A surgical retractor has a body and a socket laterally receiving a connector head. A clock is displayed on a viewing device according to a first time interval and a second time interval. Some examples also include a section where an input voltage and a reference voltage are simultaneously blocked such that switching noise may be minimized and a reference voltage may also be stably supplied. The fluid injection system can be used to inject gaseous or liquid refrigerant, lubricant or the fluid injection system can be used to vent the moving pocket to the suction area of the compressor for capacity modulation. The tension sensors generate tension data that corresponds to the measured amount of tension. Exemplary implantable stimulation devices, lead assemblies and methods are also disclosed.