Improved implantable microstimulators covered with a biocompatible polymeric coating in order to provide increased strength to the capsule thereof and to capture fragments of the microstimulator should it become mechanically disrupted and to make the microstimulator safer and easier to handle are provided here. The coating may include one or more diffusible chemical agents that are released in a controlled manner into the surrounding tissue. The chemical agents, such as trophic factors, antibiotics, hormones, neurotransmitters and other pharmaceutical substances, are selected to produce desired physiological effects, to aid, support or to supplement the effects of the electrical stimulation. Further, provided herein are systems employing the improved microstimulators to prevent and/or treat various disorders associated with prolonged inactivity, confinement or immobilization of one or more muscles. Such disorders include pressure ulcers, venous emboli, autonomic dysreflexia, sensorimotor spasticity and muscle atrophy. These systems include external control means for controlling the operation of the microstimulators, said control means having memory means for programming preferred stimulation patterns for later activation by the patient or caregiver.