National Council for Certified Personal Trainers (NCCPT) Practice Test

The term "outer unit" refers to a group of muscles that are primarily responsible for movement and stability during activities that require force production and dynamic motion. In this context, the erector spinae muscle group fits this description well, as it plays a key role in in maintaining posture, extending the spine, and supporting movements involving the back.

The erector spinae muscles, located along the length of the spine, are critical for activities that involve bending, lifting, and twisting. They work in conjunction with other muscles of the outer unit to help stabilize the body and allow for effective movement patterns.

Other options such as the hamstrings and quadriceps are important muscle groups that contribute to lower body movement and control but do not encompass the concept of the outer unit in the way that the erector spinae does. The core stabilizers are crucial for stability but focus more on the inner units of stability rather than the movement-oriented outer unit. Thus, the erector spinae muscle is the best fit for the classification of the 'outer unit' within the context of musculoskeletal function.