The factors that are being implicated in this trend are the relative deterioration and muscle weakness that occur during the off-season.The hamstrings undergo a complex dynamic process during gait, making it unsurprising that they are frequently injured. They must first contract concentrically during the end of the stance phase in order to bend the knee and allow the foot (along with dorsiflexion at the ankle) to clear the ground. At the end of the swing phase, the hamstrings must eccentrically contract while applying a braking moment to knee extension, then immediately change functions to again concentrically contract and produce hip extension. Studies have shown that "the hamstring group reaches peak elongation and acts eccentrically at the hip and knee during the late swing phases of running" and that "the hamstrings are most active and develop the greatest torques at the hip and knee during the late swing through midstance phase of running." Thus, the hamstrings reach their maximum length while attempting to forcefully contract eccentrically and switch functions to immediately produce a concentric contraction, which makes the terminal part of swing phase the most vulnerable for injury. There have been many other proposed predisposing factors to injury. These include muscle weakness, muscle imbalance, poor flexibility, fatigue, inadequate warm up, poor neuromuscular control, and poor running technique. One of the few predisposing factors that most researchers agree upon however is previous hamstring injury. Brokett et al.
Factor XIII or fibrin stabilizing factor is a zymogen found in blood of humans and some other animals. It is activated by thrombin to factor XIIIa. Factor XIIIa is an enzyme of the blood coagulation system that crosslinks fibrin. Deficiency of XIII worsens clot stability and increases bleeding tendency.Human XIII is a heterotetramer. It consists of 2 enzymatic A peptides and 2 non-enzymatic B peptides. XIIIa is a dimer of activated A peptides. Function
Within blood, thrombins cleave fibrinogens to fibrins during coagulation and a fibrin-based blood clot forms. Factor XIII is a transglutaminase that circulates in human blood as a heterotetramer of two A and two B subunits. Factor XIII binds to the clot via their B units. In the presence of fibrins, thrombin efficiently cleaves the R37–G38 peptide bond of each A unit within a XIII tetramer. A units release their N-terminal activation peptides.Both of the non-covalently bound B units are now able to dissociate from the tetramer with the help of calcium ions (Ca2+) in the blood; these ions also activate the remaining dimer of two A units via a conformational change.Factor XIIIa (dimer of two active A units) crosslinks fibrins within the clot by forming isopeptide bonds between various glutamines and lysines of the fibrins. These bonds make the clot physically more durable  and protect it from premature enzymatic degradation (fibrinolysis).In humans, plasmin, antithrombin and TFPI are the most relevant proteolytic inhibitors of the active factor XIIIa. α2-macroglobulin is a significant non-proteolytic inhibitor.