Physiological Effects of Strengthening

Neural factors which may increase muscle strength:

·         Motor units acting synchronously

·         More motor units activated during a muscular contraction

·         A reducing or counteracting inhibitory impulses (e.g. from golgi tendon organs)

Motor unit = A single motor neurone and all the muscle fibres that it stimulates.

Muscle hypertrophy:

• Transient: oedema in the interstitial and intracellular spaces of the muscle (fluid returns to blood within hours)
• Chronic: fibre hypertrophy or fibre hyperplasia

Fibre Hypertropy –

-          More myofibrils

-          More actin and myosin filaments

-          More sarcoplasm

-          More connective tissue

During exercise, protein synthesis decreases while protein degradation increases. The reverse is true in the post-exercise recovery period.

When the muscle is damaged during exercise (e.g. microtrauma) mononucleated cells in the muscle are activated by the injury, providing the chemical signal to circulating inflammatory cells. Neutrophils invade the injury site and release cytokines (immunoregulatory substances), which then attract and activate additional inflammatory cells. Neutophils possibly also release oxygen free radicals that can damage cell membranes. Macrophages then invade the damaged muscle fibres, removing debris through the process of phagocytosis. Last, a second phase of macrophage invasion occurs, which is associated with muscle regeneration (which might be the cause of muscle hypertrophy).

Fibre Hyperplasia -  

It is postulated that individual muscle fibres have the capacity to divide and split into two daughter cells, each of which can then develop into a functional muscle fibre. It has more recently been established that satellite cells (myogenic stem cells involved in skeletal muscle regeneration) are likely involved in the generation of new muscle fibres. Satellite cells are typically activated by muscle stretching and injury. Muscle injury can lead to a cascade of responses, in which satellite cells become activated and proliferate, migrate to the damaged region, and fuse to existing myofibres (hypertrophy) or combine and fuse to produce new myofibres (hyperplasia).

Wilmore, J., Costill, D. and Kenney, W. Physiology of sport and exercise. 4^th ed. (2008)