Research Review By Demetry Assimakopoulos©

Date Posted:

November 2009

Study Title:

Resistance training is accompanied by increases in hip strength and changes in lower extremity biomechanics during running

Authors:

Snyder AKR, Earl JE, O’Connor BK, Ebersole KT

Author's Affiliations:

University of Northern Iowa, University of Wisconsin-Milwaukee

Publication Information:

Clinical Biomechanics 2009; 24: 26–34.

Background Information:

30 million Americans choose running as their preferred mode of exercise (3). Of these 30 million, approximately 50-75% will sustain an injury at some point (3, 6, 5). It is currently speculated that overuse injuries can result from a lack of control over some lower extremity movements, specifically excessive hip adduction and internal rotation, knee valgus, tibial internal rotation, and foot pronation (2, 4).

Recently, many researchers have focused on the development of adequate hip strength to prevent injuries to the lower extremity. However, the use of a hip strengthening exercise protocol in an effort to stop aberrant movement patterns has not been explored. Bearing this in mind, the researchers in this study attempted to:
  1. determine if a closed-chain would increase the strength of hip adduction and external rotation; and
  2. to determine whether or not the excessive hip adduction and internal rotation, knee abduction (valgus), tibial internal rotation, and foot pronation are changed after strengthening.
Their hypothesis was that a 6 week strength training program, utilizing a closed-chain hip rotation exercise, would increase external rotation and hip adduction strength. In addition, they assert that this increase in strength would correct the motions related to lower extremity injury.

Pertinent Results:

The results indicate that the training program was successful in strengthening the subjects’ hip rotators and abductors.
  • At the end of the training protocol isometric abduction and external rotation in the trained side increased by a statistically significant amount (P = 0.009 and P < 0.0005, respectfully).
  • Tukey’s post hoc analysis showed that the adductors and external rotators were stronger at the mid- and post-intervention assessments. However, there was no significant increase between the mid- and post- intervention assessments.
  • Also, eversion range of motion, ankle inversion moment, and knee abduction moment significantly decreased, while there was only a trend for reduced hip internal rotation range of motion.
  • Additionally, hip adduction range of motion increased significantly.
  • While there were significant results for the aforementioned motions, eversion angle at heel strike, eversion velocity and knee abduction range of motion did not change significantly.

Clinical Application & Conclusions:

The strengthening protocol led to an increase in strength, and thus reductions in eversion range of motion, ankle inversion, and knee valgus motion. Additionally, there was a trend toward a statistically significant reduction in hip internal range of motion.

These results supported the researchers’ hypothesis. The exercises were chosen because they directly targeted the gluteus medius and because they train the muscles crossing the hip in a functional position. However, because of this functional position, muscles other than the hip abductors and external rotators may have increased in strength. There are several nuances to training these areas that were realized by the researchers: it is necessary to continually increase the load to overload the targeted musculature.

Further, clients and patients may compromise their posture to accommodate the increase in load. With this being said, a constant reminder to retain proper posture and to perform motions about the hip only is needed. Because of this compensation, it is likely that muscles that were not directly targeted with training were included in the movement and increased in strength as well.

The researchers concluded that strengthening the hip musculature can affect lower extremity biomechanics. There was however one measurement that was contrary to the researchers’ hypothesis: a small increase in hip adduction range of motion (the position of the thigh relative to the pelvis) and a decreased knee abduction moment. To further analyze this phenomenon, the researchers did a post-hoc analysis of the pelvic segment angle relative to the rest of the body. There were no significant changes between pre- and post-treatment (P=.81). Because of the contradiction of an increased hip adduction range of motion and a decreased knee abduction moment, the researchers assessed the angle of the ground reaction force vector in the frontal plane. A lateral shift in centre of mass would account for this phenomenon.

The researchers found no significant difference in the location of the centre of mass at heel strike (P = .90) or the location of the centre of pressure at heel strike (P = .26). With this additional experimentation, the researchers concluded that the increase in hip adduction range of motion, while statistically significant, has little clinical significant. However, they assert that further investigation in this matter is needed.

Previous research has shown that faulty hip mechanics influence rearfoot motion. This study’s findings show that strengthening the hip muscles has a significant influence on rearfoot mechanics and thus should be incorporated into the rehabilitation of such injuries. Additionally, they state that their finding of decreased knee abduction angle can alter knee joint loading and may be beneficial in the rehabilitation of patellofemoral pain syndrome (see Related Reviews below).

Study Methods:

Participants:
Fifteen women were included into this study. The inclusion criteria were: a lifestyle that included regular moderate intensity exercise (defined as “30 min of moderate intensity activity on most days of the week” by the American College of Sport Medicine) and a normal menstrual cycle. Individuals who had excessive pronation (having greater than 10 mm of navicular drop), a previous history of orthopedic surgery, lower extremity injury within a 6 month time frame, neurological impairments and used foot orthoses were excluded from the study.

Strength testing protocol:
The dominant lower extremity (defined as the leg that could kick a soccer ball the furthest distance) was used for the study. It was determined that the right side was dominant for all participants. To test the hip abduction strength, the researchers had their patients lay on their side with the test leg facing upward. The leg was placed in a neutral position. Pillows were placed between the extremities to ensure this position was not lost. A hand held dynamometer was positioned 5 cm proximal to the lateral side of the participant’s knee. It was held in this location by a strap that encircled both the knee and the table. The strength test was done isometrically from this position. To test hip external rotation strength, the subjects were seated with their hips and knees at 90° of flexion. A dynamometer was positioned 5 cm above the medial malleolus and was fixed to this position by a strap holding the dynamometer and the lower limb to a table leg, laterally. From this position, the participants were asked to, as forcefully as possible, push against the strap in the direction of the dynamometer (external rotation) for 5 seconds.

Both the tests were performed one practice trial and three experimental trials with 15 seconds in between bouts. The peak force (in kilograms) was documented and normalized to the subject’s body mass. These measurements were performed on both the dominant and non-dominant sides to ensure there was no cross-over training effect.

Strengthening Protocol:
Each subject attended supervised exercise sessions for six weeks, three times per week. Most training sessions were separated by 48 hours, but varied according to subject availability. The strengthening program utilized functional, closed-chain hip abduction and rotation exercises that can be done using a cable/pulley or elastic band. The cable/pulley was used in the study to standardize the load resistance throughout the training (as elastics have a variable load). The chosen exercises have shown large electromyographic activity in the gluteus medius (1).

Specifically, the exercises were designed to target the hip abductors and external rotators. The cable was strapped to the left side of the waist of the participant using a belt designed with a cable clip. The cable pulley was placed on the lowest height setting. To ensure universal technique, tape was placed on the floor to guide the subjects’ foot placement. Verbal corrections were given to the subjects to ensure correct posture. Initial loads were determined through 1 repetition max tests for the hip hike and toward exercises.

The participants started at 60% of their 1 repetition max. They were asked to perform as many repetitions as possible – if 12 repetitions were completed, the load was increased by 2.5 lbs; if 30 or more were completed, the load was increased by 5 lbs to ensure that the muscles were being sufficiently overloaded to develop strength. 1 repetition max tests were completed every 2 weeks to monitor progression.

Exercise description:
  • The toward exercise: Initial position was to face toward the weights. The movement included hip extension (non-dominant leg) and external rotation of the dominant leg. When they reached end range of external rotation, they lightly touched the floor with the toes of the non-dominant foot and return to starting position.
  • The away exercise: Initial position was to face away from the weights. The subjects extended the left leg (non-dominant) and externally rotated the right leg (dominant). When they reached end range of external rotation, they lightly touched the floor with the toes of the non-dominant foot and returned to starting position.
  • The hip hike: Participants stood with the weights to their left (weights beside the left leg) and raised their left hip toward the ceiling. They then returned to their initial position.
Follow up data collection:
The strength testing and movement analysis were repeated at week 3 and week 6 using the same procedures as in the initial testing session.

Study Strengths / Weaknesses:

Weaknesses:
  • What is hip strength? No definition was given. 60% of 1 repetition max is likely more geared towards muscular endurance.
  • No details about the rest periods between sets were given. Were they long enough to most properly facilitate strength gains?
  • Could it be that the development in better lower limb kinetics and kinematics were due to a more efficient motor patter that was established due to training and not necessarily due to the increase in one’s strength?
  • Small sample size
  • The study was conducted on apparently healthy individuals and not on anyone diagnosed with any aberrant biomechanical movement leading to injury. While one can assume that these exercises would help in the rehabilitation of injury of the rearfoot and patellofemoral pain syndrome, is this really the case?
  • How did they do the 1 rep max tests for the hip hike and toward exercises? Did they ensure that it was in fact their 1RM by doing the test repeatedly? There is a large learning curve to 1 RM testing and if this measurement is not done properly it can skew results once motor patterns are established.
  • The study was conducted only on women. No men were recruited.
Strengths:
  • The researchers picked apparently functional tests, which brought the participants thought the entire functional range of motion.
  • The initial tests were repeated both at midway through the study and at the end, which can provide the results with greater statistical significance.
  • They recommended that these exercises can be done with elastics for home use.

Additional References:

  1. Earl J. Gluteus medius activity during three variations of isometric single leg stance. J Sport Rehabil 2005; 14: 1-11.
  2. Ireland M. The female ACL: why is it more prone to injury? Orthop Clin North Am 2002; 33: 637-651.
  3. Kaufman KR, Brodine SK, Shaffer RA, Johnson CW, Cullison TR. The effect of foot structure and range of motion on musculoskeletal overuse injuries. Am J Sports Med 1999; 27: 585-593.
  4. Powers C. The influence of altered lower-extremity kinematics on patellofemoral joint dysfunction: a theoretical perspective. J Orthop Sports Phys Ther 2003; 33: 639-646.
  5. Taunton J et al. A retrospective case-control analysis of 2002 running injuries. Br J Sports Med 2002; 36: 95-101.
  6. van Mechelen W. Running injuries. A review of the epidemiological literature. Sports Med 1992; 14: 320–335.