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Lower Limb and Trunk Muscle Activation with Back Squats and Weighted Sled Apparatus

Abstract: The back squat is a traditional resistance training exercise whereas the resisted sled exercise is a relatively new resistance exercise. However as there are no studies comparing muscle activation between the exercises, the objective of this study was to examine activity of leg and trunk muscles for the two exercises. Ten healthy resistance trained males participated in a randomized cross-over design study consisting of two preparation sessions and two testing sessions. Electromyographic (EMG) activity of the rectus femoris, biceps femoris, gastrocnemius, lower erector spinae and the transversus abdominus/internal obliques (TrA/IO) were monitored during a twenty step maximum (20SM) push with the weighted sled apparatus and a 10 repetition maximum (10RM) with a bilateral back squat. There were non-significant trends for the rectus femoris (p=0.092: 8.6%-16.7%) and biceps femoris (p=0.09: 10.5%-32.8%) to demonstrate higher activity with the sled and squat exercises respectively. There were main effects for condition with 61.2% greater gastrocnemius EMG with the sled exercise (p=0.01) and 74.5% greater erector spinae EMG activity with the squat (p=0.002). There were no significant differences between the exercises for the TrA/IO. In summary the sled and squat exercises provided similar EMG activity for the quadriceps, hamstrings and TrA/IO. The squat provided higher lower erector spinae activation, while the sled had superior gastrocnemius activation. Dependent upon the movement training specificity of the sport, either exercise may be employed in a training program while acknowledging the differences in gastrocnemius and erector spinae activity.


Alex’s Notes: The back squat is a ubiquitous movement in physical culture. It seems that no matter what the goal, the back squat is relied upon to accomplish it. But while strength gains are easily transferable to any sport, the squat’s vertical, stationary movement pattern is not ideal. Athletes competing in American football, rugby, and wrestling for example, move in a more horizontal plane with the body at an incline (e.g. blocking; tackling; sprinting). This movement pattern is easily mimicked by a far less well known exercise – the weighted sled.

Ten healthy college-age males with at least two years of resistance training and back squat experience, who were also familiar with performing the sled exercise (although squatting was much more common), were recruited for this study. They underwent two preparation and two testing sessions separated by 2-5 days each with the order of testing (squat or sled) decided by having them pull slips of paper out of a hat. The prep sessions were to determine the 10RM on the squat and 20RM on the sled, while the testing sessions used these values with all the EMG equipment attached. The 20RM was chosen for the sled because it corresponded to 10 steps per leg.

To ensure that all participants had great neural drive to the legs, the testing sessions began with a task-specific warm-up in which the subject performed 5 reps (10 steps) with 25%, 50%, and 75% their 10RM (20 step max). Each set was separated with three minutes of rest and the final test came afterwards. Also, the EMG activity was compared during the concentric portion of the exercises

For all muscle groups except the transverse abdominis (TVA; functions like a corset) there was an increase in EMG activity and neural drive as the participants progressed through the warm-up towards the final load. When comparing the squat and sled exercises, the squat provided 33.8% greater EMG activity to the erector spinae, the sled provided 61.2% greater EMG activity to the gastrocnemius, and there was no difference for the quadriceps, hamstrings, or TVA. Additionally, the sled load was 124% significantly greater than the squat load.

These results bring to mind two thoughts. First, we now need a study comparing activation of the glutes and surrounding stabilization muscles (ab-/adductors; gluteus minimus). I’m actually a bit surprised the researchers didn’t test the gluteus maximus given how important it is with hip extension. The second thought is that weight sled pushing is an incredible unilateral exercise for the legs. With similar thigh activation as the back squat but with only one leg working per step and greater time spent under load, it stands to reason that the weighted sled would not only provide a killer metabolic stimulus, but also greatly enhance all the stabilizer muscles that future research should test.

The differences in erector spinae activation are likely due to simple torso position. The stable and more forward incline of the torso while pushing the sled allowed for force to transfer through the spine to the legs, while the squat requires these muscles hold the torso upright during the movement. While the ability of the squat to activate many leg and torso muscles is a reason for its ubiquitous use, lower back fatigue because of it represents a programming challenge. The sled is thus a great substitute on days when the lower back is already getting worked significantly, such as following heavy squats or deadlifts.

Bottom line: Both are excellent exercises that can and should be integrated into a proper strength training program.

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