Upper limb muscle volumes in adult subjects

Abstract

Muscle force-generating properties are often derived from cadaveric studies of muscle architecture. While the relative sizes of muscles at a single upper limb joint have been established in cadaveric specimens, the relative sizes of muscles across upper limb joints in living subjects remain unclear. We used magnetic resonance imaging to measure the volumes of the 32 upper limb muscles crossing the glenohumeral joint, elbow, forearm, and wrist in 10 young, healthy subjects, ranging from a 20th percentile female to a 97th percentile male, based on height. We measured the volume and volume fraction of these muscles. Muscles crossing the shoulder, elbow, and wrist comprised 52.5, 31.4, and 16.0% of the total muscle volume, respectively. The deltoid had the largest volume fraction (15.2%±1%) and the extensor indicis propius had the smallest (0.2%±0.05%). We determined that the distribution of muscle volume in the upper limb is highly conserved across these subjects with a three-fold variation in total muscle volumes (1427–4426 cm³). When we predicted the volume of an individual muscle from the mean volume fraction, on average 85% of the variation among subjects was accounted for (average p=0.0008). This study provides normative data that forms the basis for investigating muscle volumes in other populations, and for scaling computer models to more accurately represent the muscle volume of a specific individual.

Introduction

Humans vary greatly in size and shape, yet biomechanists often use generic musculoskeletal models with average parameters to evaluate muscle function and coordination. While this approach allows researchers to investigate general principles underlying human movement, it is unclear how conclusions derived from studies of generic models apply to individuals of different sizes.

Muscle force-generating properties used in models are often derived from cadaveric studies of muscle architecture. However, cadaveric specimens may not accurately reflect absolute or relative sizes of muscles in young, healthy subjects. Cadaveric studies of muscle architecture often focus on individual muscle groups; this is especially true for the upper limb, where muscle parameters have been measured separately for the shoulder (Langenderfer et al., 2004), elbow (An et al., 1981; Murray et al., 2000), and forearm and wrist (Lieber et al., 1990; Jacobson et al., 1992; Lieber et al., 1992). Thus, there are excellent data describing the relative size of muscles acting about a single joint in cadaveric specimens, but the relative sizes of muscle across joints in living subjects has not been evaluated.

Several fundamental questions remain unanswered. What are the relative sizes of muscles in the upper extremity? Are relative sizes of muscles consistent across subjects with different total muscle volume? How is muscle volume distributed among muscles crossing the shoulder, elbow, and wrist? We answered these questions by measuring volumes of 32 muscles of the upper limb in young healthy subjects using magnetic resonance imaging (MRI). This study provides the most comprehensive evaluation of muscle volumes in the entire upper extremity to date.