Over-exercise of one muscle group can also result in a condition called chronic exertional compartment syndrome, also known as exercise induced compartment syndrome.
Compartment syndrome in general is usually caused by an accident or injury (like a fracture), but in the case of CECS overworking of the muscles leads to increased muscle volume in a compressed area. Where the muscles have no room to expand, they cause an increase in pressure to the area, which can lead to a disruption of the blood supply to the extremity beyond that point. Most often CECS occurs in the forearm or lower leg, particularly in the anterior tibialis muscle (the muscle at the front of the shin, covering the tibia).
CECS is most common "in athletes who train rigorously in activities that involve constant repetitive actions or motions" (Wikipedia). The recommended cure for CECS is rest of the affected muscle or muscle group.
Muscles relax and contract all the time. Even at rest, the muscles still contract. Only after serious injury, for example, to the spinal chord, or possibly after a stroke, will a muscle become completely slack. Anyone who has had a massage understands how much effort is required just to get up and perhaps pour some water. Arms, even the whole body, feel limp. This is because the muscles are relaxed. If they were not contracted at all, however, imagine the time and effort required to make them respond. Consider new-born babies. They have poor muscle tone and their heads need to be supported at all times until such time as their muscles contract sufficiently and voluntarily to allow them to hold their heads up.
Exercising the same muscle group on a daily basis is counter-productive and causes more harm than good because muscles need to rest in order to grow. Someone who wants to build muscle must factor recovery time into their routine otherwise exhaustion will likely result and motivation will suffer.
Myositis is a condition that can result from working the same muscle groups daily due to inflammation of the muscles. It is usually associated with auto-immune conditions but can also result from infection or injury. Muscle pain causes inflammation and the aggravated condition could lead to myositis. Fortunately, when exercise and subsequent injury is the cause, the myositis will resolve itself with sufficient rest and a chance to recover.