The present invention relates to a medical injection device (1) for use in combination with a medicament filled cartridge (2), the injection device being provided with a spring-assisted injecting mechanism where energy released from the spring moves a plunger (3) of a held cartridge (2). Sensor circuitry is adapted to detect speed related data during injection so at to detect an abnormal speed condition, such as a needle blocking condition, a priming condition or an air purge condition. The invention also relates to a medical injection device comprising sensor circuitry and user communicating means to provide a recommended needle retraction waiting period which is dependent on the speed of injection.