, the anterior half of crest of ilium, and the inguinal ligament; it inserts into the lower three ribs, the sheath of rectus muscle, and the inguinal aponeurotic fold. The TA is the most internal abdominal muscle, and it lies in the lateral and ventral abdominal wall between the internal surface of the IO and the costal cartilage (26). The TA originates from the 7th through the 12th costal cartilage, the lumbar fascia, the iliac crest, and the inguinal ligament and inserts into the xiphoid cartilage and the linea alba. Contraction of abdominal muscles during expiration causes an inward displacement of the abdominal wall and an increased abdominal pressure, which displaces the diaphragm into the thorax and decreases lung volume (9). Thus abdominal muscle activation appears to improve the pressure-generating capacity of the diaphragm and increase its ability to expand the lower rib cage (9). For anesthetized supine dogs quietly breathing, studies show that the TA is the primary active muscle and the only abdominal muscle involved during expiration (8). Furthermore, the TA shows activity during speech, laughter, voluntary contractions of the abdominal wall, and hyperoxic hypercapnia (7). During inspiratory elastic loading, the TA helps in the reduction of the diameter of abdominal anteroposterior, which helps to push the incompressible abdominal viscera against the diaphragm and decrease functional residual capacity (7). Because the IO is physically attached to the TA, any contractile forces generated by TA may be transmitted directly to the IO muscle through connective tissue that is tightly connected to both TA and IO muscles.

Traditionally, the myofibers of parallel-fibered strap muscles in vertebrates have been assumed to run the full length of the muscle (12, 19). However, our laboratory (2, 4) and others (16) demonstrated that the majority of muscle fibers of the diaphragm in either the dog or the cat are short fibers. These fibers are attached to either insertion and terminate by tapering gradually to a very fine thread before reaching the other insertion. This architecture of muscle fibers was also shown in the cat strap muscles (24, 28). Furthermore, in the RA of the rat, a mixed population of spanning (SPF) and nonspanning fibers (NSF) was found in different parts of the muscle (18).

In this paper, we tested the hypothesis that the abdominal muscles of the IO and TA have discontinuous fiber architecture with more short fibers that are tapered and end intrafascicularly. Additionally, we examined whether length of the abdominal muscles has an effect on muscle-fiber architecture. The TA is about twice as long as the costal muscle of the diaphragm, whereas the IO is about the same length as the costal diaphragm. To answer these questions, first we stained the IO and TA muscles for acetylcholinesterase activity to determine the macroscopic organization of neuromuscular junctions (NMJ) and gain insight into their fiber architecture. Then we dissected single muscle fibers from either the TA or the IO muscles. In addition, we analyzed the shape of these fibers to assess the mechanism by which force is transmitted across the interface between muscle fibers and the surrounding endomysial component of connective tissue. Our results demonstrate that these abdominal muscles have a discontinuous fiber architecture, as shown by the multiple NMJ bands that are visible on the surfaces of the IO and TA muscles and the length of dissected single fibers from either muscle. Muscle length of the abdominal muscles appears to be a major determinant of their fiber architecture. About 10% of those fibers dissected from the IO spanned the entire length of the muscle, whereas SPF were <1% of all fibers dissected from the TA. The NSF of the shorter muscle, the IO, were mostly attached to one insertion and ended intrafascicularly by tapering to a very fine strand. Only ~6% of total fibers in the IO were not attached to either insertion and tapered at both ends. In contrast, the TA had nearly a seven times greater percentage of fibers that were not attached to either insertion.

	METHODS

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We were able to harvest abdominal muscle from 12 mongrel dogs that were being used for other studies at the time they were euthanized. Below we describe the methodology used in both the experimental protocol for the NMJ study and the single-muscle-fiber dissection study.

NMJ histochemistry. An established technique was used for histochemically testing the intact muscle for acetylcholinesterase activity to detect motor end-plate distribution on muscle surface (34). This technique is based on the assumption that most muscle fibers are singly and focally innervated (22). Six adult mongrel dogs, weighing from 13 to 17 kg each, were killed by pentobarbital sodium overdose (1 ml/kg). The chest was opened by a bilateral thoracotomy across the sternum, and the abdominal wall was opened by a midline laparotomy. Within 15-20 min of death, both the IO and TA muscles were excised from the left lateral side of each animal with careful separation of the connective fascia between the two muscle layers. Six IO and six TA muscles were excised from the left lateral sides of six dogs, for a total of 12 individual abdominal muscles. The connective fascia between the two muscle layers was carefully separated. Each muscle was sutured onto a rigid polyester screen to limit both muscle shrinkage and muscle distortion during subsequent histochemical processing. Muscles were incubated overnight in 0.5 mg/ml acetylthiocholine iodide in 0.1 M phosphate buffer (pH 6.4), containing 0.1 M sodium citrate, 30 mM cupric sulfate, and 5 mM potassium ferricyanide at room temperature. The samples were then rinsed in distilled water and counterstained in 1% ammonium sulfide until groups of end plates appeared as dark spots. Care was taken to avoid too long a reaction time in the acetylcholinesterase stain, since the reaction product tends to diffuse and migrate to the connective tissues adjacent to the fiber (13). Rinsing the muscle in distilled water and immersing the muscle in 5% formalin blocked the reaction. To obtain the distribution of NMJ on muscle surface of both the IO and TA, we put a transparent plastic sheet on the flat excised muscle without deforming the tissue. The boundary of the muscle and the locations of the NMJ were carefully traced.

Single-muscle-fiber dissection. Six mongrel dogs (19-23 kg) were killed with an intravenous injection of pentobarbital sodium (1 ml/kg). The IO and TA muscle tissues were excised in their entirety from the left lateral side of six dogs, for a total of 12 individual abdominal muscles. Approximately 103 muscle fascicles were removed from the IO and 139 from the TA in strips of ~2 in. wide. These strips were dissected from regions that span the entire muscle. Muscle fascicles appear to run the muscle length and are separated by connective tissue. These fascicles contain ~20-30 muscle fibers.

Assessment of muscle shrinkage. The length of the freshly excised muscle, as well as the length of the same muscle after histochemical treatment, was measured carefully. The shrinkage factor for both muscles was calculated from these measurements.

	RESULTS

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NMJ distribution. The distribution of NMJ of the IO and TA muscles of a typical dog after acetylcholinesterase staining is shown in Fig. 1, A and B, respectively. The acetylcholinesterase-positive zones stain as irregularly spaced bands on both surfaces of the IO and TA and are associated with the NMJ of innervating motoneurons. These bands are scattered across most of the muscle surface, as would be required to innervate the short muscle fibers distributed at various positions along the course of the muscle. Direct observation of any NMJ band under high power shows that it is composed of numerous individual spots, each representing a single motor end plate. Some spots are immediately adjacent, but most are separated by gaps that are multiples of cell diameters. A line parallel to the fascicle traverses up to six NMJs distributed along the TA muscle from origin to insertion. In the IO, up to four NMJs traverse the muscle.

View larger version (16K): [in this window] [in a new window]   Fig. 1.   Distribution of neuromuscular junction (NMJ) of the internal oblique (IO; A) and transverse abdominis (TA; B) tracked on the surfaces of the abdominal muscle of a mongrel dog. The directions along the muscle fibers and transverse to the muscle fibers are shown for both muscles. The near midline insertion and lateral origin are to the right and the left side of each diagram, respectively. The dots represent the acetylcholinesterase-positive zones associated with the NMJ of innervating motoneurons. Note the transverse multiple bands of NMJ, which run in irregular lines on surfaces of both the IO and TA muscles. These data illustrate that NMJ are numerous and may number, from origin to insertion, as many as 4 across the surface of the IO and as many as 6 across the TA muscle.

Muscle-fiber architecture. The incidence of SPF and NSF in muscle fascicles from the IO muscle of six mongrel dogs is shown in Table 1. SPF spans the entire length of the muscle from origin to insertion. Most NSF were attached to the origin or insertion of the fascicle and terminated intrafascicularly within the muscle. NSF that tapered at both ends were only 6.2 ± 1.0% of all examined fibers in the IO muscle, whereas SPF accounted for 9.8 ± 1.5% of all dissected fibers.

View larger version (78K): [in this window] [in a new window]   Fig. 2.   A: single spanning fiber (SPF) isolated from the IO muscle is shown. A1: the entire muscle fiber at ×1.9 magnification. A2 and A3: blunt ends of the same muscle fiber at ×250 magnification. B: single nonspanning muscle fiber (NSF) isolated from the IO is shown. B1: the entire muscle fiber at ×3.5 magnification. B2: blunt end of the muscle fiber at ×250 magnification. B3: intramuscular tapered end of the fiber at ×250 magnification. C: single NSF isolated from the IO is shown. C1: the entire muscle fiber at ×2.7 magnification. C2 and C3: intramuscular tapered ends of muscle fiber at ×250 magnification.

View larger version (82K): [in this window] [in a new window]   Fig. 3.   A: single SPF isolated from the TA is shown. A1: the entire muscle fiber at ×2 magnification. A2 and A3: blunt ends of the same muscle fiber at ×250 magnification. B: single NSF isolated from the TA is shown. B1: the entire muscle fiber at ×2.4 magnification. B2: a blunt end of the muscle fiber at ×250 magnification. B3: intramuscular tapered end of the fiber at ×250 magnification. C: single NSF isolated from the TA is shown. C1: the entire muscle fiber at ×2.6 magnification. C2 and C3: intramuscular tapered ends of muscle fiber at ×250 magnification. Note that the tapered ends of the fiber in B3, C2, and C3 in Figs. 2 and 3 are morphologically similar to their nontapering regions. Striations are visible throughout the tapering region. No special attachments or other structures related specifically to the tapered free ends could be resolved by light microscopy.

View larger version (37K): [in this window] [in a new window]   Fig. 4.   A: 6 NSF that are tapered at both ends and adjacent to a single SPF from the TA muscle with a magnification of ×2.5. B: 3 bundles of NSF with both ends tapered, taken from the TA, with a magnification of ×2.0 are shown. From right to left, a bundle of 5 fibers appear to be attached to 2 bundles of 2 fibers. C: TA bundle of NSF, with 1 end tapered, is shown with a magnification of ×2.6. The 4 fibers are attached to each other at their tapered ends. D: bundle of fibers taken from the IO with a magnification of ×2.6 is shown. Five NSF with 1 end tapered are seen attached to one another at their tapered ends.

View larger version (15K): [in this window] [in a new window]   Fig. 5.   Schematic representation of the actual proportion of fibers that span the entire muscle fascicle (SPF) and NSF with different sites of insertion. A: IO. B: TA. C: midcostal diaphragm muscle. NSF taper to a very fine strand within each muscle. In the TA, SPF make up <1% of the total no. of fibers and are not shown in the schematic. The dimensions of the schematics and the length of fibers within each schematic are proportional to the actual measurements of the IO, TA, and midcostal diaphragm muscles.

	DISCUSSION

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Our histochemical and morphological results demonstrate that IO and TA muscles of the abdominal wall have discontinuous fiber architecture. That is, most fibers in these abdominal muscles were shorter than the entire length of their fascicle. Muscle fibers in the IO were a mixed population of SPF and NSF, whereas essentially all fibers in the TA were NSF. Short fibers in either muscle were tapered along a significant portion of their length, and the absolute total length of the tapered portion of muscle fibers was longer in TA than in IO.

Skeletal muscle is composed of muscle fibers held together by connective tissue, and both the fibers and the connective tissue contribute to the overall mechanics of the muscle (32). A common assumption for architecture composed of muscle fibers that span the entire length of the muscle (SPF) is that the fibers only transmit tensile forces along their lengths. In other words, force is only transmitted along the length of the fiber between the tendon at the origin and the tendon at the insertion. The frog semitendinosus muscle is relatively short and therefore is most likely composed principally of SPF. In a classic study by Street (29) using the frog semitendinosus, both active and resting forces were shown to be transmitted from myofibers to adjacent intramuscular connective tissue. That is, forces are transmitted in a pathway that is parallel to the myotendinous junction (MTJ) pathway (20). Street isolated one end of a single fiber from a bundle and left the other end immobilized within the transected bundle, called the splint. If force was only transferred along a fiber from its origin to its insertion, a splinted fiber should shorten at its free end without transmitting force to its fixed end (27). However, the splinted fibers transmitted 76-100% of the force of a fiber held at both ends. Monti et al. (27) have reasoned that the myofibrils within muscle cells may be linked at various points along the sarcolemma, and force may be transmitted as fibers pull against each other along these portions. Others (24, 28) speculated that force is transmitted across the membrane of active muscle fibers to the connective tissue of the endomysium, which then transmits it to adjacent muscle fibers. Forces may be transmitted from sarcomeres to the extracellular matrix through transmembrane protein chains located at costameres, subsarcolemmal domains regularly spaced along muscle cells (30). These proteins are localized not only at costameres but also at the MTJ as well, which indicates that they have a role in force.

In mammals, longer muscles are composed mostly of short tapered fibers, which show many motor end-plate bands arranged in rows perpendicular to the longitudinal axis of the fascicles (11, 14, 24). A discontinuous fiber architecture is important to ensure synchronous contractions in a long muscle, since myoelectrical transmission is slow (~3 m/s) (24). For example, propagation of an electrical signal from the center to the ends of a 12-cm fiber should be complete within ~20 ms, which is in the same order as the twitch rise time. Therefore, this delay could cause contractions to occur out of phase and lead to a disruptive mechanical instability (24). However, although the length of the IO muscle is about one-half that of the TA muscle, both abdominal muscles have discontinuous fiber architecture. It is possible that shear is an important mechanism of force transmission in muscles that are submaximally activated in vivo. Therefore, architecture with fibers that are tapered and shorter than the length of their muscles is important to carry stresses between adjacent muscle fibers in shear.

The transfer of shear forces is affected by the amount and characteristics of the connective tissue matrix surrounding the fiber and the fascicles and by the shear properties of both active and passive muscle fibers (33). A discontinuous architecture contains tapered fibers that terminate intrafascicularly within the connective tissue (NSF), and these NSF cannot exert purely tensile forces against a conventional tendon. Instead, their forces must be distributed laterally into the surrounding fibers through the connective tissue by means of interfacial shear (17). Tapering fibers ending intrafascicularly are inconsistent with force transmission occurring only at the fiber ends. If this occurred, force would be constant along the muscle bundle, and stress at the sarcomere level would be inversely proportional to the cross-sectional diameter of myofibril. Furthermore, if there were myofibrils of different lengths, maximal contraction of the longest myofibers could exceed the shortening of the shorter fiber, unloading them completely. These considerations suggest that muscle force of individual fibers would be transmitted through MTJ at the ends of the fiber (2). However, in discontinuous fiber architecture, as seen in the IO and TA, muscular force can be transmitted not only through the MTJ but also through the transmembrane proteins at costameres along the muscle fibers. These structural proteins include the integrin complex and the dystrophin complex.

Acetylcholinesterase activity staining has correlated discontinuous muscle-fiber organization with the presence of multiple end-plate bands (2), a study that has been supported by others using muscles with short tapered fibers (14, 24). The multiple NMJ bands visible in the IO and TA muscles are consistent with muscles composed of fibers with a variety of lengths. The distribution of the NMJ bands shows the relative location of muscle fibers in the muscle, and that is based on the assumption that a myoneural junction is located near the middle of a muscle fiber. The average spacing between two adjacent NMJ along the same muscle bundle is smaller in the TA than that for the IO muscle. These results are consistent with the presence of a greater fraction of muscle fibers in the TA that does not reach either attachment of the muscle. Furthermore, the observation that NMJ spacing was greater in the IO than in TA suggests that the ratio of NSF length to SPF length may be greater in the IO than in TA. This is consistent with single-fiber length measurements of NSF and SPF in these muscles.

Because forces may be transmitted by intramuscular endings to the overlying endomysium along the length of the tapered region of a fiber (32), the shape and length of the tapered region are considered important determinants of muscle force transmission. In particular, these determinants affect the stress distribution within the muscle cell and at the interface between the endomysial tissue and the muscle fibers. The magnitude of a taper angle in the tapered short fibers determines the amount of amplification of the surface area of the fiber end relative to the cross-sectional area at the onset of tapering (31). Furthermore, the taper angle determines the mechanism of force transmission between the tapered region of the fiber and connective tissue. Lubkin (25) determined that, for taper angles of ~1°, the shear load parallel to the plane of the membrane is proportional to the cosine of the angle, whereas the tensile load normal to the membrane plane is proportional to its sine. Therefore, forces generated by a fiber (T_fiber) can be transmitted by two components: 1) tensile force that is perpendicular to the surface (T = T_fiber sin²) and 2) shear force that is parallel to the surface of the fiber (T = T_fiber sin cos), where  is the angle of taper at the end of a muscle fiber. Thus a small taper angle creates an interface, which is loaded mostly in shear and maintains constant force throughout the muscle, whereas short, blunt fibers generate a more localized force at the end of the fiber (5). If the force generated by a skeletal muscle fiber was not transmitted across the tapered region, the myofilaments may be required to carry higher loads as the fiber tapers (31). That is, the force per unit cross-sectional area of the filaments at the tapered region would be greater than those of the nontapered region. With the use of the taper angles that we calculated (Table 4), a typical fiber in the IO transmits 7.80 × 10⁶ ± 1.21 × 10⁷ of T_fiber in tension and 2.79 × 10³ ± 3.49 × 10⁴ of T_fiber in shear. Therefore, shear force is over 350 times greater than tensile force. In the TA, the total force transmitted by a fiber has a tensile component of 2.56 × 10⁵ ± 2.74 × 10⁷ of T_fiber and a shear component of 5.06 × 10³ ± 5.24 × 10⁴ of T_fiber. Thus shear force is almost 200 times greater than tensile force. Therefore, this confirms that contractile forces are transmitted among fibers essentially in shear. Muscle fibers in the IO appeared to have smaller taper angles than those in the TA, and tapering regions made up a higher proportion of the total fiber length (Table 4). Compared with the TA muscle, the IO is expected to transmit a higher proportion of muscle force in shear. However, compared with the TA, the IO has a much smaller fraction of NSF that tapers at both ends. Therefore, in the TA, there is an increase in the proportion of the total muscle force that is transmitted in shear. The surface of the tapering region is typically very uneven (Fig. 6). The shape of this region gives the shape of the cell membrane along the muscle fiber across which force is transmitted to the extracellular matrix.

View larger version (14K): [in this window] [in a new window]   Fig. 6.   Plot of fiber radius against length of fiber in the tapering region of representative fibers from the IO and TA. The tapering region is located between the tip of a fiber and the first point of measurement where the diameter reached 90% of its maximum (5). TA fibers have a shorter tapering region and larger tapering angle than IO fibers (see also Table 4).

Fiber length, an important mechanical determinant of muscle architecture, imposes constraints on muscle function in three ways: 1) the length of the fiber determines the number of sarcomeres in series and the speed of muscle shortening; 2) fibers shorter than fascicle length must transmit their forces in shear to connective tissue within muscle; and 3) long muscles may require more than a single innervation per SPF to excite all regions of the fiber simultaneously. Our muscle-fiber length measurements showed that average length of a SPF in the TA is ~75% longer than SPF in the IO muscle, whereas the average length of a NSF was ~50% longer for the TA. The length of a SPF from the IO muscle was very similar to that of a SPF from the diaphragm muscle of the dog (4), although somewhat longer. The differences in length of the diaphragm, TA, and IO muscles are also reflected by the percentage of NSF in each muscle. About 99% of all fibers in the TA are NSF, compared with 90% for the IO and 79% for the diaphragm. NSF that taper at one end make up 58.1 ± 2.2% of total fibers in the TA, 83.4 ± 1.4% of the fibers in the IO, and 78.3 ± 2.3% of the fibers in the diaphragm. In addition, those fibers that do not span either insertion appear to be greater in the TA (41.0 ± 2.3%) than in either the IO (6.2 ± 1.0%) or diaphragm (1.0 ± 0.4%). Furthermore, more fibers were found in the TA along the length of the muscle than in either the IO or diaphragm. This suggests that the probability that a single fiber will span the length of the fascicle is inversely proportional to the fascicle's length (5). That is, the shorter the muscle, the higher the probability that its fibers span the entire length of the muscle.

The mean diameter of all NSF in the IO and TA was 81.26 ± 5.38 and 124.21 ± 7.77 µm, respectively. Fibers with large diameters have more contractile units in parallel and, consequently, a greater capacity for contractile force development (10). The diameter of a muscle fiber may be determined more by usage than species specificity (5), and our results are consistent with physiological observations that the TA is the most active muscle during expiration (1).

If individual muscle fibers insert diffusely into an epimysial extracellular matrix, then it is possible that the extracellular series elasticity in the muscle can change the way the stretch is distributed within the muscle (24). This allows for possible sliding of discontinuous muscle fibers with respect to one another. Another important source of internal compliance is the passive muscle fibers (31). For example, fibers of the cat tenuissimus muscle that belong to a single motor unit are usually adjacent to fibers belonging to other motor units (23), which implies that the fibers have no members of their own motor unit group as neighbors. In such a muscle, when cells contract, force is dispersed through collagen fibers and noncontracting cells in the muscle (15). Therefore, both passive muscle fibers and connective tissue may contribute to the transmission of stress generated by the contracting muscle fiber (31). Because abdominal muscles are submaximally activated during quiet breathing, it is likely that a large proportion of muscle fibers in the IO and TA are noncontracting, and, therefore, these fibers could play an important role in force transmission among muscle fibers.

Unlike the diaphragm in which fibers are arranged in a single muscle layer, the abdominal muscles are arranged in multiple layers. It is conceivable that there is a myofascial force transmission between the IO and TA via the connective tissue network separating the two muscles. Thus there may be mechanical linkage between the IO and TA. Muscle force generated by the contracting TA can be readily transmitted directly to the adjacent IO muscle via a connective tissue network at the interface between the two muscles. Alternatively, a contracting IO muscle can transmit muscle force to a passive TA via the same interface. In addition, a stretched TA muscle during inspiratory efforts may transmit passive muscle forces to the IO muscle via the interfacial connective tissue. It is noteworthy that the extensibility of the abdominal muscle of the TA and IO as a unit was found to be significantly less than that of any of the individual muscle layers of the IO and the TA (6). We tested the hypothesis that the IO and the TA muscles form a composite laminate structure, which has a greater passive stiffness than either TA or IO alone (6). In this study, we measured length-tension relations using muscle strips from the TA, IO, and the two muscles together as a composite. Our results showed that the length-tension relationship of the composite muscles of IO and TA is shifted to the right compared with the length-tension relationship of either layers of the IO or TA (6). In particular, the composite laminate structure increased stiffening only along the direction of the fibers, and muscle anisotropy was less pronounced in the composite than individual muscles. These results are consistent with the hypothesis that during inspiration, there is myofascial passive force transmission between the IO and TA via the interfacial connective tissue.

In summary, the data in this study demonstrated that the internal abdominal muscles of IO and TA of the dog have a discontinuous fiber architecture with NSF tapered along either or both ends of the muscle fiber to a very fine strand, as determined by single-fiber dissection and morphological measurements. When the fiber architectures between these two muscles are compared, fibers in the TA are essentially all NSF. However, the IO muscle has a mixed population of SPF and NSF. Furthermore, the total absolute length of the tapered portions of fibers along the length of the TA muscle is longer than that of the IO. Therefore, it is expected that, during submaximal activation in quiet breathing and maximal expiratory efforts, a greater shear force is transmitted among muscle fibers in the TA than in the IO. This is consistent with the TA being the primarily recruited expiratory muscle during expiration.