It is often difficult to find safe and effective strength and conditioning exercises for the hamstring muscles that mimic the dynamic loading patterns found in many sports. For this reason, most coaches and athletes select a combination of exercises to target the hamstrings across both joints and through their full range of movement.
These include, Romanian deadlifts, where the hamstrings work as hip extensors in outer range, Nordic curls, lying hamstring curls, and stability ball curls where the hamstrings work as knee flexors in inner range, and on the glute-ham machine that can be adjusted to target the muscles across both joints.
In terms of injury prevention, it appears that targeting the hamstrings eccentrically (particularly using Nordic curls) can produce a number of benefits for injury prevention, namely, an increase in the eccentric peak torque (9,11) and the angle at which it occurs (5,6,10), leading to a reduction in hamstring strains (2,3,12).
The acute training variable that is often neglected when training the hamstring is velocity, particularly during the eccentric portion of the muscle action. This is particularly relevant for sports that require rapid eccentric muscle activation, often at the end range of the movement. For example, the ability to decelerate the limb when kicking or reaching for a football—or toward the end of the leg swing phase during sprinting—is crucial for injury prevention, protecting the joint structures during dynamic movement (1,3,4,12,13,14), and performance, allowing a faster limb to be decelerated quicker (8).
One exercise that can be adapted to target the hamstrings in this way is the kettlebell swing (Figure 1). Kettlebell swings involve extension of both the knee and hip to propel the kettlebell upward and flexion of both knee and hip to decelerate the kettlebell before the next swing. The timing of the hip and knee flexion during the lowering (deceleration) phase and the hip and knee extension during the up (acceleration) phase results in the hamstrings being stretched simultaneously over one joint while shortening over one joint while shortening over another. With proper timing of the phases in this exercise, the muscle will not undergo rapid lengthening or shortening.
In contrast, the modified kettlebell swing (MKBS) is performed from a stiff leg position, such as in the stiff-legged deadlift exercise (Figure 2). This variation emphasizes a rapid eccentric loading of the hamstrings, followed by a rapid concentric contraction of the same muscle group to accelerate the kettlebell back toward its highest position (Figure 3). Training the stretch-shortening cycle in this manner fulfills several functional principles often advocated for effective hamstring injury prevention and rehabilitation (7). We recognize that the MKBS does not fully mimic those activities that involve hip flexion and knee extension in an open chain (such as kicking); however, the joint positions and ballistic nature of this movement does provide a stepping stone to more functional activities by allowing the hamstrings to be targeted at higher velocities.
PERFORMING THE EXERCISE
- The kettlebell will need to be approximately 30% lighter than would normally be used for a traditional swing.
- Stand feet shoulder width apart holding the kettlebell in both hands.
- Movement occurs at the hip joint.
- Keeping the knees slightly flexed, begin to flex and extend the hip while allowing the kettlebell to swing forward and backward, slowly building up to maximum amplitude over several swings.
- At the top of the movement, the kettlebell should swing to a point just above shoulder height with the hips fully extended.
- The bottom position is achieved at a point where the hips are flexed, and the hamstrings are approaching full stretch. Because the knees remain slightly flexed throughout the whole movement, but do not flex or extend from this position, the bottom position will vary depending on the hamstring flexibility of the athlete.
- Focus on the quality of movement with a quick transition from eccentric to concentric muscle action of the hamstrings at end of range (lowest position).
- Maintain a neutral spine by not allowing any flexion/extension of the lumbar and thoracic spine to occur.
Because this is a ballistic movement that rapidly targets the hamstrings eccentrically—and then concentrically—toward end of range, it is not uncommon for an athlete to experience delayed onset muscle soreness after the first few times this exercise is performed. A conservative and progressive approach to load increases is recommended when progressing this exercise.
To maintain movement quality, we recommend no more than 6 full-amplitude swings with a load that allows the athlete to perform the exercise with perfect technique. If the athlete begins to flex and extend the knee, or lumbar spine, then the load is too heavy. Once the athlete can perform 6 perfect full-amplitude swings, then progress to the next heaviest kettlebell. To maintain perfect technique with the new load, fewer repetitions may need to be performed. Again, emphasize quality over quantity.
In summary, the MKBS provides an excellent alternative to traditional hamstring exercises because it places a greater emphasis on developing rapid eccentric control of the posterior chain muscle groups. Therefore, the authors recommend its inclusion alongside other methods of training as a means of preparing the muscles for the loading conditions encountered during sport and to potentially mitigate injury risk.
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