6 Structure and Function of Muscles
6.1 Overview
The functional abilities of the hand and upper extremity occur by the integration of several systems. Movement is accomplished by contraction of muscles that interact with the skeletal system to achieve a vast array of motions and joint configurations. The astounding dexterity of the hand is the result of the great diversity of skeletal muscles. In this chapter, macroscopic and microscopic aspects of muscle structure are discussed, with emphasis on their relationships to muscle function and their clinical significance.
6.2 Muscle Architecture
Gross examination reveals that most skeletal muscles have a common thematic arrangement: the bulk of the muscle is composed of fibers that project from proximal to distal tendons, which in turn arise from and insert onto bony structures. However, the manner in which these fibers are organized is hugely variable and has profound effects on the physiological properties of muscles. 1 , 2 This arrangement of muscle fibers, relative to the axis of force generation, is referred to as “muscle architecture.”
The categories of muscle architecture are numerous. For simplicity, it is easiest to consider broad organization of muscle fibers in one of three categories. Muscles with fibers arranged parallel to the axis of motion are termed “longitudinal.” Longitudinal muscles make up a relatively small proportion of the muscle in the body; many muscles have fibers that are oriented obliquely to the muscle’s line of action. The angle between this line of action and the muscle fibers is termed the “pennation angle,” and these muscles are classified as “unipennate” and “multipennate” depending on whether their fibers have single or variable pennation angles, respectively.
Based on geometric considerations, it is easy to understand why differences in fiber pennation affect force production capacity between muscles. Consider a simple longitudinal muscle in which every fiber can produce the same amount of force. Adding more fibers in parallel would result in greater force production capacity. However, anatomic space limitations prevent the unlimited addition of fibers in parallel. By placing fibers at an angle, it becomes possible to increase the number of fibers without changing the total volume occupied by the muscle. Modulation of pennation angle allows the placement of muscles in a way that best fits within the anatomic constraints that arise due to the bony skeleton.
6.2.1 PCSA and Fiber Length
Although changing pennation angle increases fiber number while maintaining a constant volume, these fibers do not contract in a line that is parallel with the line of action of the muscle, and they will also necessarily be shorter than the original fibers. These two alterations have physiologic consequences. To summarize the overall effects of muscle fiber arrangement, two important architectural parameters are used: physiological cross-sectional area (PCSA) and fiber length (L f). These are considered the most important parameters because they are the most predictive of muscle biomechanical function.
L f determines the excursion of a muscle. Muscle fibers are composed of many repeating subunits (sarcomeres) that can expand or contract in length. Placing more sarcomeres in series (i.e., having a longer muscle fiber) changes its total excursion capability because excursion of the fiber may be calculated as the sum of the excursions of the individual sarcomeres in series.
PCSA is an important determinant of muscle function because it is the only architectural parameter that is directly related to the maximum force capacity of a muscle. 3 PCSA cannot be measured directly—it is a conceptual “cross-sectional area” that represents the total functional cross-sectional area of all the muscle fibers. It is calculated using the equation
which has been validated in a guinea pig hindlimb model. 3 In this equation, ρ represents muscle density 4 and θ represents pennation angle. Inspection of this equation reveals the basis for its form. Mass/density gives muscle volume, and dividing the volume by fiber length yields a cross-sectional area. However, due to pennation angle, fibers do not pull strictly in line with the axis of force generation, so not all of the force produced is transmitted to the tendons and into joint movement. To estimate the percentage of total force that contributes to functional motion, the term is scaled by the factor of the cosine of the pennation angle. This term arises from a geometric consideration of fiber pennation and standard trigonometric relationships.
By considering these two factors simultaneously, it is possible to compare architectural organization among muscles. Presentation in graphical form (▶Fig. 6.1) makes it possible to visualize groupings and differences among upper-extremity muscles. While many variables may be considered when comparing skeletal muscles, it has been determined that PCSA and L f are the two strongest discriminating factors—that is, that plotting muscle data on the PCSA–L f plane will provide the greatest spread between the data. 5 As part of this analysis, a “score” for quantitatively describing the difference between two muscles was developed and is termed the architectural difference index (ADI). To reconstruct lost muscle function (secondary to trauma, nervous system injury, tendon rupture, etc.), a functional donor muscle is transferred to recapitulate the function that has been lost. Donor selection is a key surgical choice, and ADI may be a reasonable benchmark for selecting potential donor muscles or deciding between two potential transfers (ADI for potential upper extremity pairings are summarized in Table 6.1). It would be unwise to transfer a donor muscle whose force production potential or excursion is vastly different from that of the target muscle. In fact, architectural mismatch has been blamed for the poor clinical results observed in the flexor carpi ulnaris to extensor digitorum communis tendon transfer. 6
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