Final answer:
The concept demonstrated is muscle synergism when a tight psoas decreases the neural drive of the gluteus maximus.
Step-by-step explanation:
The concept demonstrated when a tight psoas decreases the neural drive of the gluteus maximus is muscle synergism.
Muscle synergism refers to the cooperation between muscles to perform a specific movement. In this case, the psoas and gluteus maximus are both involved in hip extension. When the psoas is tight, it can inhibit the activation and neural drive of the gluteus maximus, leading to a decreased ability to extend the hip effectively.
In summary, the concept demonstrated here is muscle synergism, where the tightness of one muscle affects the neural drive and function of another muscle.