A way of delivering an attack by propelling the body and hence the weapon forward. Generally this is done by propelling the weight forward onto the leading leg with or without stepping.
Ways of Lunging
There are several mechanical ways to deliver a lunge:
- Propelled Lunge: This lunge is performed by propelling the front foot through a sudden straightening of the back leg, as if you were kicking the ground. This lunge is described by Salvator Fabris. 
- Stepping Lunge: This lunge is delivered by first placing the lead foot into the extended position (increasing the pace) while maintaining the weight of the body over the rear leg. The hips and body are then shifted forward over the front leg by straightening the back leg. This lunge is described by Capo Ferro. 
- Combination Lunge: This lunge combines both methods, whereby the upper body is extended forward, weight begins to shift over the front foot, which midway through the motion steps to support the torso and continue the lunge to full extension. This lunge is described by Agrippa. 
- Firm Lunge: This lunge is delivered by reaching out with the weapon, shoulders, and hips, shifting the weight from the rear leg to the front leg without moving either foot. This lunge is described by Capo Ferro (Italian pie fermo).
- Passing Lunge: This lunge is delivered by carrying the weapon forward by stepping through with the rear foot. This lunge is described by most rapier masters. 
Mechanics of the Propelled and Combination Lunge
- Fully extend the arm.
- Bring the shoulders forward while keeping the weight back (see second guard for an example of a forward position).
- Propel the hips forward by rapidly extending the rear leg (as if you were kicking the ground) and stepping with front foot one foot-length.
- The lunge ends when the knee is ahead of the lead toe.
- To recover, bend the back knee to begin shifting the hips back, then push with the lead foot to return to the shoulders-forward position.
- Then recover out of distance or into a rear guard such as Capo Ferro's third guard.
The difference between the propelled and combination lunge is simply the point at which you step with the lead foot. In the propelled lunge you step immediately at the beginning of the rear leg's extension, in a sense jumping forward. With the combination lunge you allow the hips to begin shifting before stepping with the lead leg (see the figure of Agrippa's lunge below).
- The sword is the first thing to move forward and the last thing to move back.
- Why? This ensures that the sword's guard and strong protect you from the point that you initiate your attack to the point that you recover out of measure. It also forces your opponent to deal with the threat of your weapon before considering their own counter-attack.
- Extend your shoulders as far forward as possible before attacking and during the attack and recovery.
- Why? This ensures that your head is well hidden behind your guard and that your flank is out of your opponent's easy reach.
- Do not hyper-extend your elbow.
- Never lock your arm joints. Extend them to the fullest point before full extension. This helps prevent injury and causes the muscles to tire less quickly.
Mechanics of the Passing Lunge
The passing lunge is shown in many different forms in historical manuals. The following two example show finishing positions that the passing lunge can take as described and shown by Capo Ferro:
Passing Lunge with Full Hip Rotation
When passing the passing foot lands with the toe in the direction of movement, this is facilitated through the turning of the hips so the left side of the fencer is leading.
Passing Lunge with Maintained Hip Profile
When passing the passing foot continues to point toward the fencer's left. This allows the hips and fencer's body to maintain an orientation where the right side and hence the sword stay in the lead.
Capo Ferro's Extraordinary Pace
This figure from the Gran Simulacro demonstrates the reach that is gained through use of the lunge. Capo Ferro describes this position being reached first by expanding the pace and then shifting the weight of the body (a stepping lunge). 
See Gran Simulacro plate 5 for a detailed description of the figure.
This figure from Agrippa's Opera Nova shows in detail the progression of the lunge from arm extension through to the movement of the torso, hips and feet.
Many practitioners of modern fencing and of Western Martial Arts insist that pushing the knee past the heel of the lead foot is dangerous to the joint and or ligament (this motion is seen for example in the extraordinary pace figure above). The only dangerous part of this type of extension is if the knee does not follow the line of the toe, thus deviating to the right or left and causing strain on the ACL. To avoid this type of strain ensure strengthening of the supporting muscles through exercises such as squats, aerobic lunges, or slow lunging.