Hip muscle activation and knee frontal plane motion during weight bearing therapeutic exercises.

Lubahn AJ, Kernozek TW, Tyson TL, Merkitch KW, Reutemann P, Chestnut JM. Int J Sports Phys Ther. 2011 Jun;6(2):92-103.

As the kinetic chain concept has taken hold throughout the sports medicine community, it has become more and more important for the clinician to understand how to treat not only the injury itself, but accurately treat and identify discrepancies of the entire kinetic chain. Weakness of the hip musculature has been a common finding among athletes with knee injuries. Therefore, research identifying the most effective exercises to activate the hip musculature is of critical importance. The purpose of this study was 2-fold. First, to determine which of 3 exercises (double-leg squat, single-leg squat, front step-up) most effectively activated the hip musculature and allowed maintenance of a neutral frontal plane knee alignment. Second, to determine what effect a medial pull applied to the knees would have on gluteus maximus and gluteus medius activation, while attempting to maintain neutral frontal plane knee alignment. In this study, 18 healthy females participated (18 to 26 year old). For the double-leg squat a green theraband was placed around both knees, while a load of 15% body weight was applied in a medial direction to the knee via a cable column machine for the single-leg squat and front step-up. Hip muscle activation during the trial was measured with surface EMG. Frontal plane knee alignment was measured with high speed cameras. Data was collected from each participant’s dominant leg, which was defined as the leg that they used to kick a soccer ball. Each participant executed five repetitions of each exercise in the weighted and un-weighted conditions while keeping pace with a motronome set at 40 beats/minute. Each repetition lasted a total of 3 seconds. The participants were allowed to rest 10 to 15 seconds between reps and 45 to 60 seconds between each exercise. For the single-leg squat and front step-up exercises the subjects were allowed to use a vertical pole to maintain their balance.

The results of this study show that the single-leg squat, regardless of condition, elicited the highest muscle activation for both the gluteus maximus (47.4% maximum voluntary isometric contraction [MVIC]) and gluteus medius (65.6% MVIC). A previous study (Andersen, 2006) has stated that muscles must reach 40 to 60% of maximal effort in order for adaptive strength changes to be reached. The single-leg squat clearly reaches that, but the double-leg squat and front step-up fall short. With that being said, the authors don’t rule out continuing to utilize the double-leg squat and front step-up in rehabilitative efforts. They suggest using these exercises to train the gluteal muscles for endurance. By utilizing these exercises in this manner, the patient should be able to limit the amount of excessive internal rotation that occurs at the hip while running, which can lead to patello-femoral pain syndrome. Adding a medial pull to the double leg squat exercise did increase the amount of muscle activation by 6% MVIC, but it did not increase with the single-leg squat exercise. A muscle activation decrease of 11.9% MVIC was seen in the gluteus medius during the loaded single-leg squat trial. Although, there was a pronounced decrease in muscle activation comparing the unloaded to the loaded gluteus medius trial, the muscle activation during the loaded condition was still better than the front step-up and double leg squat by 8% and 30% MVIC, respectively. The authors believe that the load applied might have been too great and that postural compensations occurring during the exercises could possibly account for this decrease in muscle activation. This study also demonstrated that there is a significant increase knee abduction angle during each exercise whether there is medially directed load or not. This is important to note because the gluteal muscles help maintain proper frontal plane knee alignment which has been linked to ACL injuries, patello-femoral pain syndrome and iliotibal band syndrome. This study is clinically important for several reasons. First it clearly shows that of the 3 exercises the single-leg squat is the most effective exercise for recruiting the gluteus maximus and medius. By virtue of having increased muscle activity, as well as an increased knee abduction angle to work against, it is conceivable that the individual might be able to improve their knee mechanics during athletic activity and thus prevent future knee injuries. Once the exercises that best activate the hip musculature have been identified, the clinician can then institute them into training programs that focus on preventing knee injuries that result from overuse, deceleration and cutting. Are you addressing the hip in your injury prevention programs and if so, what types of outcomes are you seeing? What has and hasn’t worked for you?

Written by: Mark Rice
Reviewed by: Stephen Thomas

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