The principle of Specificity, often referred to as the SAID (Specific Adaptation to Imposed Demands) principle, states that body will specifically adapt to the type of demand placed on it.
We know that Type I (slow twitch) muscle fibres and Type II (fast twitch) muscle fibres function differently. When the client or athlete engages in prolonged repetitive exercise (cardio) he/she becomes more efficient at recruiting Type I muscle fibres. Similarly, by engaging in resistance training (weight-lifting) he/she will become more efficient at recruiting Type II muscle fibres for this activity.
The degree of adaptation that occurs is directly related to the mechanical, neuromuscular and metabolic specificity of the exercise programme. In order to effectively achieve programme goals, a trainer needs to consistently evaluate the need to manipulate the exercise routine to meet the specific training goals.
The body can only adapt if it has a reason to adapt. In order to elicit the desired adaptations the trainer must ensure that the client or athlete is engaging in exercise that elicits a Specific Alarm Reaction that leads to Resistance Development.