Final answer:
A pacemaker is the medical apparatus that can directly assist a poorly functioning muscle, specifically the heart, by using electrical pulses to maintain a normal heart rhythm.
Step-by-step explanation:
The medical apparatus that can directly assist a poorly functioning muscle in your body is a pacemaker. A pacemaker is a device that's implanted in the chest or abdomen to help control abnormal heart rhythms. This device uses electrical pulses to prompt the heart to beat at a normal rate and rhythm, essentially assisting the heart muscles when they are not functioning properly. On the other hand, a defibrillator is used in cases of life-threatening cardiac dysrhythmias, where it delivers a shock to the heart. An orthopedic brace supports the skeletal system, and a dialysis machine is used when the kidneys fail to filter the blood.