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At increased ionic strength, G-actin molecules polymerize into filaments, known as F-actin. F-actin is the main component of the thin filaments in sarcomeres of muscle cells. Muscle sarcomeres use ATP hydrolysis to produce conrtractions by sliding the thin actin filament

past thick myosin filaments. The entire process is initiated by the slow formation of actin dimmers and trimers. These serve as a base for filament elongation. During elongation more actin monomers associate to than dissociate from either of the two ends, which results in the net growth of both filament ends. The steady-state phase is characterised by a dynamic equilibrium where the length of the actin filaments remains constant. This means while monomers are associating, other monomers are dissociating from the ends. In this dynamic equilibrium a stationery population of free actin monomers is established and is called the critical concentration. The polarity of the ends of the actin filaments play a key role in elongation. The ends of the filament are referred to as "barbed" and "pointed" according to the polarity of the arrowhead-like structure generated on binding with myosin subfragment 1. ) (Structure and Function of Actin by Wolfgang Kabsch) The barbed or plus end binds actin monomers faster than the pointed or minus end. Although its ATPase activity is not crucial for actin polymerisation, actin self-assembly is associated with the ATPase cycle, which powers this treadmilling process (Conformational Dynamics of Actin: Effectors and Implications for Biological Function) After the actin molecule is changed into a filament, the bound ATP is hydrolysed to ADP and inorganic phosphate is realeased. (Structure and Function of Actin by Wolfgang Kabsch) The formation of the F-actin is what stimulates the actin ATPase. The release of the  inorganic phosphate happens more slowly than the formation of the filament so that the growing filament has a cap of ATPactin at its barbed end, while monomers containing ADP and inorganic phosphate accumulate in the rest of the filament.  

The mature filament contains only ADP-actin. The tendency of actin to polymerize depends upon the affinity of actin monomers for filament ends. There is an actin monomer concentration below which actin will not polymerize known as the Critical Concentration (CC). At monomer concentrations above the CC, the actin will polymerize until the free monomer concentration is equal to the CC. (http://www.ncbi.nlm.nih.gov/books/NBK28299/) It is thought that the strong binding to the ATP is due to the amide proton of a serine 14 amino acid forming hydrogen bonds with the phosphate of ATP, thus stabilizing the nucleotide protein complex. (ACTIN: General Principles from Studies in Yeast). Pure actin can be switched between these states in the test-tube by altering the salt concentration. Actin in low salt conditions is in the G-state, while adding salts causes the actin to polymerise.  (http://www.bms.ed.ac.uk/research/others/smaciver/lectures/Cs1.htm Because actin polymerization is reversible, filaments can depolymerize by the dissociation of actin subunits, allowing actin filaments to be broken down when necessary. Therefore an apparent equilibrium exists between actin monomers and filaments, which is dependent on the concentration of free monomers. At this critical concentration, monomers and filaments are in apparent equilibrium.(Structure and Organization of Actin Filament)