It’s well-accepted that varying exercise parameters can cause different activation patterns within muscle compartments. This means changing the angle that you work the muscle, the pattern of loading that you use and many other factors. This is vital in a hypertrophy-based program, where promoting uniform growth of muscle tissue is essential for maximizing overall muscle size and density. This works because muscles can have different attachment sites that provide greater leverage for different angles of movement.
Differences within various muscles can impact their response to exercise choice. For example, slow and fast twitch fibres are often scattered across a muscle, so that a slow-twitch fibre can be activated while an adjacent fast-twitch fibre is inactive and vice versa. Muscles are also sometimes divided into neuromuscular components—distinct regions of muscle each of which is innervated by its own nerve branch—suggesting that portions of a muscle can be called into play depending on the activity.
The effects of muscle partitioning on movement are seen in the biceps, where both the long and short heads have areas that are innervated by private branches of the primary neurons. Studies investigating muscle activity of the long head of the biceps show that muscle fibres in the lateral aspect are recruited for elbow flexion, muscle fibres in the medial aspect are recruited for supination, and centrally located muscle fibres are recruited for nonlinear combinations of flexion and supination. It also shows that the short head appears to be more active in the latter part of an arm curl, whereas the long head is more active in the early phase. These variances muscle contraction give support for the need to adopt a multi-planar, multi-angled approach to hypertrophy training using a variety of different exercises. Moreover, given the need to fully stimulate all fibres within a muscle, it would seem that a frequent exercise rotation is warranted to maximize the hypertrophic response.
Moreover, multi-joint movements tend to require significant stabilization of the entire body, thereby involving numerous muscles that otherwise might not be stimulated in the performance of single-joint movements. On the other hand, single-joint exercises allow for a greater focus on individual muscles compared to multi-joint movements. During performance of multi-joint movements, certain prime movers may take precedence over others, creating a hypertrophic imbalance between muscles. The use of single-joint exercises can selectively target underdeveloped muscles, improving muscular symmetry. Moreover, the unique architecture of individual muscles suggests employing single-joint movements can elicit differing neuromuscular activation patterns that heighten overall muscular development.