The Hips are extended by the Gluteus Maximus and Adductor Magnus. In the lower half the lift, the Hamstrings act as a Dynamic Stabilizers moving through the hips and knee with little change in length. The Hamstrings act more as a Synergist through the upper half of the movement. See Involvement of Hamstring below.
The knee is extended by the Quadriceps. The Soleus Planter Flexes the ankle allowing the shin to become upright from the forward angled position at the bottom of the deadlift. The Gastrocnemius acts as a Dynamic Stabilizer, traveling through the ankle and knee with little change in length.
The Spine is held ridged by the Erector Spinae acting as a Stabilizer with the Rectus Abdominis and Obliques acting as Antagonist Stabilizers countering the pull of the Erector Spinae. Under very heavy loads, the spine may tend to flex forward under the weight of a load. The flexion of the spine temporarily decreases the moment arm consisting of the hip and the barbell increasing leverage at this more challenging portion of the lift. If the spine buckles under the weight of the barbell it typically occurs in the thoracic spine. It does not represent a loss of position if this braced flexed position is held until the hips extend near the top of the motion. At this point, the flexed spinal position creates a secondary moment from the shoulders to the hips. This final moment arm gap is resolved by straightening the spine in an upright posture which is facilitated by pulling back the shoulders. This final extension of the flexed spine position requires the Erector Spinae to act as synergists near the top of the motion.
Although many may have issue with such extraneous movement (see Controversial Exercises), the spine structures and the accommodating musculature can adapt to these forces with adequate training. To condition the spine in these positions both Dave Tate, world famous champion powerlifter, and Louie Simmons of Westside Barbell recommends both arched back and rounded-back Good-morning in addition to Reverse Hyper-extensions. Also see Adaptation Criteria.
With the torso angled forward in the lower portion of the lift, the shoulder girdle is primarily stabilized by the Middle Trapezius and the Rhomboids, to some degree. When the torso is more upright the shoulder girdle is stabilized by the Middle and Trapezius, Upper as well as the Levator Scapulae.
The Latissmus Dorsi is the primary muscle that pulls the bar closer to the body in effort to decrease the moment arm consisting largely of distance between the barbell and hip.
At first glance of the Deadlift, the Hamstrings appear to work as a Dynamic Stabilizers essentially like they do in the Squat (see Torque Forces During Squat). The beginning and ending positions of the hip and knee are somewhat similar to the squat although the angle of the knee is less in the lower position of the deadlift. However there are subtle differences between the squat and conventional-style Deadlift which make the hamstrings acts as both a synergist and a dynamic stabilizer.
The Hamstrings appear to contract through the hip in the Deadlift, but obviously not to the same extent as they would if the knees where kept totally straight through out the movement (ie: Straight-leg deadlift) since the quadriceps are obviously extending the knee during the deadlift.
The hips begin in a nearly full flexion where as the knees may start in a 75% flexed position (90 degree flexion / 120 degree full-range). Observing beginning and ending joint positions only, that hamstrings appear to act as both a synergist (25%) and a dynamic stabilizer 75% since the net contraction is 100% hip extension (hamstring shortens) minus 75% knee extension (hamstrings lengthens). With this information, it could be argued that the hamstrings could be classified more as a Dynamic Stabilizer than a Synergist since the actual contractions through the hamstrings is relatively small.
|Straight-leg Deadlifts||Full Movement|
|Stiff-leg Deadlift, Romanian Deadlift*||Top 2/3||Bottom 1/3|
|Deadlift||Top Half||Bottom Half|
|Squat, Trap Bar Squat||Full Movement|
*Romanian Deadlift 35-70 degree knee bend (30-60% ROM) , 115-130 degree hip bend (100% ROM)
However, when observing the actual movement of the deadlift (between beginning and ending positions), the majority of knee extension actually occurs early in the lift (allowing the bar to clear the knees), leaving a significant portion of the remaining hip extension to occur nearer the end of the lift with less concomitant knee extension. When the bar clears the knees, the knees and hip flexion are approximately 30-40 degrees (25-33% flexed) and 70 degrees (60%) respectively. These means at this position (bar just above knees), the knees travel from 75% to about 30% flexion (40% full ROM). In this same position, the hips travel from near 100% flexion to 70% flexion (<30% Full ROM). As the lift continues, the knees become nearly straight (increasing hamstring efficiency in effort to partially resolving a active insufficiency of the Hamstrings while the hips continue to straighten.
However since the Hamstrings have entered partial active insufficiency as the hips approach full extension with knees bent, the glutes are still the primary hip extensor although they too are contracting beyond their optimal tension potential.
This means in the early phase of the Deadlift (bar below knees), the Hamstrings act as a 'Dynamic Stabilizer'. However, as the movement progresses (bar above knees), the Hamstrings act increasing like a Synergist (greater net contraction) although a small countering movement through the knees still occurs.
In the lowering phase, the hip and knee movements become more distinct, with significantly less knee bend, particularly until the bar was lowered below the knees. With less knee bend in the initial portion of the in the lowering phase, the hamstrings act more like a synergist during the eccentric phase of the movement.
In the case a competitive powerlifter, the knees never regain their original bend during the actual descent, only bending a maximum of 35 degrees during the lowering phase, as opposed to a 90 degree bend in the initiation of the lift. Since powerlifters don't need to train the lowering phase of the deadlift they commonly either drop the weight part way down or rapidly lower the weight in what could be described as a controlled drop. The 90 degree initial knee bend is only reset after the weight is at rest on the floor. Without the hip and knee bending simultaneously, the hamstrings can act as a synergist in the eccentric phase, however the eccentric portion of a competitive powerlifter's deadlift can be nearly non-existent due to their style of unloading.
The distinction between Dynamic Stabilizer and Synergist is not cut and dry (as with many other classification systems). Other muscles besides the Hamstrings exhibit qualities of both a synergist and dynamic stabilizer. See analysis of the biceps involvement during different variations of the pull-up and chin-up.
The Sumo Deadlift offers various mechanical trade-offs as compared to the conventional deadlift. The wide stance of the Sumo Deadlifts allows the bar to positioned closer to the hips. The sumo position, along with the lower positioning of the hips, allows the torso to begin in a slightly more upright posture. Although this allows for a shorter moment arm between the hips and barbell, the lower position of the hips creates longer moment arms between the knees and hip.
The wide stances of the Sumo Deadlift also positions the body slightly lower to the ground thereby reducing the need to bend over as far. It could be said that the sumo stance decreases the distance the bar needs to be lifted but in effect it actually decreases the moment arm distances on the sagittal plane (< spine angle) while positioning moment arms more in the coronal plane (hip adduction). The legs push both outward and downward into the floor at lateral angles creating a converging reactive force driving the hips upward.
The Sumo Deadlift uses similar muscles as the Conventional Deadlift with a few notable variations. The Sumo Deadlift has a greater reliance on the powerful hip musculature with relatively less emphasis on the spinal musculature.
The wider stance and deeper initial squatting position relies heavily on the Gluteus Maximus and particularly the Adductor Magnus. Like the Squat, the hamstrings act as Dynamic Stabilizers moving through the hips and knee with little change in length. The hamstring may act as a Synergist nearer the top of the lift if the knees extend significantly sooner than the hips, which may be caused by a narrower sumo stance or long femura, possibly requiring the knees to be extended early in the upper mid position thereby permitting the bar to clear the knees.
Although the knees do not extend as far beyond the ankle with the wide stance with knees pointing outward, significant knee torque is still generated by the bilateral forces angled slightly outward and downward, eliciting quadriceps evolvement.
The Erector Spinae and their antagonist stabilizers, Rectus Abdominis and Obliques, while still heavily involved in stabilizing the spine are proportionally less evolved in the Sumo Deadlift due to the more upright posture of the torso as compared to the more bent over positioning of the Conventional Deadlift. This more upright posture also decreases the tendency for the spine to flex forward under the weight of a load.
The Sumo Deadlift is typically relatively easier nearer the top of the motion as compared to the Conventional Deadlift. However placing the arms narrower than shoulder width creates unneeded torque near the top of the motion potentially interfering with a full upright lockout since the bar cannot held as close to the center of gravity at the top of the lift with the arms positioned anteriorly on the body.