Muay Thai is the "Art of 8 Limbs". It is a sport that emphasizes physical conditioning and strong basics. Punching, kickboxing, knees and elbows are allowed. The clinch fighting of Thai boxing is what makes it very different from other forms of kickboxing.
In Shidokan Karate, the 2nd concept of the triathlon is kickboxing (the first being Full Contact Karate and the 3rd Grappling). When Soeno (Shidokan founder) visited Thailand to test his Kyokushin against Muay Thai he fought against top Thai fighters and even though he won, found the fights to be tough. So, he began to study that art and would later incorporate it into his system.
The clinch and knees of Muay Thai are a powerful weapon. Even in Shidokan Karate tournaments, this is often what decides the winner. Because grabbing is allowed, you can wear an opponent down. Here is a clip of my favorite Muay Thai fighter, Saenchai Sor Kingstar. Enjoy!