WHAT IS THE ROLE OF THE HIP EXTENSORS IN LATERAL MOVEMENTS?
Although all the above movements occur primarily in the sagittal plane, 2 recent studies have investigated the role of the hip extensors in sidestepping. Inaba et al. (6) found that hip extension moments increased significantly with increasing side step distances but hip abduction moments did not. Similarly, Shimokochi et al. (12) reported that hip abductor function did not seem to be the critical factor for lateral movements but rather that faster hip extension motions were the key to more explosive movements in the frontal plane.
WHY DOES THE ROLE OF THE HIP INCREASE WITH INCREASING INTENSITY?
The findings of these studies indicate that the ratio of hip-to-knee extensor moments or related variables increases with heavier loads during squats, lunges, deadlifts, and hex-bar deadlifts (Table 6). Additionally, the studies indicate that the ratio of hip-to-knee extensor moments or related variables increases with greater speeds in running and with greater heights in jumping. This may happen as a result of different movement strategies being used with different loads and speeds, as indeed Frost et al. (4) recently observed. Frost et al. (4) found that when performing lifting or squatting tasks with a faster movement speed, subjects used a less upright posture and shifted to a more hip-dominant pattern involving an increase in trunk inclination toward the horizontal. Such alterations in kinematics would be expected to lead to corresponding changes in kinetics. Indeed, Hay et al. (5) observed a simultaneous change in trunk inclination along with a proportionally greater increase in hip extension moment during squat exercises with increasing load. However, exactly what drives this change in the kinematics and kinetics of the movement patterns as intensity increases is unclear. It is possible that as the demand of the movement increases, individuals find they are able to move faster or lift heavier by leaning forward more, which may then lead to a shift in the proportion of hip and knee extension moments in favor of the hip. Because athletes typically display significantly greater hip extension moments than knee extension moments during maximal isometric testing (e.g., Buśko and Gajewski (3)), it may be the case that this shift occurs because the hip muscles are stronger than the knee muscles and therefore provide greater scope for increased moments. However, because maximal isokinetic moments do not demonstrate such clear differences between the magnitude of hip and knee extension moments at all angular velocities (e.g., Smith et al. (13) and Blazevich and Jenkins (1)), the exact reasons for the changes remain unknown.
The 5 examples shown in this article demonstrate that as loads get heavier (squat, lunge, conventional deadlift, and hex-bar deadlift), running gets faster, and jumps get higher, hip extension moments or related variables contribute proportionally more to the movement and knee extension moments or related variables contribute proportionally less (Figure 6). Similarly, faster lateral movements require greater increases in hip extension moments than hip abduction moments. This means that for athletes training to produce maximal power and speed, whether in the sagittal or frontal plane, developing the muscular strength of the primary hip extensors (i.e., the gluteus maximus and hamstrings) must be a primary concern.
For athletes training with exercises that closely resemble their competitive activities (e.g., sprinters sprinting or powerlifters squatting), it is likely that submaximal performances will involve significantly lower hip-to-knee extension moments. Therefore, strength coaches will want to address this deficit. The deficit could be addressed in several ways. First, additional work could be performed for the hip extensors. This could take the form of assistance work in the gym, comprising hip-dominant compound movements such as Romanian deadlifts, good mornings, back extensions, reverse hypers, barbell hip thrusts and/or kettlebell swings, and waist-attached sled dragging, which has been shown to be superior to shoulder-attached sled dragging in terms of hip extensor moments (7). Second, maximal work could be emphasized and submaximal work de-emphasized, to the degree that this is possible within the periodized program that the coach considers necessary for the long-term development of the athlete. Finally, although most researchers have emphasized the optimal load for power training as being the load that leads to the greatest overall power output, it may be more fruitful to train with the load that maximizes power output at the hips. Moir et al. (9) recently reported that power output was maximized at the hip during jump squats at 42% of 1RM, whereas system power output was maximized with no load (i.e., 0% of 1RM). Future research should be conducted to determine the optimal loads for hip extension power with other explosive lifts, and to determine whether training at these loads is more beneficial for performance than training at optimal loads for system power.
As we have seen in the previous examples, the ratio of hip-to-knee extensor moments or related variables increases with heavier loads during squats, lunges, deadlifts, and hex-bar deadlifts. Additionally, the ratio of hip-to-knee extensor moments or related variables increases with greater speeds in running and with greater heights in jumping. In these movements, this represents a shift from knee-dominance at lower intensities to hip-dominance at higher intensities. It is not surprising that the hips produce greater moments as intensities increase during these movements. However, the fact that the hip moments rise to a greater extent than knee moments shows that the mechanics (i.e., form or technique) are altered during these movements in a manner that relies more on the hips and less on the knees for propulsion. As force increases, therefore, the role of the hip extensors becomes much more important and their contribution to the movement increases. This has important ramifications for training and performance.
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Keywords:© 2014 by the National Strength & Conditioning Association
hip extensors; hip extension torque; hip extension moments; squat exercise; lunge exercise; deadlift exercise; sprint running; vertical jumping; hamstrings; gluteus maximus