To get rid of the commercial hits from companies promoting their own massage ideas or massage products, filter for the hits that come from well known sources, such as domains where scientific papers are published.
Try to google for example "physiology of massage ncbi.nlm.nih.gov"
One starting point is for example this review paper
The Role of Massage in Sports Performance and Rehabilitation: Current Evidence and Future Direction
"Background
Massage is a popular treatment choice of athletes, coaches, and sports physical therapists. Despite its purported benefits and frequent use, evidence demonstrating its efficacy is scarce.
..
Purpose
To identify current literature relating to sports massage and its role in effecting an athlete's psychological readiness, in enhancing sports performance, in recovery from exercise and competition, and in the treatment of sports related musculoskeletal injuries.
..."
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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2953308/
and
The mechanisms of massage and effects on performance, muscle recovery and injury prevention
"Massage can produce mechanical pressure, which is expected to increase muscle compliance resulting in increased range of joint motion, decreased passive stiffness and decreased active stiffness (biomechanical mechanisms). Mechanical pressure might help to increase blood flow by increasing the arteriolar pressure, as well as increasing muscle temperature from rubbing. Depending on the massage technique, mechanical pressure on the muscle is expected to increase or decrease neural excitability as measured by the Hoffman reflex (neurological mechanisms). Changes in parasympathetic activity (as measured by heart rate, blood pressure and heart rate variability) and hormonal levels (as measured by cortisol levels) following massage result in a relaxation response (physiological mechanisms). A reduction in anxiety and an improvement in mood state also cause relaxation (psychological mechanisms) after massage. Therefore, these benefits of massage are expected to help athletes by enhancing performance and reducing injury risk."
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https://www.ncbi.nlm.nih.gov/pubmed/15730338
Start with those and i am sure you can navigate further on from those.
/Fredrik