Strength training helps prevent thrombosis
One of the health risks associated with a sedentary lifestyle is thrombosis: the formation of blood clots that can constrict crucial blood vessels. Researchers at the University of Connecticut have discovered that strength training can help protect against thrombosis. A single session has an immediate effect, and the effect is stronger in people for whom weight training is a habit.
Blood clotting & thrombosis

In healthy people the systems that are responsible for activating blood clotting and those that are responsible for inhibiting clotting and breaking down clots are in balance. In people with thrombosis the balance has been disturbed. The enzyme Plasminogen Activator Inhibitor-1 [PAI-1] may be too active, or the enzyme Plasminogen Activator [tPA] may not be active enough.
Explanation: tPA breaks down blood clots. PAI-1 inhibits tPA, and thus encourages the formation of blood clots.Researchers already know that long periods of moderately intensive exercise stimulate the blood-clot breakdown system, and high-intensity endurance training activates both blood clotting and the blood-clot breakdown system. People at risk from thrombosis therefore benefit from a lifestyle that includes, for example, lots of long walks.
But what about strength training? Is it good or not good for people who want to prevent thrombosis?
The researchers answered this question by setting up an experiment in which they got ten people with a sedentary lifestyle and ten well-trained strength athletes to perform six sets of squats using a weight with which they could just manage 10 reps. The researchers took blood samples from the subjects before and after the weight-training session.

When the researchers analysed the blood they noticed that in both the inactive people and the strength athletes the activity of PAI-1 decreased and that of t-PA increased.

Strength training, it would seem, causes a shift in clotting balance in a favourable direction for people who are at risk from thrombosis. The effect is stronger in the experienced athletes.
The strength training had no effect on the blood-clot forming fibrinogen, but did boost the concentration of prothrombin fragment 1 + 2 and thrombin-antithrombin complexes. These are blood-clotting markers.
The researchers also measured the activated partial Thromboplastin time. This is the result of a test that says something about the balance between clotting and anti-clotting factors. The researchers discovered that the workout shifted the balance in the direction of the anti-clotting factors.
"Our findings suggest that habitual resistance training may limit the inhibitory activity of PAI-1 and enhance tPA activity, thereby allowing a favorable fibrinolytic state", the researchers conclude.

Thromb Res. 2013 Jun;131(6):e227-34.