A swing knee in Muay Thai is the knee you throw when you're too close for the straight stabbing knee. Think clinch range — locked up with your opponent, hips tight, no room to drive forward. The swing knee solves that problem by going around instead of through.
Here's the mechanic. Open your hips like a door and blast the knee in from the side. Pivot hard on your base foot — Scott Sullivan stresses that you need to be on the ball of your foot, getting as much leverage as possible. The power comes from turning your entire hip over, not from lifting the leg.
You've got two angles to choose from. Some fighters throw it tight, angling the knee inward toward the body. Others open it up and slap with the inside of the thigh and knee, making more of a sweeping contact. Both hurt. Both score. It's personal preference.
But here's what a lot of people don't realize about the swing knee — you don't only throw it to the body. Target the thighs. Especially against the ropes. Beat up the legs and you take away their stance, their kicking, their base, their balance. Everything. Scott makes this point clearly in the video: the more damage you do to the legs, the less they can do to you.
Works every time on the ropes. Get them backed up, lock the clinch, and just chew through those legs with swing knees. By round three they can barely stand.
The swing knee is a clinch fighter's best friend. If you want to dominate the inside game, this is where you start.
For the full clinch breakdown, check out our Muay Thai clinching guide. Get the full course in the Ultimate Muay Thai Training System.