The static hold limits the blood flow in the muscles during the work and encourages the hypertrophy of the slow fibers.
Blood Flow "Occlusion"
Blood Flow Occlusion/Kaatsu Training amount to wrapping your leg or arm with a tourniquet. On a scale of 1 - 10, you want to be around a 6 - 7.
This allows arterial blood flow from the heart to the working muscle and minimize ventricle blood back to the heart; ballooning the muscle up. This produces a anabolic effect for muscle growth.
Muscle Contractions
Muscle contractions with moderate to high repetition exercises, Isometric Actions and Stretching also occlude blood flow. However, pumping blood to the muscles with moderate to high repetition is the most effect method.
"The Size Principle"
There is a "Pecking Order" during exercise when it comes to Muscle Fiber.
Slow Twitch/Type I fire first, followed by Fast/Type IIa and finally "Super" Fast/Type IIb.
When blood flow is restricted to the muscle, it innervates the Fast Twitch Muscle Fiber. "The Size Principle" in the Muscle Firing Sequence still follows the Slow > Fast > "Super" Fast sequence. However, blood flow restriction bring the Fast Twitch Muscle Fiber into play much faster and working and exhaust them quickly.
Engaging the Fast Twitch Muscle Fiber, working them and exhausting them is one of the primary reason for blood flow restriction training methods; Fast Twitch Muscle Hypertrophy Training.
Slow Twitch Muscle Fiber
Once the Fast Twitch Muscle Fiber are exhausted in a movement, the Slow Twitch Muscle Fiber take over.
Static Hold and Blood Flow Occlusion
Static Hold minimally occlude blood flow to the muscles. It doesn't really produce much of "The Pump/Metabolic Stress" necessary for Hypertrophy.
Static Hold for Extending "Metabolic Stress/The Pump"
Of the three components for Muscle Hypertrophy (Mechanical Tension, Metabolic Stress, Muscle Damage), "Metabolic Stress/The Pump" is the dominate factor. It involves flooding blood into the the working muscle via moderate to high repetition.
Once the moderate to high repetition set is completed, performing an Isometric/Static Hold provides a "Tourniquet Like Effect"; maintaining "The Pump" in the muscle, restricting ventricle blood back the heart.
With the said, Static Hold alone provide a minimal amount of Hypertrophy Training. However, performing them after a moderate to high repetition exercise set enables you to maintain "The Pump/Metabolic Stress on the muscle, magnifying the anabolic effect.
Kenny Croxdale