Video narration
Does contracting harder always make a muscle produce more force? Bend your elbow while raising your arm: the biceps brachii crosses both the shoulder and elbow. Shoulder flexion and elbow flexion shorten it together. The problem is not too little effort. It is still contracting, but now it is too short to produce its best force. That force limitation is active insufficiency. So raised arm, bent elbow: the biceps is shortened across two joints and cannot produce peak force.