Research has established that strenuous exercise, such as marathon running, activates clot formation by increasing markers of coagulation. In response, clot breakdown activates in coordination with the coagulatory system following exercise. This phenomenon where in healthy athletes, post exercise clot breakdown is approximately equal to clot formation, is termed homeostatic activation.
While exercise induced homeostatic activation is not harmful for most individuals, marathon running and other strenuous exercise may disproportionately activate the coagulatory system, increasing the risk for venous thrombosis (VTE) and contributing to reports of Deep Venous Thrombosis (DVT), Pulmonary Embolism (PE) or both. This is of increasing concern, given marathon participation has increased 40% in the last 10 years.
The superimposition of car, bus, train, or air travel on an athlete who has recently engaged in endurance exercise, for example, may shift the hemostatic balance in athletes post competition, thereby increasing the risk of VTE in certain individuals.
Data provided the first evidence that the combination of marathon running and air travel disrupts the hemostatic balance and favors a coagulatory response, which appears to be exacerbated with increasing age. Other factors specific to endurance athletes that could additionally exacerbate VTE risk include oral contraceptive use, presence/family history of a clotting disorder, sex, injury, bradycardia, atrial fibrillation, or previous history of VTE.
Compression socks during a marathon.
The Evidence-Based Clinical Practice Guidelines from the American College of Chest Physicians suggests the use of properly fitted compression socks to mitigate blood clot risk in high-risk populations. The use of compression socks, or mechanical prophylaxis, to maintain hemostatic balance has been studied with participants at rest and has been shown to be effective in reducing VTE in some clinical populations (eg, patients with a previous history of DVT or recent surgery), but contraindicated in others (eg, patients with arterial insufficiency).
Performance, Recovery and VTE risk
Athletes wear compression socks for a variety of reasons beyond reduction of blood clot risk, and thus their influence on noncoagulatory outcomes deserves further mention. Compression socks are increasingly popular with athletes due to perceived enhancement of exercise performance and recovery. To date, the research regarding the efficacy of compression socks to enhance performance, aid in recovery, or both has been equivocal. This is partially due to the difficulty of conducting placebo-controlled trials and the use of subjective qualitative reporting as primary outcome measures. Studies that have measured objective physiological markers of muscle damage (ie, creatine kinase, a marker of muscle damage, and lactate, a metabolic byproduct) have been limited and inconclusive, perhaps because the studies are vastly heterogeneous in terms of a) the type of compression garment used (eg, whole body, sleeves, knee-high compression) and b) the modality of exercise being tested (eg, resistance or aerobic).
Hypothetical mechanisms underlying performance and recovery benefits of compression socks differ depending on their timing of use (ie, during or after exercise), but are similar in that all theorize that the mechanism of action targets components of Virchow’s triad. (Virchow’s triad is composed of three factors—venous stasis, endothelial cell injury, and hypercoagulability—that augment blood clot risk)
Compression socks worn during exercise are thought to reduce microtrauma and enhance venous return by applying an external circumferential pressure gradient that reduces swelling space, improves blood flow, and in turn improves performance.
Compression socks worn during recovery are thought to accelerate metabolic waste clearance, attenuate edema and swelling, and improve oxygen delivery to muscle.
Awareness of VTE in endurance athletes has grown significantly in the past few years, and, consequently, running associations and events are increasingly urging athletes to wear compression socks during flight and competition to diminish DVT risk. Although these informal (albeit common-sense) recommendations are grounded in evidence derived from clinical populations, the efficacy of compression socks to attenuate marathon-induced hemostatic activation has been tested only recently.
In conclusion, with the exception of one study, the data does not appear to reveal any adverse consequences of compression socks, and in some cases suggest socks may result in psychological advantages that translate into performance gains. Assuming that socks are properly sized, marathoners can consider compression socks a sports garment that has preliminary evidence to support its use for preserving hemostatic balance during exercise and hastening recovery from exercise, but not for enhancing performance.