Biomechanical analysis of Deadlift

28.03.2019

The deadlift is a multijoint resistance exercise that is performed in a variety of training settings. The exercise requires the lifter to grasp a barbell at mid-shank level in a squat position and elevate the load by extending the lower back, hip, knee, and ankle joints. The deadlift is most frequently used to develop maximum strength based on the hypothesis that heavy loads lifted will generate large muscular forces and stimulate adaptation.
Large muscular moments can be produced with the greatest values recorded at the hip, followed by the lumbar spine, ankle, and knee. Brown and Abani reported net joint forces ranging from approximately 1,450-1,550 N at the hip, knee, and ankle joints for adolescent powerlifters during competition. The authors noted that competitors who lifted the heaviest loads experienced the greatest internal forces. Studies that have included linked segment models designed to estimate forces at the lumbar spine have reported extremely large disk compression forces during the deadlift. Cholewicki et al. recorded L4/L5 disk compression forces ranging from 14,350 to 17,192 N for male and female powerlifters during a national-level competition. For world-class athletes lifting extremely heavy loads, lumbar disk compression forces as large as 36,400 N have been reported. As a consequence of the very large internal forces and moments imposed during the deadlift, the potential for injury exists.

To minimize the likelihood of sustaining an injury during the deadlift, athletes are instructed to position the barbell close to the body throughout the movement. Maintaining the barbell in close proximity to the body decreases the overall resistance of the external load by reducing the moment arm at the individual joints. When performing the deadlift with a conventional straight barbell, the moment arm of the external resistance can be reduced up to the point where the barbell impinges on the body. To overcome this restriction and further reduce the resistance moment arm, a nonconventional barbell was created. The premise of the barbell was to enable the athlete to position the load closer to the joints by creating a frame that the athlete could lift within. Originally, the barbell was trapezoidal in shape and commonly referred to as the trap bar. In subsequent years, the shape of the barbell was altered from trapezoidal to hexagonal to provide greater stability and space. The hexagonal barbell is now a standard resistance training implement used widely in the strength and conditioning training of athletes and is most commonly used to provide a variation of the deadlift exercise. It has been theorized that performing the deadlift with the hexagonal barbell reduces lumbar stress.
The theory is based on the assertion that the hexagonal barbell enables the lifter to adopt a more upright lifting posture and reduce the resistance moment arm at the lumbar spine. To our knowledge, there have been no published reports of the kinematics and kinetics of the deadlift performed with the hexagonal barbell or empirical data supporting the theory. Information regarding potential differences in the kinematics and kinetics of deadlift variations would be of practical significance to coaches in their exercise selection.

Experimental Approach to the Problem

A cross-sectional, repeated measures design was used to quantify and compare kinematics and kinetics of the deadlift exercise using 2 distinct barbells. Joint moments were calculated to investigate whether the choice of barbell had an effect on the muscular effort and internal stresses developed when lifting a given load. External kinematics and kinetics (e.g., vertical ground reaction force, velocity, and power) were calculated across a range of submaximal loads to investigate whether the deadlift could be used to obtain a biomechanical stimulus effective for developing muscular power.

Data were collected for each subject over 2 sessions separated by 1 week. The first session was performed in the gymnasium and involved 1RM testing in the straight barbell deadlift (SBD) and the hexagonal barbell deadlift (HBD). During the second session, subjects reported to the laboratory where they performed the SBD and HBD across loads of 10-80% of their predetermined SBD 1RM. Kinematics and kinetics were analyzed in the second session only.

No main effects of load were found for the orientation of the torso, hip, knee, or ankle at the start of the concentric phase of the deadlift movement. Therefore, joint angles for the SBD and HBD were averaged across loads and are presented in Table.

The pattern of movement at each joint was assessed by measuring joint angles over 10% intervals of the vertical barbell displacement. Statistical analyses revealed no main effects of load for angles generated at the torso, hip, or knee during the deadlift movement. A significant main effect of load was obtained for the ankle joint (p < 0.05, η2 P = 0.87). The results showed that as load increased the maximum amount of ankle plantar flexion achieved at the conclusion of the concentric phase decreased.

Analyses of deadlift variation revealed significant main effects for peak moments obtained at the lumbar spine (p < 0.05, η2 P = 0.53), hip (p < 0.05, η2 P = 0.43), and knee (p < 0.05, η2 P = 0.85).

Performing the deadlift with the hexagonal barbell significantly increased the peak moment at the knee and significantly decreased the peak moment at the lumbar spine and hip compared to the deadlifts performed with the straight barbell. The effect of deadlift variation on peak moments was explained by the barbell path associated with each variation and the corresponding resistance moment arm at the individual joints.

Exercise selection is a key variable in resistance training design. Selected exercises should recruit the desired muscles and provide an appropriate biomechanical stimulus when combined with suitable acute program variables.

The results of this study show that the biomechanical stimulus of the deadlift can be altered by performing the movement with different barbells.
The deadlift variation that should be selected for a resistance training program will depend on the stimulus required. If the training objective is to target the lumbar area and maximally recruit the erector spinae muscles then it is recommended that the SBD is performed. Because the HBD more evenly distributes the load between the joints of the body, practitioners may find deadlifts performed with the hexagonal barbell to be an effective alternative to the squat and an appropriate exercise to use in the final stages of low back rehabilitation.

This is the first study to demonstrate that the deadlift can be combined with submaximal loads to generate large power outputs. The finding suggests it may be advantageous to include the deadlift in structured periodized models aimed at developing muscular power. The results of the study also demonstrate that the HBD can produce significantly greater peak force, peak velocity, and peak power values than the SBD. Strength and conditioning coaches should be aware of the enhanced mechanical stimulus created with the hexagonal barbell when selecting a deadlift exercise.