The S.A.I.D. Principle

The Specific Adaptation to Imposed Demand (S.A.I.D.) Principle states that the body will adapt only to the specific type of stresses the body undergoes. What does this mean for training? Training is a form of stress. Apply too much and the body will become overtrained and will not adapt well. Apply too little and there will be no signal demanding an adaptation. The body is designed to adapt to meet the needs of the environment in which it lives. From this idea, we can imply that the body will only adapt to the stressors of that environment.

Specificity in Training

Types of muscle contractions

Although there is some carry over from the different types of contractions, increases in strength are generally specific to the type of muscle contraction performed. Different types of contractions create different structural changes within the trained muscle(s). For example, dynamic training the transverse striations of the muscle fibers become more prononced, while the nuclei become oval, motor end-plates extend the length of the muscle fibers, and the endomysium and perimysium become thinner. Static training, the sarcoplasmic content of muscle fibers increase, nuclei become more rounded, motor endplates expand transversally to the muscle fibers, and the endomysium and perimysium become thicker. Types of muscle contractions:

  • Isometric - Muscle is active but does not change in length.
  • Eccentric - Muscle is active and lengthening.
  • Concentric - Muscle is activly shortening.

Movement Pattern and Range of Motion

Strength will be developed most in the movement patterns trained. Even if the muscle groups used within two different patterns are the same. Joint angles will affect how well a muscle group is loaded and how that muscle group will contribute to the movement. Intramuscular (within the muscles) and intermuscular (between the muscles) coordination will be improved in the patterns used but less in other movement patterns. The length of a muscle group while being contracted will also create specific adaptations and affect how well a muscle can contract at a given length and joint angle.

Velocity of Movement and Force of Contraction

F=Mass * Acceleration. This means that high force is dependent on both the load and the speed the load is moved. Heavier weights move slowly while light weights can be moved quickly. The same is true for distance, longer distances are traveled slower than shorter distance. The speed of a movement will create the most significant adaptations to similar speeds. Faster movement speeds generally condition the nervous system while slower speeds are generally better for hypertrophy and slow strength. The load effects what type of training adaptation will occur. If a load is moved for greater than 15 reps the adaptation will be muscular endurance, 8-12 reps muscular hypertrophy, 1-3 maximal strength (power if weight is lessened slightly and move quickly). 

Muscle Fibre Recruitment

Only muscle fibers recruited and fatigued will be trained. Slow twitch fibers produce less force but are more resistant to fatigue, while fast twitch fibers produce higher force but are quick to fatigue.  Intensity, duration, movement pattern will have an effect on how and which muscle fibers are recruited.


We have three pathways for producing energy (ATP/Glyocolitic/Aerobic). ATP-CP system provides energy to the working muscles quickly, but exhausts quickly. The glycolic system uses glycolysis (breaking down of glucose) and lactate to provide fuel at intermediate intensities and durations. The glycolic system does not require oxygen and minimal steps to produce ATP compared to the aerobic system. The aerobic system requires oxygen and can use both glucose and fat to produce ATP. The aerobic system has the most steps required to produce ATP making it slow, but can produce the greatest amount of ATP. Although there is always a combination of these pathways at work, the intensity and duration of the activity will determine the demand placed on each pathway. It may be better to describe each system as high intensity-short duration system (ATP), the intermediate system (glycolytic), and long-term system. Most sports will rely most heavily on the ATP-CP and Glycolytic systems. Since metabolism is specific to intensity and duration, the over development of one system can have detrimental effects on the others. 

Biochemical Adaptations

Different training methods will create different biochemical responses, affecting the adaptations created. Examples of different biochemical adaptations:

  • Strength work - muscular hypertrophy
  • Training at high speeds - increased non-oxidative resynthesis of ATP
  • Endurance work - increased oxidative resynthesis of ATP 


The measure of the maximum range a joint is capable of moving. Since many joints are capable of multiple directions ROM for each joint is specific. There are multiple types of flexibility, such as static active, static passive, ballistic, and active. Each develop from different training methods. More is not always better and should be enhanced only based on the needs of the sport.


The type of work done can create different types of fatigue, such as central or peripheral fatigue. Central fatigue refers to the central nervous system, decreased motivation and impulses of the spinal nerve impulses and impaired recruitment of spinal motor neurons. Peripheral fatigue refers to the muscular system and neuromuscular system. Differences in peripheral fatigue can also be found, such as mechanical or metabolic fatigue. Different types of fatigue will have varying times of recovery, therefore varying times until adaptations are achieved. Some examples of different fatigue:

  • Muscles and Muscle fiber fatigue at different rates
  • Type of work (mechanical or elastic)
  • High intensities - low volume and low intensities - high volumes
  • Speeds of movement
  • Mental


Supertrainning by Yuri Verkhoshanky and Mel Siff

Science and Practice of Strength Training by Vladamir Zatsiorsky and William Kraemer




    Mychael Swenning