The apparatus detects contraction of muscles of living animals, the contraction causing an elastomer body whose impedance changes when flexed, positioned adjacent muscle to flex when the contraction occurs. When a bias voltage is applied to the elastomer body, any change in impedance is readily detectable. The change in impedance may produce a rhythmically repeated elecrical signal, as, for example, when the elastomer body is used to monitor heart activity. This electrical signal can be used for triggering or controlling purposes. For example, this signal can be used to control a pulse generator so that a cardioverting or pacing pulse is generated only after a specified period of time has elapsed since the latest heart activity was sensed by the elastomer body.