Final answer:
A TENS unit is the medical device designed to directly assist poorly functioning muscles by using electrical currents to stimulate nerves, offering pain relief and helping muscles relax.
Step-by-step explanation:
The medical apparatus that can directly assist a poorly functioning muscle in your body is a TENS unit (Transcutaneous Electrical Nerve Stimulation unit). This device uses electrical currents to provide pain relief by stimulating nerves, which can indirectly help muscles relax and reduce muscle spasms. A TENS unit can be particularly useful for patients with chronic pain or muscular injuries. On the other hand, devices like a thermometer, which measures body temperature, or a blood pressure monitor such as a sphygmomanometer, are diagnostic tools that do not assist muscles directly. Similarly, a stethoscope is used for auscultation, allowing the technician or physician to listen to the internal sounds of the body, but it does not offer direct assistance to the muscles.