What is the theory of specificity in muscle contraction?

From the Guidelines

The theory of specificity in muscle contraction is a fundamental principle that states muscles adapt specifically to the type of demands placed upon them during exercise, with the most recent and highest quality evidence supporting this concept found in the 2007 update from the American Heart Association 1. This means that the physiological adaptations a muscle undergoes are directly related to the specific type of training it receives.

Key Points to Consider

• If you train with heavy weights and low repetitions, your muscles will develop more strength and power, while training with lighter weights and higher repetitions will promote muscular endurance.
• This specificity extends to the type of muscle contraction as well - concentric (shortening), eccentric (lengthening), or isometric (static) contractions each produce different adaptations.
• The physiological basis for this specificity involves neural adaptations, changes in muscle fiber recruitment patterns, and alterations in the metabolic pathways that support different types of muscle activity. Some of the key evidence supporting this principle includes:
• A statement from the American Heart Association in 2007, which highlights the importance of resistance training in individuals with and without cardiovascular disease, and emphasizes the need for specificity in exercise training 1.
• The American Thoracic Society/European Respiratory Society statement on pulmonary rehabilitation in 2006, which notes that improvement is specific to those muscles trained, and recommends incorporating upper limb exercises into training programs to improve daily living activities 1.

Application of the Principle

To apply this principle effectively in training, you should design your exercise program to closely match the specific movement patterns, energy systems, and muscle contraction types required for your particular goals or sport. This specificity principle explains why swimmers develop swimming-specific strength that doesn't fully transfer to running, and why resistance training must mimic the demands of a particular activity to maximize performance benefits, as supported by the American Heart Association's 2001 statement on exercise standards for testing and training 1.

From the Research

Theory of Specificity in Muscle Contraction

The theory of specificity in muscle contraction refers to the idea that different types of muscle fibers are specialized for specific types of contractions.

• There are two main types of muscle fibers: slow-twitch (type I) and fast-twitch (type II) fibers 2.
• Slow-twitch fibers are designed for low-intensity, long-lasting contractions, while fast-twitch fibers are designed for high-intensity, short-duration contractions 2.
• The differences in contractile properties between slow-twitch and fast-twitch fibers are due to differences in molecular characteristics, such as myosin heavy chain isoforms and mitochondrial function 3, 2.

Muscle Fiber Type and Exercise

• Human muscle fibers can be classified into different types based on their contractile properties, with slow-twitch fibers being more abundant in endurance athletes and fast-twitch fibers being more abundant in power athletes 3.
• Exercise training can induce changes in muscle fiber type, with some studies suggesting that fibers can shift between hybrid and pure fiber types, as well as between slow and fast fiber types 3.
• The context and extent to which this occurs, along with the limitations of current evidence, are still being researched 3.

Muscle Fiber Type and Recovery

• Muscle fiber type can influence the time to recover from high-intensity exercise, with fast-twitch fibers being more prone to fatigue and delayed recovery compared to slow-twitch fibers 4.
• A non-invasive method using proton magnetic resonance spectroscopy can estimate muscle typology and predict the extent of fatigue and time to recover following repeated all-out exercise 4.

Regulation of Muscle Contraction

• Myosin binding protein C (MyBP-C) is a regulator of contraction that plays a crucial role in skeletal muscle function, with its phosphorylation state regulating sarcomere contraction by modifying cross-bridge recruitment and cycling kinetics 5.
• The regulation of myofilament force and loaded shortening by skeletal MyBP-C is important for understanding the mechanisms of muscle contraction and relaxation 5.

Skeletal Muscle Fiber Type and Disease

• Skeletal muscle fiber type can have a profound impact on muscle diseases, including muscular dystrophies and sarcopenia, with certain fiber types being more susceptible or resistant to disease 6.
• Shifting fiber type characteristics from slow to fast or fast to slow phenotypes may be a potential therapeutic strategy for treating muscle diseases, depending on the disease 6.