The exercises described below will add strength to the muscles around the knee, particularly the hamstrings. 

Exercise 1:The Six-Way Lunge with Arm Drop

This exercise stretches and strengthens the hamstring muscles on the back of the thigh in all three planes of motion (sagittal, frontal, and transverse).

• Strong and flexible hamstring muscles assist the ACL in its task of controlling the knee joint and preventing the tibia from moving excessively during knee flexion.
• Begin by standing with feet parallel and hip-width apart, arms bent at the elbows so that your hands are directly in front of your shoulders;
• Take a long step forward with your right foot, as for a lunge, leaning your upper body forward by approximately 45 degrees at the waist. As your right foot makes contact with the ground, drop your hands on either side of your right knee;
• Quickly extend your right knee (ie straighten your right leg) and return to your starting position. Repeat with the left leg;
• From the starting position, step directly to your right with your right foot in a lateral-lunge position, upper body facing right and leaning forward over your right leg at approximately 45 degrees. As your right foot makes contact with the ground, drop your hands on either side of your right knee, as above, with left foot pointing forward;
• Quickly extend your right knee and return your body to the starting position. Repeat the same motion with your left leg moving to the left;
• From the starting position, twist your body around at the hips and step diagonally and to the rear with your right foot in a backward-lateral-lunge position, upper body facing to the right-rear (at about 'four o'clock' position) and inclined over your right leg at approximately 45 degrees. As your right foot makes contact, drop your hands, as above, with left foot pointing forward;
• Quickly extend your right knee and return your body to the starting position. Then, repeat the overall motion with your left leg, moving it to the left-rear ('eight o'clock') position;
• Repeat the entire sequence described above three times for a total of 18 repetitions of stepping. Repeat twice more, with 30-60 seconds rest between each set of 18 reps.
• Progressively toughen the exercise over a period of 4-6 weeks by increasing the resistance held in your hands (small dumbbells weighing up to 10 pounds) and the speed of the exercise.

Exercise 2: The One-Leg Squat with Lateral Hop

Sudden changes in direction while running and jumping can cause injury to the ACL by placing increased stress on the knee. Lateral hopping movements help prepare the ACL and muscles around the knee for these sudden (and often unpredictable) movements in the frontal (side-to-side) plane. Even if you're not very worried about your ACLs, this is a great exercise!

• Stand with your left foot forward and right foot back, feet about one shin-length apart from front to back and hip-width apart from side to side;
• Place the toes of your right foot on a step or block 6-8 inches high, with most of your body weight directed through the heel of your left foot. Bend your left leg and lower your body until there is an angle of about 90 degrees between the thigh and lower leg;
• Then hop upwards and laterally, so that your left foot lands about 6-8 inches to the left;
• On landing, immediately go into another squat, then hop back to your take-off point;
• Now repeat with the same leg but hopping to the right. Maintain upright posture throughout and keep your hands at your sides;
• Complete a total of 12 lateral hops (to the left and right) with your left leg before repeating with your right leg. Perform a total of three sets with each leg, with 30-60 seconds of rest between sets.
• Be sure to perform this exercise only on a resilient service with some 'give', eg an aerobics floor, a wooden gym floor, a grassy surface, soft dirt or a rubberised track. Hopping repeatedly on concrete or asphalt may increase the risk of overuse injuries to the lower leg.

Exercise 3: Zig-Zag Runs

This exercise helps develop the balance and body control required to move in multiple directions at various speeds. The runs force the knee joints to move through a number of different angles and directions, thus mimicking movements which can lead to ACL injury in unprepared athletes.

• Start by running straight ahead at half speed for about five metres;
• At the 5m mark, cut quickly to your left for several strides by pushing off your right foot and moving in a left-forward (diagonal) direction;
• Then cut back to the right for several strides by pushing off your left foot and moving in a right-forward (diagonal) direction;
• Repeat this sequence for a total of 8-10 cuts (4-5 to the right and 4-5 to the left);
• Perform 3-5 sets of this exercise, with a break of 30-45 seconds between each run.
• Gradually increase the intensity of your zig-zag runs over 4-6 weeks by running faster and also by changing the number of strides between direction changes (vary the number of strides between one and five). If you participate in sports which involve running backwards (eg football, rugby, and basketball), complete some of the sets in a backward direction.

Exercise 4: The High-Bench Step-Up

This old 'stand-by' for runners is great for warding off ACL problems.

• Stand on a bench about knee-high, with your body weight on your left foot and shifted towards the heel, right foot free and held slightly behind the body;
• Lower your body in a controlled way until the heel of the right foot touches the ground, but still supporting all your weight on your left foot;
• Return to the starting position by driving down with the left heel and straightening your left leg; * Repeat 10-12 times, then switch over to the right leg, maintaining upright body posture with your trunk at all times, with hands at your sides (with or without dumbbells).
• Make this exercise progressively more difficult over time by increasing the resistance (with dumbbells up to 20lb) and raising the height of the step (up to mid-thigh height). The higher the step the greater the involvement of the hamstring muscles on the back of the thigh, which work in tandem with the ACL to stabilise the knee joint. Strong yet flexible and coordinated hamstrings help minimise the risk of ACL injury.

Over time, you should aim to increase the speed with which you carry out all of the above exercises, thus making your hamstrings specifically strong at the rates of movement which are most likely to produce injury. These exercises should help keep your knees sound and your ACLs intact. They are likely to be particularly useful as part of regular strength training if you play basketball, soccer, volleyball or any other sports which involve jumping or cutting.