3.1.2: The Scientific Status of Both Models
Neuromechanical Matching: Plausible but Messy
As we discussed in Section 3.1.1, the idea that the brain recruits specific regions of a muscle based on leverage (Neuromechanical Matching) is very well-supported in respiratory muscles. Research on the human diaphragm shows a clear pattern: the body selectively sends more neural drive to the regions that have the best mechanical advantage for a specific breathing task [12, 13].
In the arms and legs, however, the picture is more complicated. While we see regional growth, it is harder for researchers to prove that leverage is the only cause. Several factors “muddy the waters” in limb muscles:
- Fiber Type Gradients: Some regions of a muscle may simply have more growth-prone Type II fibers than others.
- Passive Tension: Some regions might experience more stretch, triggering growth via the SMH model rather than NMM.
- Individual Anatomy: Leverage varies significantly between people, making it hard to find a universal “best” angle in studies.
As a recent meta-analysis (Varovic et al., 2025) noted, the differences in regional growth between exercises are often small [15]. Therefore, NMM is best treated as a theoretical framework. It is mechanistically sound and a high-probability strategy for exercise selection, but it should be applied with an experimental mindset rather than as absolute law.
Stretch-Mediated Hypertrophy: Real Results, Debated Mechanics
Stretch-Mediated Hypertrophy (SMH) has an incredibly strong evidence base. We have known since animal studies in the 1970s that passive stretching alone can trigger massive growth [5, 16]. In humans, we see this reflected in the superior results of “lengthened partials”—doing only the bottom half of a rep where the muscle is stretched [10, 11].
The real debate in the scientific community isn’t if it works, but how it works.
- Length vs. Thickness: One prominent model (Beardsley) suggests that passive tension primarily adds “sarcomeres in series.” This makes the muscle longer (longitudinal hypertrophy) but not necessarily thicker [1, 7].
- Conventional Growth: Other researchers (Henselmans) argue that loading the stretched position also triggers conventional “transverse” growth (thickness) that lifters are actually looking for [14]. Multiple human studies support this, showing significant increases in muscle thickness and cross-sectional area from lengthened training [9, 10].
The Terminology Problem: Part of the confusion in this field comes from a lack of agreed-upon definitions. Some researchers use “SMH” only to refer to growth from passive tension, while others use it to describe any growth that happens because you trained at a long muscle length [14].
Bottom Line for the Lifter: regardless of the exact mechanism, training in a stretched position consistently produces good hypertrophy results. So even if the science is still debating how it works, try to include exercises that load the muscle in a lengthened position to maximize your gains. On top of this, if there is enough room for more volume to be allocated, covering the entire contractile range with a variety of exercises is a good idea to ensure comprehensive development.