Hamstring curls performed with an exercise ball provide several advantages over their prone, machine-curl counterparts.

Because they are performed on an unstable surface (the ball), they develop the lower back and abdominal core, improve proprioception, and develop the smaller stabilizer muscles. They also allow you to diagnose muscle strength imbalances between the left and right hamstrings.

How to Perform

• Lie on your back and place the medicine ball beneath your heels, with your legs outstretched.
• Tighten the core and lift the hips upward until the body is straight.
• Then, with the feet in a neutral position (neither pointed nor dorsiflexed), pull the feet back towards the buttocks, making sure the ball rolls straight back.
•  Repeatedly pull then extend, focusing on making the ball roll back and forth in a straight line while keeping the hips up and the body straight.

Changing the Level of Difficulty
There are a number of variations that change the difficulty of the exercise.

1. Arm Position
    You can change the difficulty of the exercise by changing the position of the arms. From easier to harder, the positions are:

• Arms outstretched at 90 degrees.
• Arms at 45 degrees
• Arms at the sides.
• Arms at sides, hands up in the air (elbows-only     touch the ground).
• Arms crossed over the chest.

2. Ball Position
    A starting position that is closer to the buttocks will make the exercise easier. For example, placing the ball under the calf muscles instead of the feet.

3. Ball Inflation
The exercise is easier with a less-inflated ball.

4. Single Leg
Performing the exercise with only one leg increases the difficulty even further.

Diagnosing Leg-Strength Imbalances
There are several ways to identify a difference in strength between the hamstrings of the left leg and the right leg.

1. Direction of Ball Movement
The direction of ball movement is a cue because it's difficult to move the ball back and forth in a straight line if there is a leg-strength discrepancy.  If the ball moves to the left, then the left hamstring is probably stronger. If it moves to the right, it's probably the right hamstring that has greater strength.

2. Position of Feet
Also, the position of the feet on the ball can take indicate differential weaknesses. For example, if one foot dorsiflexes more than the other, then that hamstring is probably weaker.

Why? The calf muscles play a role in bending the knee and the extra dorsiflexion is an attempt to activate the calf muscles to help that weaker hamstring. So, if the left foot is more dorsiflexed, the left hamstring is probably weaker. (Both feet should look the same – in a neutral position, facing the ceiling).

The same holds true for a foot that is splayed to the side. It's an attempt to use other muscles to assist a weaker hamstring.

 

References: From a workshop with Tony Scott (RMT & Certified Flexibility Therapist), a private stretching consultant and massage therapist, formerly with the Toronto Maple Leafs and Toronto Blue Jays), of Newmarket, Ontario; and Dave Harris, (BSc in Kinesiology), Personal Trainer and Strength and Conditioning Coach, Aurora, Ontario.

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