Skip to main content
NCBI home page
Journal of Anatomy logoLink to Journal of Anatomy
. 2020 Aug 13;238(1):53–62. doi: 10.1111/joa.13289

Fiber type composition of contiguous palmaris longus and abductor pollicis brevis muscles: Morphological evidence of a functional synergy

Colin W Moore 1, Jacob Fanous 2, Charles L Rice 2,3,✉
PMCID: PMC7754940  PMID: 32790091

Abstract

The palmaris longus (PL) tendon is used in surgical opponensplasty to restore functional hand movements in thenar paralysis. Although successful PL autologous tendon transfer has been attributed to an established synergistic relationship between the PL and abductor pollicis brevis (APB) muscles in vivo, this functional relationship may be dependent on the quality of their spatial relationship and properties of their constituent muscle fibers. The purpose was to compare the proportion of type I and type II muscle fibers in the APB based on its contiguous morphological relationship with the PL tendon for indirect insight into their functional synergy, contractile capacity, and digastric arrangement. Twenty‐four contiguous PL and APB specimens were harvested from the upper limbs (12 right and 12 left) of twelve formalin‐embalmed cadavers (mean age: 74 ± 10 years). The fiber type composition of these muscles was determined by labeling serial cross sections with myosin heavy chain (MyHC) type I and type II monoclonal antibodies. The PL consisted of a relatively heterogeneous fiber type composition irrespective of the presence of a discrete (type I: 41 ± 11%; type II: 55 ± 12%; hybrid: 4 ± 3%) or rudimentary (type I: 49 ± 10%; type II: 45 ± 9%; hybrid: 6 ± 4%) tendinous connection with the APB. The APB fascicles arranged contiguously with the PL through a discrete tendon had significantly greater proportions of type II fibers (41 ± 19%) compared to those with rudimentary PL connections (type II: 15 ± 8%). Therefore, the APB fascicles arranged in a digastric relationship with the PL may have the capacity to produce more powerful contractions than those with rudimentary PL tendons based on the known contractile properties of type II muscle fibers. Knowledge of the spatial relationship between the PL and thenar musculature prior to PL autologous tendon transfer may be a useful indicator of the quality of established synergy in vivo.

Keywords: abductor pollicis brevis, functional anatomy, immunohistochemistry, muscle fiber typing, muscle function, muscle morphology, myosin heavy chain, palmaris longus

The major finding of this study suggests that the abductor pollicis brevis consists of significantly greater type II muscle fibers when arranged contiguously with the palmaris longus tendon. This study supports evidence of the palmaris longus and abductor pollicis brevis acting as a functional digastric unit capable of enhancing thenar muscle actions.

graphic file with name JOA-238-53-g004.jpg

1. INTRODUCTION

The palmaris longus (PL) is known for its variant morphology and is absent in approximately 14% of forearms in the population (Moore et al., 2017a). Although considered a weak wrist flexor and tensor of the palmar aponeurosis (Gilroy, 2017; Moore et al., 2017a), the PL may provide significant contributions to thenar abduction strength based on its morphological relationship with the thenar musculature (Gangata et al., 2010). Despite common depictions of the abductor pollicis brevis (APB) as a thin, subcutaneous muscle of the proximolateral thenar eminence (Napier, 1952; Ross, 2016), Simard and Roberge (1988) described it as consisting of three muscular heads with several discrete fascicular subdivisions representing a substantial proportion of the thenar muscle mass. Of the three APB muscular heads, the superficial head consisted of a discrete fusiform fascicle continuous with the PL (Simard and Roberge, 1988). These discrete fascicles are considered homologous to lumbricals (Fahrer and Tubiana, 1976) or interossei (Le Double, 1897) based on their insertion into the dorsal aponeurotic expansion and presumed functional role in the extension of the interphalangeal joint of the 1st digit. Discrete APB fascicles may originate from several PL tendon locations including a bifurcated PL tendon, a region proximal to the palmar aponeurosis, or from an accessory abductor pollicis longus tendon (Fahrer and Tubiana, 1976; Fahrer, 1977; Kaplan, 1984; Moore et al., 2018). This morphological arrangement indicates that the APB and PL may act as a functional digastric unit for synergistic contributions to thenar muscle contractions.

The omohyoid, occipitofrontalis, and the digastric proper are examples of muscles engaged in functional synergistic relationships. The nomenclature of the digastric reflects its morphological arrangement indicating the presence of two discrete muscle bellies separated by an intermediate tendon. The functional relationship between the digastric muscle bellies has been investigated histologically by determining the fiber type identity of its constituent muscle fibers. A predominance of type II muscle fibers exists among both anterior (type I: 37%; type II: 63%) and posterior bellies (type I: 36%; type II: 64%) indicating a functional relationship irrespective of innervation and site of embryological development (Monemi et al., 1999). Because type II muscle fibers have greater shortening velocity and fatigability compared to type I muscle fibers, the predominant type II muscle fiber type consistency among digastric bellies reflects its gross function in performing powerful movements of the jaw (Monemi et al., 1999; Pette and Staron, 2000). Similarly, the medial and lateral heads of the quadratus plantae in the foot demonstrate a fiber type homogeneity indicative of a shared function in bipedal gait despite their variable absence in 20% of the population (Schroeder et al., 2014). The quadratus plantae fiber type homogeneity persists among its heads despite differences in their phylogenetic origins with the lateral head common with mammals and the medial head found only in humans (Sooriakumaran and Sivananthan, 2005; Schroeder et al., 2014). The PL and APB are arranged in a similar morphological arrangement as the digastric muscle through terminal tendon of the PL (Fahrer, 1977). Although the APB fiber type composition has been shown to consist of a predominant proportion (>60%) of type I muscle fibers (Johnson et al., 1973), the APB fiber type composition has not been investigated with respect to its contiguous morphological arrangement with the PL muscle.

In severe carpal tunnel syndrome, median nerve compression can impair the functional actions of the APB including thenar abduction, metacarpophalangeal (MCP) joint rotation, and true pulp‐to‐pulp contact of the digits (Napier, 1952). Restoring functional hand movements in patients with thenar paralysis can be achieved using an autologous tendon transfer of the PL to the 1st digit (Camitz, 1929; Rymer and Thomas, 2016). Opponensplasty success has been attributed to an intrinsic synergy of the PL with the APB such that no specific muscular retraining is needed upon tendon transfer (Kato et al., 2014). This synergy has been demonstrated in young participants in vivo, in which, synchronous electromyographic (EMG) activity was recorded between the PL and thenar musculature during abduction, flexion, opposition, and circumduction movements (Moore et al., 2018), yet this synergistic relationship may not be adequately established in all individuals due to morphological differences. Evidence of an intra‐individual fiber type homogeneity between contiguous APB and PL muscles may provide morphological support of their functional interrelationship. Knowledge of the morphological connection between the APB and the PL tendon could be indicative of the quality of synergy established in vivo and therefore useful in predicting the success of the PL in opponensplasty tendon transfer.

The purpose of this study was to investigate whether differences in the proportions of type I and type II muscle fibers exist among the APB fascicles originating from the PL tendon. When arranged in a digastric manner with the PL, the APB may be capable of producing more powerful contractions due to greater type II muscle fiber proportions, thus contributing to the significant thenar abduction strength attributed to the presence of PL musculature (Gangata et al., 2010). We hypothesized that the APB fascicles with discrete continuity with the PL will have significantly greater type II fiber type proportions compared to the APB musculature with rudimentary connections, or nonexclusive origins, with PL musculature. Knowledge of the APB fiber type composition with respect to its morphological relationship with the PL could be useful to further characterize the complexity of thenar contractile function and assist surgeons in functional restoration of thumb prehension and dexterous hand movements.

2. METHODS

2.1. Cadavers

Twenty‐four contiguous PL and APB muscles were harvested from the forearms and hands (left limb: 12; right limb: 12) of twelve embalmed cadavers (6 males and 6 females). The mean age of the cadaveric specimens was 74 ± 10 years (range: 55–87). The PL was present bilaterally in all cadavers. Cadaveric specimens were obtained from the body bequeathal program at the University of Western Ontario and approved for research use by the Committee for Cadaver Use in Research (REF# 21092016). The cadavers are received and embalmed within 24 hr postmortem. The embalming formula consisted of formalin (1.5%), ethanol (45.6%), phenol (1.8%), propylene glycol (24.0%), methanol (2.8%), alkylphenol ethoxylate (0.3%), and EDTA (0.3%). The cadaveric tissue samples were harvested within approximately 6 months of receiving the bodies through the body bequeathal program. To ensure muscle fiber type proportions were not influenced by other comorbidities, cadaveric specimens were excluded if neuromuscular diseases, rheumatoid, or osteoarthritis were indicated in the cause of death report, or by visual evidence of hand deformation.

2.2. Morphological classification of the APB and PL

The forearms and hands of each cadaveric specimen were dissected and photographed by a single investigator (CWM) to examine the PL tendon morphology and its continuity with the abductor pollicis brevis muscle based on the observations by Fahrer and Tubiana (1976). The twenty‐four hands were stratified into two groups based on morphology of the APB and its relationship with the PL tendon. The APB muscles were classified into two groups based on the following morphological criteria: (a) APB muscle with discrete PL tendon connections (APBD) or (b) APB muscle with nondiscrete, or rudimentary, PL tendon connections (APBND). The APBD fascicles were classified as discrete if they were relatively mobile with origins from robust tendinous connections from the PL muscle. Conversely, the APBND fascicular divisions were classified as nondiscrete if they were affixed primarily to the carpal bones and had only rudimentary connections with the PL tendon through thin fascial extensions. Visual confirmation of continuity of the APB with the PL tendon was obtained by applying tension to the PL tendon to observe whether the tension was transferred to the APB fascicle.

2.3. Immunohistochemistry

Whole PL muscle tissue sections were harvested from the midpoint of the belly, which was determined by measuring half the distance between the medial epicondyle and PL myotendinous junction. In each hand, the superficial muscular fascicles of the APB muscles were identified and harvested by measuring half the distance between the scaphoid and the proximal phalanx of the thumb. At their midpoints, standardized 0.5 cm lengths of PL and APB muscle tissue were excised for immunohistochemical analysis. Overall, the approximate dimensions (length × width × thickness) of the excised PL and APB tissues were 0.5 cm × 1.5 cm × 0.5 cm and 0.5 cm × 1.0 cm × 0.4 cm, respectively. The PL and APB tissue specimens were processed using previously established immunohistochemical labeling procedures as per Moore et al. (2017b). Immunohistochemical labeling of type I and type II fibers from fixed cadaveric muscle tissue has been validated against myosin ATPase staining techniques (Behan et al., 2002) and has been used to study the fiber type composition of several skeletal muscles (Lovering and Anderson, 2008; Kim et al., 2013; Moore et al., 2017b). The specimens were immediately immersed in a 10% formalin solution for a minimum of 24 hr upon harvesting. All tissues were paraffin‐embedded and serially sectioned at a thickness of 5 μm using a microtome (Microm HM‐325). The tissue slides were warmed to 37°C for a minimum of 24 hr prior to immunohistochemical procedures. Antigen retrieval was performed in citrate buffer (pH 6.0) in a de‐cloaking chamber. Slides were blocked in 10% horse serum and subsequently incubated with mouse monoclonal antibodies specific to either MyHC type I (Sigma‐Aldrich NOQ7.5.4D) or MyHC type II (Sigma‐Aldrich MY‐32) at a dilution of 1:3200 for 1 hr at room temperature as established by previous experimentation (Moore et al., 2017b). The antibodies NOQ7.5.4D and MY‐32 label all type I (slow‐twitch) fibers and all type II (fast‐twitch) fibers, respectively. The secondary antibody, ImmPRESS Anti‐Mouse Ig Peroxidase Polymer Detection Kit (Vector Laboratories, Cat. No. MP‐7402), was applied prior to labeling with DAB (DAB Peroxidase Substrate Kit, 3,3′‐diaminobenzidine, Vector Laboratories, Cat. No. SK‐4100). Specimen‐matched negative control sections underwent identical procedures, except for the application of the primary antibody. Hematoxylin counterstain was applied to the negative control and to immunohistochemical sections labeled for MyHC type I and MyHC type II (Figure 1). Hematoxylin was applied to the immunohistochemical sections for 1 min and subsequently dehydrated through a series of graded alcohols, cleared in xylene, and mounted on microscope slides. The hematoxylin stain is beneficial for enhancing the contrast between the dark appearance of labeled muscle fibers and unlabeled muscle fibers (Kiernan, 2000; Behan et al., 2002; Taylor and Rudbeck, 2013).

FIGURE 1.

FIGURE 1

Open in a new tab

Histological appearance of contiguous abductor pollicis brevis (APB) and palmaris longus (PL) muscles. The APB and PL were labeled with both NOQ7.5.4D and MY‐32 antibodies allowing for identification of muscle fibers composed of myosin heavy chain type I and type II isoforms, respectively. Both the APB and PL muscles consisted of a heterogeneous proportion of type I and type II muscle fibers. Representative type I (yellow), type II (red), and hybrid (blue) muscle fibers are identified in the serial histological cross sections. Note: the histological sections were harvested from a single cadaveric specimen with the PL tendon serving as a discrete origin to the APB. Scale bar: 100 μm

2.4. Image acquisition and muscle fiber quantification

High‐resolution images of the PL and APB tissue sections were captured using ToupView computer software (OMAX, Ver. X64, 2.7.5849) and a digital USB microscope camera (OMAX, model: A35140U3; 14 megapixels) attached to a Leitz Laborlux S Microscope. Two to three sites from each slide were imaged at 40× magnification such that a minimum of 2000 representative muscle fibers was visible for quantitative analysis per specimen, except in one APB sample (1861 fibers) due to a smaller tissue volume. Serial slide cross sections of the PL and APB were labeled for MyHC type I or MyHC type II and then compared side by side to classify the muscle fibers as type I, type II, or hybrid fibers (Figure 1). Fiber quantification was performed using the counting tool in Adobe Photoshop CC software (2015.5.0 Release). In each PL and APB specimen, the fiber type proportions were calculated as a percentage by dividing each fiber type number (type I, type II, and hybrid) by the total number of muscle fibers in each immunohistochemical section.

2.5. Statistical analysis

Primary data tabulation and calculations were performed using Excel software (version 13.5.8, 2011, Microsoft Corporation). Statistical analysis was performed using SPSS statistical software (version 25, SPSS Inc.). Statistical analysis was performed on the type II muscle fiber proportions. The type II fiber proportions were transformed using a logit transformation, ln (p) = [(p/(1 − p)]. A general linear mixed model was used to determine the effect of sex (male, female), hand (left, right), morphology (APBD, APBND), and muscle (APB, PL) on the type II muscle fibers. A random factor, cadaver, was included in the statistical model to account for the correlation between muscles taken from the same cadaver. Post hoc comparisons were performed with a Bonferroni correction. All descriptive statistics are presented as mean ± SD.

3. RESULTS

3.1. Morphological classification

The superficial fascicle of the APB originated from a bifurcated PL tendon in 9/24 hands (37%) or directly from the PL tendon in the remaining 15/24 hands (63%). The APB fascicles from 11 hands (46%) were classified as discrete based on their distinct continuity with the PL tendon. Conversely, the APB fascicles from 13 hands (54%) were classified as nondiscrete due to rudimentary or minimal connectivity with the PL tendon (Figure 2).

FIGURE 2.

FIGURE 2

Open in a new tab

Morphology of the “lumbrical”‐like fascicular divisions of the abductor pollicis brevis (APB). Upper Row: APB fascicular divisions with discrete origins from the palmaris longus tendon (PL); Lower row: APB fascicular divisions with nondiscrete/rudimentary origins from the palmaris longus tendon. The discrete APB fascicular divisions were relatively mobile and originated primarily from a robust bifurcated PL tendon (n = 11). Conversely, the nondiscrete APB fascicular divisions were affixed primarily to the carpal bones and consisted of rudimentary connections with the PL tendon through thin fascial extensions (n = 13). APB fascicles were also classified as nondiscrete if there was no visual deformation of the APB muscle upon applying tension to the contiguous PL tendon

3.2. Muscle fiber quantification

In 24 cadaveric upper limbs, a total of 55,267 PL (left forearms: 28,412; right forearms: 26,855) and 52,042 APB (left hands: 26,537; right hands: 25,505) muscle fibers were examined throughout the serial histological sections. The mean proportions of type I, type II, and hybrid fibers identified in the PL and APB are summarized in Table 1.

TABLE 1.

Percentage of all fiber types within the palmaris longus and abductor pollicis brevis

Palmaris longus tendon connection Palmaris longus fiber type (%) Abductor pollicis brevis fiber type (%)
Type I Hybrid Type II Type I Hybrid Type II
Discrete

41 ± 11

(9814)

4 ± 3

(983)

55 ± 12

(12,871)

44 ± 16

(9952)

15 ± 10

(3505)

41 ± 19

(9269)
Nondiscrete

49 ± 10

(15,205)

6 ± 4

(2053)

45 ± 9

(14,341)

75 ± 9

(22,210)

10 ± 6

(2748)

15 ± 8

(4358)
Open in a new tab

Values are expressed as means ± standard deviations (absolute number of muscle fibers).

The results of the general linear mixed model indicated a statistically significant interaction effect between morphology (APBD, APBND) and muscle (PL, APB) on the logit proportion of type II muscle fibers examined [F(1, 32) = 5.748, p = 0.023)] (Table 2). Simple main effects revealed a significant difference in the logit proportion of type II fibers between APB and PL muscles [F(1, 32) = 37.316, p < 0.001] and between discrete and nondiscrete connectivity [F(1, 32) = 18.147, p < 0.001]. There was no significant difference in the logit proportions of type II fibers between discrete and rudimentary connectivity in the PL muscles but there was for the APB muscles [t(32) = 4.744, p = 0.0000418]. These statistical results are summarized graphically in Figure 3. The random effect, cadaver, was not significant [z =0.261, p = 0.794].

TABLE 2.

Fixed‐effects table from the four‐factor general linear model

Source F Df1 Df2 Sig.
Corrected Model 4.907 15 32 0.000
Sex 0.079 1 32 0.781
Hand 0.655 1 32 0.424
Morphology 18.147 1 32 0.000
Muscle 37.316 1 32 0.000
Sex * Hand 0.029 1 32 0.865
Sex * Morphology 0.075 1 32 0.787
Sex * Muscle 0.298 1 32 0.589
Hand * Morphology 1.683 1 32 0.204
Hand * Muscle 0.018 1 32 0.895
Morphology * Muscle 5.748 1 32 0.023
Sex * Hand * Morphology 0.015 1 32 0.902
Sex * Hand * Muscle 1.142 1 32 0.293
Sex * Morphology * Muscle 0.491 1 32 0.488
Hand * Morphology * Muscle 1.869 1 32 0.181
Sex * Hand * Morphology * Muscle 0.905 1 32 0.349
Open in a new tab

FIGURE 3.

FIGURE 3

Open in a new tab

Mean proportions of type II fibers within the palmaris longus (PL) and abductor pollicis brevis (APB) with contiguous and noncontiguous connections through the PL tendon. The APB contains significantly greater type II muscle fibers when arranged through a discrete tendinous connection with the PL. Note the figure presents mean percentages from original data; however, statistical analysis was performed on transformed data. *p < 0.05

4. DISCUSSION

The PL is regarded as a muscle whose clinical importance as an autologous tendon graft may supersede its functional purpose in vivo. Recent investigations have presented evidence that the PL may contribute significant strength to thenar musculature based on a functional synergy with the APB (Gangata et al., 2010; Moore et al., 2018). By determining the fiber type proportions of the APB based on its morphological arrangement with the PL, a better understanding of the complexity of thenar function may be gained. This knowledge could assist in surgical restoration of opposition movements, for example, in cases of severe thenar paralysis. In the present study, the APB and PL muscles from 24 cadaveric limbs were examined to determine whether their morphological arrangement influenced the APB fiber type proportions. Using immunohistochemical techniques, a differential proportion of type I and II muscle fibers were found among APB musculature with contiguous discrete (APBD), and rudimentary, nondiscrete (APBND), morphological connections with the PL tendon. This may provide further evidence of the quality of digastric relationship and functional synergy established in vivo.

4.1. “Lumbricals” of the thumb

Textbooks typically describe the APB as originating from the scaphoid tubercles, trapezium, and the flexor retinaculum prior to its insertion into the base of proximal phalanx of the 1st digit (Gilroy, 2017; Moore et al., 2017a). Comprehensive morphological studies describe the APB as consisting of multiple divisions with a superficial division consisting of discrete fascicles that insert into the dorsal aponeurotic expansion of the 1st digit (Simard and Roberge, 1988). In 44 dissected upper limbs with PL musculature, Fahrer (1977) observed several “lumbrical”‐like APB fascicles arising from the tendons of extrinsic musculature including the PL and abductor pollicis longus tendons. A discrete APB fascicle originated from the PL tendon in 23 (52%) hands, and the remaining APB fascicles (48%, 21/44) originated from a fibrous arch between the PL and abductor pollicis longus tendons (Fahrer, 1977). In the absence of the PL, a radial APB muscle belly originated consistently from the abductor pollicis longus tendon (Fahrer, 1977). In the present study, the APB fascicles originated from either a distinct lateral PL terminal tendon (37%) or directly from the PL tendon proximal to the palmar aponeurosis (63%); however, only in 11/24 hands did the PL serve as an exclusive origin to a relatively mobile APB fascicle (Figure 2). In the remaining hands (n = 13), the APB was primarily affixed to its carpal origins with the PL providing only a rudimentary, or nondiscrete, connection (Figure 2). Compared to Fahrer (1977), the fibrous arch between the PL and abductor pollicis longus was not observed in our sample, but was illustrated in a previous investigation (Moore et al., 2018).

Lumbricals are known for their unique worm‐like appearance (Latin, lumbricus: earthworm) and function in both digital flexion and extension of the MCP and interphalangeal joints, respectively (Moore et al., 2017a). The fiber type composition of the lumbrical acting upon the index finger is relatively heterogeneous composition of type I (43%) and II (57%) muscle fibers (Hwang et al., 2013) (Table 3), which was consistent with the fiber type composition of the “lumbrical”‐like APBD fascicles in continuity with the PL tendon (type I: 44%; type II: 56%1, Table 1). Conversely, we observed a predominance of type I (75%) muscle fibers in the APBND fascicles, which is consistent with type I APB (63%) fiber proportions harvested from tissues of young cadavers (range: 22–30 years) (Johnson et al., 1973). The APBD fascicles share a morphological and functional homology with proper lumbricals based on their tendinous origins from extrinsic musculature, assistance in MCP joint flexion and interphalangeal joint extension, and a consistency in phenotypic muscle fiber type profile.

TABLE 3.

Fiber type composition of select musculature of the head, forearm, and hand

Muscle Fiber type Reference
% Type I % Type II
Palmaris Brevis 72% 28% a Moore et al. (2017b)
Adductor Pollicis 80% 20% Round et al. (1984)
Abductor Pollicis Brevis 63% 37% Johnson et al. (1973)
Abductor Digiti Minimi 52% 48% Johnson et al., 1973)
Lumbrical (Index finger) 43% 57% Hwang et al. (2013)
Flexor Digitorum Profundus 47% 53% Johnson et al. (1973)
Digastric Muscle (Anterior) 37% 63% Monemi et al. (1999)
Digastric Muscle (Posterior) 36% 64%
Open in a new tab
a

includes hybrid fibers.

4.2. Thenar muscles as a series of digastric complexes

4.2.1. Palmaris longus and abductor pollicis brevis

The thenar eminence has been described as a system of extrinsic–intrinsic musculature consisting of the APB, PL, and abductor pollicis longus (Fahrer and Tubiana, 1976). When considered as a functional unit, the PL and APBD share similar morphological features with the digastric muscle including the presence of two muscular heads interconnected by an intermediate tendon (Figure 1). The anterior and posterior bellies of the digastric muscle from aged cadavers (mean: 73 years) both consist of a predominance of type II (anterior: 63%; posterior: 64%, Table 3) muscle fibers, despite independent cranial innervation patterns (Monemi et al., 1999).

In healthy human aging, a loss of type I and II motor units and decrease in muscle fiber diameter contribute to muscle atrophy and weakness associated with old age (Berger and Doherty, 2010). Preservation of muscle function may occur through collateral reinnervation processes in which denervated type II muscle fibers are reinnervated by adjacent slower type I motor units producing hybrid muscle fibers co‐expressing both slow and fast MyHC isoforms (Andersen et al., 1999; Hepple and Rice, 2016). The APBD muscles consisted of a relatively large proportion of hybrid fibers (15%, Table 1), which may be indicative of age‐related type II motor unit loss and collateral reinnervation processes. Although hybrid fibers co‐express multiple MyHC isoforms, their contractile properties are “fast‐like” compared to pure type I fiber types (Bottinelli et al., 1996; Pette and Staron, 2001). If the percentage of APBD hybrid fibers (I/II) are pooled with type II fibers, the contiguous APBD and PL muscles share similar type I (APBD: 44%; PL: 41%) and type II (APBD: 56%; PL: 59%) fiber type proportions; a feature consistent with the digastric muscle. Compared to the predominant type I muscle fiber proportions typical of thenar musculature (Table 3), the heterogeneous APBD fiber composition could represent a functional advantage allowing for more powerful thenar abduction contractions for activities of daily living, fine thenar motor control, and hand dexterity.

The lumbricals share a similar heterogeneous fiber type composition with the flexor digitorum profundus (Table 3), which acts as the origin to the lumbricals of the 2nd to 5th digits. Electrical stimulation of the lumbricals produces interphalangeal joint extension prior to flexion of the metacarpophalangeal joints (Backhouse and Catton, 1954). Furthermore, the lumbricals also contract in concert with the extensor digitorum muscle during voluntary finger extension movements (Backhouse and Catton, 1954). The lumbricals are engaged in a functional interrelationship with two extrinsic forearm muscles, likely indicative of a proprioceptive role of the lumbricals during pinch and grasp actions of the hand (Ranney and Wells, 1988). The APB has almost double the mean muscle spindle density of the lumbricals (APB: 29 spindles/gram of muscle vs. lumbricals: 17 spindles/gram of muscle), suggesting the thumb requires more precise sensory feedback for fine thenar coordination (Peck et al., 1984; Wang et al., 2014). If the APBD is considered a distal muscle belly of the PL when contiguously arranged, the APBD could provide a similar proprioceptive role as the lumbricals for crucial sensory feedback during the performance and coordination of complex thumb joint movements.

The functional relationship between the PL and thenar musculature has been investigated in vivo in young participants using indwelling fine‐wire electromyography and ultrasound imaging techniques (Moore et al., 2018). In response to maximal thenar abduction contractions, an increase in PL muscle thickness (21%) and PL muscle activity (46%) was recorded, indicating the PL functions as an extrinsic thenar muscle in vivo (Moore et al., 2018). Comparing those with congenital PL absence, Gangata et al. (2010) observed significantly greater thenar abduction strength in those with PL musculature and attributed the PL tendon as the means for transmitting additional force to the thenar eminence. In the APBD fascicles, the greater proportion of type II muscle fibers could further contribute to the contraction power along with the additional strength contributions from the PL muscle mass itself.

In severe carpal tunnel syndrome, open carpal tunnel release in conjunction with PL opponensplasty allows for restoration of functional, dexterous hand movements to perform activities of daily living during recovery of thenar muscle atrophy (Camitz, 1929; Foucher et al., 1991; Terrono et al., 1993; Rymer and Thomas, 2016). In a study of 21 patients, moderate to abundant muscle contractions were observed in the PL post‐tendon transfer using ultrasound imaging during opposition (90%, 19/21) and abduction (81%, 17/21) movements; however, PL muscle contraction was minimal or absent in the remaining patients (Durban et al., 2017). The surgical outcome of PL opponensplasty is likely multifactorial and may depend on individual factors such as PL muscle and tendon morphology, and the extent of synergistic relationship established in vivo between the PL and APB prior to tendon transfer. In our sample of hands, rudimentary, or nondiscrete, connections of the PL with the APBND were observed in 54% of cases and were accompanied by a predominant proportion of type I muscle fibers (75%). In a portion of these cases, the synergistic relationship between the PL and APBND fascicles may be minimal due to rudimentary PL tendon extensions to the thenar eminence (Figure 2). In a previous functional investigation, the absence of synchronous synergistic EMG activity between the PL and APB was attributed to variant PL tendon morphology at the wrist (Moore et al., 2018). If an adequate synergy fails to develop in vivo, the variant anatomy of the PL tendon could influence the functional recovery and opponensplasty success. Furthermore, Fahrer and Tubiana (1976) proposed surgical mobilization of the thenar “lumbricals” in conjunction with the PL terminal tendon as a complex to restore functional thenar abduction movements in patients with thenar paralysis; however, an established synergy, viable PL tendon, and adequate PL muscle mass likely are necessary to achieve adequate force transmission to restore functional thenar movement capacity.

Muscle fiber types can undergo ordinal phenotypic changes in their MyHC profiles (MyHC I ⇔ MyHC IIa ⇔ MyHC IIx) in response to altered muscle activation patterns (Pette and Staron, 2001). For surgically transferred muscles, their constituent muscle fibers may shift toward a slower or faster phenotype depending on their surgically imposed function. In autologous PL tendon transfer for opponensplasty, the fiber type transitions may be less apparent in those muscles with previously established functional synergistic relationships with thumb musculature (ex: PL muscles with robust tendons to the APB). Comparing other hand muscles used in surgical opponensplasty, the abductor digiti minimi (Cawrse and Sammut, 2003) may experience alterations in its phenotypic fiber type profile (Table 3) in response to differential muscle activation patterns induced through an imposed functional and anatomical relationship with thumb musculature. Therefore, the muscle fiber types are dynamic and likely capable of further phenotypic transitions depending on whether a muscular synergy is established during normal embryological development or imposed following a surgical transfer.

4.2.2. Abductor pollicis longus and abductor pollicis brevis

Beyond the evidence demonstrating continuity of the PL with the APB, other known connections among extrinsic and intrinsic thumb musculature are found between the APB and abductor pollicis longus (van Oudenaarde and Oostendorp, 1995). The abductor pollicis longus located on the posterior forearm is divided into superficial and deep divisions (van Oudenaarde and Oostendorp, 1995). Whereas the superficial division of the abductor pollicis longus inserts primarily on the 1st metacarpal, the deep division can have several insertions into the trapezium, joint capsule, and capsular ligaments (van Oudenaarde and Oostendorp, 1995). Most notably, the deep division of the abductor pollicis longus consistently inserts into a radial muscle belly of the APB through an accessory tendon in 64‐84% of cases (Baba, 1954; Fahrer, 1977; van Oudenaarde and Oostendorp, 1995; Moore et al., 2018). Although Le Double (1897) considered the connection between the APB and abductor pollicis longus as a malformation, surgical observations in stenosing tenosynovitis at the wrist (De Quervain's disease) indicate that variation in the abductor pollicis longus tendon is the rule rather than exception with ≥2 accessory tendons to the APB occur in 76% of reported cases (Bahm et al., 1995). Failure to adequately release all abductor pollicis longus accessory tendons from the first dorsal compartment could result in incomplete tendon decompression leading to persistent wrist pain after surgical treatment (Patel et al., 2013). Along with receiving radial arterial branches, the radial APB muscle belly can receive radial innervation from the superficial branch as observed in 4 cases of a small sample (n = 10) of dissected hands (Fahrer, 1977), indicating the possibility of the APB receiving dual motor innervation from both the median and radial nerves in some individuals. Therefore, the several APB muscle bellies originating from the tendons of extrinsic forearm musculature suggest that fine thenar motor movements function through a series of digastric muscular complexes in vivo.

5. LIMITATIONS

A potential limitation of this study was the use of muscle tissue harvested from aged cadavers, which could lead to an increase in hybrid fibers and potential fiber type grouping due to collateral reinnervation processes (McNeil and Rice, 2018). Furthermore, muscle fiber cross‐sectional areas could not be measured due to the myofiber shrinkage associated with using embalmed tissue. By comparing muscle fiber cross‐sectional areas between type I and type II fiber proportions, further insights into the potential force‐generating capacity of the APBD type II muscle fibers and their overall contributions to muscle power could be gained. Although there were no significant differences in muscle fiber proportions between hands and sexes among the morphological arrangements of the PL and APB, foreknowledge of the handedness of the cadaveric donors would have provided further insight into whether the PL tendon morphology is related to hand dominance. Future studies should investigate the PL and APB type I and type II fiber proportions in unembalmed muscle from younger adult cadavers.

6. CONCLUSION

The disparate fiber type proportions in the APBD compared to APBND fascicles provide support of the PL and APB in enhancing strength contributions to the thenar eminence based on a digastric relationship in vivo. The presence of a rudimentary PL morphological tendon relationship with APB musculature may prolong the motor learning and functional retraining of thenar movements from PL opponensplasty, if a sufficient functional synergy fails to develop in vivo.

AUTHOR CONTRIBUTIONS

C.W.M. harvested all the PL and APB muscle tissues and performed the immunohistochemical procedures. C.W.M. performed the primary analyses including fiber quantification. C.W.M. and C.L.R. conceputalized the study and all authors (C.W.M., J.F., and C.L.R.), interpreted the data, and prepared the figures and manuscript for publication.

ACKNOWLEDGMENTS

We wish to thank Linda Jackson from the Department of Oral Pathology for providing supplies and facilities to perform the muscle fiber typing procedures. We would like to thank Dr. Tyler Beveridge for assistance and guidance in the development of the immunohistochemical protocol. Furthermore, we would like to thank Dr. Brian Allman who also contributed additional supplies to complete the immunohistochemical procedures. We appreciate the helpful statistical consultation and advice from John J. Koval, PhD, P.Stat., Department of Epidemiology and Biostatistics at the University of Western Ontario. We also thank the individuals who generously donated their bodies for science and research. Without their selfless gift, this study would not have been possible. This study was supported by a grant from the Natural Sciences & Engineering Research Council of Canada (NSERC).

Moore CW, Fanous J, Rice CL. Fiber type composition of contiguous palmaris longus and abductor pollicis brevis muscles: Morphological evidence of a functional synergy. J. Anat. 2020;238:53–62. 10.1111/joa.13289

Footnotes

1

Includes hybrid fibers (hybrid fibers: 15% + type II: 41%).

REFERENCES

  1. Andersen, J.L. , Terzis, G. and Kryger, A. (1999) Increase in the degree of coexpression of myosin heavy chain isoforms in skeletal muscle fibers of the very old. Muscle and Nerve, 22, 449–454. [DOI] [PubMed] [Google Scholar]
  2. Baba, M.A. (1954) The accessory tendon of the abductor pollicis longus muscle. Anatomical Record, 119, 541–547. [DOI] [PubMed] [Google Scholar]
  3. Backhouse, K.M. and Catton, W.T. (1954) An experimental study of the functions of the lumbrical muscles in the human hand. Journal of Anatomy, 88, 133–141. [PMC free article] [PubMed] [Google Scholar]
  4. Bahm, J. , Szabo, Z. and Foucher, G. (1995) The anatomy of de Quervain's disease. A study of operative findings. International Orthopaedics, 19, 209–211. [DOI] [PubMed] [Google Scholar]
  5. Behan, W.M. , Cossar, D.W. , Madden, H.A. and McKay, I.C. (2002) Validation of a simple, rapid, and economical technique for distinguishing type 1 and 2 fibres in fixed and frozen skeletal muscle. Journal of Clinical Pathology, 55, 375–380. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Berger, M.J. and Doherty, T.J. (2010) Sarcopenia: prevalence, mechanisms, and functional consequences. Interdiscip Top Gerontol, 37, 94–114. [DOI] [PubMed] [Google Scholar]
  7. Bottinelli, R. , Canepari, M. , Pellegrino, M.A. and Reggiani, C. (1996) Force‐velocity properties of human skeletal muscle fibres: myosin heavy chain isoform and temperature dependence. Journal of Physiology, 495(Pt 2), 573–586. [DOI] [PMC free article] [PubMed] [Google Scholar]
  8. Camitz, H. (1929) Surgical treatment of paralysis of opponens muscle of thumbs. Acta Chir Scand, 65, 77–81. [Google Scholar]
  9. Cawrse, N.H. and Sammut, D. (2003) A Modification in technique of abductor digiti minimi (Huber) opponensplasty. Journal of Hand Surgery, 28, 233–237. [DOI] [PubMed] [Google Scholar]
  10. Durban, C.M. , Antolin, B. , Sau, C.Y. , Li, S.W. and Ip, W.Y. (2017) Thumb function and electromyography result after modified camitz tendon transfer. The Journal of Hand Surgery (Asian‐Pacific Volume), 22, 275–280. [DOI] [PubMed] [Google Scholar]
  11. Fahrer, M. (1977) On the form and function of the abductor pollicis brevis muscle. Australian and New Zealand Journal of Surgery, 47, 243–247. [DOI] [PubMed] [Google Scholar]
  12. Fahrer, M. and Tubiana, R. (1976) Palmaris longus, anteductor of the thumb. Hand, 8, 287–289. [DOI] [PubMed] [Google Scholar]
  13. Foucher, G. , Malizos, C. , Sammut, D. , Braun, F.M. and Michon, J. (1991) Primary palmaris longus transfer as an opponensplasty in carpal tunnel release. A series of 73 cases. Journal of Hand Surgery, 16, 56–60. [DOI] [PubMed] [Google Scholar]
  14. Gangata, H. , Ndou, R. and Louw, G. (2010) The contribution of the palmaris longus muscle to the strength of thumb abduction. Clinical Anatomy, 23, 431–436. [DOI] [PubMed] [Google Scholar]
  15. Gilroy, A. (2017) Anatomy An Essential Textbook, 2nd edition. New York: Thieme Medical Publishers. [Google Scholar]
  16. Hepple, R.T. and Rice, C.L. (2016) Innervation and neuromuscular control in ageing skeletal muscle. Journal of Physiology, 594, 1965–1978. [DOI] [PMC free article] [PubMed] [Google Scholar]
  17. Hwang, K. , Huan, F. and Kim, D.J. (2013) Muscle fibre types of the lumbrical, interossei, flexor, and extensor muscles moving the index finger. Journal of Plastic Surgery and Hand Surgery, 47, 268–272. [DOI] [PubMed] [Google Scholar]
  18. Johnson, M.A. , Polgar, J. , Weightman, D. and Appleton, D. (1973) Data on the distribution of fibre types in thirty‐six human muscles. An autopsy study. Journal of the Neurological Sciences, 18, 111–129. [DOI] [PubMed] [Google Scholar]
  19. Kaplan, E.B. (1984) Kaplan's Functional and Surgical Anatomy of the Hand. Philadelphia, PA: Lippincott Williams & Wilkins. [Google Scholar]
  20. Kato, N. , Yoshizawa, T. and Sakai, H. (2014) Simultaneous modified Camitz opponensplasty using a pulley at the radial side of the flexor retinaculum in severe carpal tunnel syndrome. Journal of Hand Surgery (European Volume), 39, 632–636. [DOI] [PubMed] [Google Scholar]
  21. Kiernan, J.A. (2000) Histological and Histochemical Methods: Theory and Practice. Oxford: Butterworth Heinemann. [Google Scholar]
  22. Kim, S.Y. , Lunn, D.D. , Dyck, R.J. , Kirkpatrick, L.J. and Rosser, B.W. (2013) Fiber type composition of the architecturally distinct regions of human supraspinatus muscle: a cadaveric study. Histology and Histopathology, 28, 1021–1028. [DOI] [PubMed] [Google Scholar]
  23. Le Double, A.F. (1897) Traité des variations du système musculaire de l'homme et de leur signification au point de vue de l'anthropologie zoologique. Paris: Schleicher frères. [Google Scholar]
  24. Lovering, R.M. and Anderson, L.D. (2008) Architecture and fiber type of the pyramidalis muscle. Anatomical Science International, 83, 294–297. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. McNeil, C.J. and Rice, C.L. (2018) Neuromuscular adaptations to healthy aging. Applied Physiology, Nutrition and Metabolism, 43, 1158–1165. [DOI] [PubMed] [Google Scholar]
  26. Monemi, M. , Thornell, L. and Eriksson, P. (1999) Diverse changes in fibre type composition of the human lateral pterygoid and digastric muscles during aging. Journal of the Neurological Sciences, 171, 38–48. [DOI] [PubMed] [Google Scholar]
  27. Moore, K.L. , Dalley, A.F. and Agur, A.M.R. (2017a) Clinically Oriented Anatomy, 8th edition. Philadelphia, PA: Lippincott Williams & Wilkins. [Google Scholar]
  28. Moore, C.W. , Beveridge, T.S. and Rice, C.L. (2017b) Fiber type composition of the palmaris brevis muscle: implications for palmar function. Journal of Anatomy, 231, 626–633. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Moore, C.W. , Fanous, J. and Rice, C.L. (2018) Revisiting the functional anatomy of the palmaris longus as a thenar synergist. Clinical Anatomy, 31, 760–770. [DOI] [PubMed] [Google Scholar]
  30. Napier, J.R. (1952) The attachments and function of the abductor pollicis brevis. Journal of Anatomy, 86, 335–341. [PMC free article] [PubMed] [Google Scholar]
  31. Patel, K.R. , Tadisina, K.K. and Gonzalez, M.H. (2013) De quervain's disease. Eplasty, 13, ic52. [PMC free article] [PubMed] [Google Scholar]
  32. Peck, D. , Buxton, D.F. and Nitz, A. (1984) A comparison of spindle concentrations in large and small muscles acting in parallel combinations. Journal of Morphology, 180, 243–252. [DOI] [PubMed] [Google Scholar]
  33. Pette, D. and Staron, R.S. (2000) Myosin isoforms, muscle fiber types, and transitions. Microscopy Research and Technique, 50, 500–509. [DOI] [PubMed] [Google Scholar]
  34. Pette, D. and Staron, R.S. (2001) Transitions of muscle fiber phenotypic profiles. Histochemistry and Cell Biology, 115, 359–372. [DOI] [PubMed] [Google Scholar]
  35. Ranney, D. and Wells, R. (1988) Lumbrical muscle function as revealed by a new and physiological approach. Anatomical Record, 222, 110–114. [DOI] [PubMed] [Google Scholar]
  36. Ross, A.C. (2016) Wrist and hand In: Rolfe B. and Standring S. (Eds.), Gray's Anatomy: The Anatomical Basis of Clinical Practice, 41st edition. New York, NY: Elsevier; pp. 1567. [Google Scholar]
  37. Round, J.M. , Jones, D.A. , Chapman, S.J. , Edwards, R.H. , Ward, P.S. and Fodden, D.L. (1984) The anatomy and fibre type composition of the human adductor pollicis in relation to its contractile properties. Journal of the Neurological Sciences, 66, 263–272. [DOI] [PubMed] [Google Scholar]
  38. Rymer, B. and Thomas, P.B. (2016) The Camitz transfer and its modifications: a review. Journal of Hand Surgery (European Volume), 41, 632–637. [DOI] [PubMed] [Google Scholar]
  39. Schroeder, K.L. , Rosser, B.W. and Kim, S.Y. (2014) Fiber type composition of the human quadratus plantae muscle: a comparison of the lateral and medial heads. Journal of Foot and Ankle Research, 7, 54. [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Simard, T. and Roberge, J. (1988) Human abductor pollicis brevis muscle "divisions" and the nerve hila. Anatomical Record, 222, 426–436. [DOI] [PubMed] [Google Scholar]
  41. Sooriakumaran, P. and Sivananthan, S. (2005) Why does man have a quadratus plantae? A review of its comparative anatomy. Croatian Medical Journal, 46, 30–35. [PubMed] [Google Scholar]
  42. Taylor, C. and Rudbeck, L. (2013) Immunohistochemistry staining methods In DAKO Education Guide Immunohistochemical Staining Methods, 6th edition. Denmark: Dako; pp. 10–93. [Google Scholar]
  43. Terrono, A.L. , Rose, J.H. , Mulroy, J. and Millender, L.H. (1993) Camitz palmaris longus abductorplasty for severe thenar atrophy secondary to carpal‐tunnel syndrome. The Journal of Hand Surgery, 18a, 204–206. [DOI] [PubMed] [Google Scholar]
  44. van Oudenaarde, E. and Oostendorp, R.A.B. (1995) Functional‐relationship between the abductor pollicis longus and abductor pollicis brevis muscles ‐ an emg analysis. Journal of Anatomy, 186, 509–515. [PMC free article] [PubMed] [Google Scholar]
  45. Wang, K. , McGlinn, E.P. and Chung, K.C. (2014) A biomechanical and evolutionary perspective on the function of the lumbrical muscle. The Journal of Hand Surgery, 39, 149–155. [DOI] [PMC free article] [PubMed] [Google Scholar]

Articles from Journal of Anatomy are provided here courtesy of Anatomical Society of Great Britain and Ireland

RESOURCES