Final answer:
In a very short AK, the quadriceps muscle group is least effective or lacking.
Step-by-step explanation:
In a very short AK, the muscle group that is usually least effective or lacking is the quadriceps or the muscles in the front of the thigh.
The quadriceps are responsible for extending the knee and lifting the leg.
In a very short AK, the muscles in the front of the thigh may not have enough strength or coordination to perform these actions effectively.