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Muscle Activity in Single- vs. Double-Leg Squats

Abstract: Muscular activity, vertical displacement and ground reaction forces of back squats (BS), rear-leg elevated split squats (RLESS) and split squats (SS) were examined. Nine resistance-trained men reported for two sessions. The first session consisted of the consent process, practice, and BS 1-repetition maximum testing. In the second session, participants performed the three exercises while EMG, displacement and ground reaction force data (one leg on plate) were collected. EMG data were collected from the gluteus maximus (GMX), biceps femoris (BF), semitendinosus (ST), rectus femoris (RF), vastus lateralis (VL), vastus medialis (VM), tibialis anterior (TA), and medial gastrocnemius (MGas) of the left leg (non-dominant, front leg for unilateral squats). Load for BS was 85% one repetition maximum, and RLESS and SS were performed at 50% of BS load. Repeated measures ANOVA was used to compare all variables for the three exercises, with Bonferroni adjustments for post hoc multiple comparisons, in addition to calculation of standardized mean differences (ES). Muscle activity was similar between exercises except for biceps femoris, which was significantly higher during RLESS than SS during both concentric and eccentric phases (ES = 2.11; p=0.012 and ES= 2.19; p=0.008), and significantly higher during BS than the SS during the concentric phase (ES = 1.78; p=0.029). Vertical displacement was similar between all exercises. Peak vertical force was similar between BS and RLESS and significantly greater during RLESS than SS (ES = 3.03; p=0.001). These findings may be helpful in designing resistance training programs by using RLESS if greater biceps femoris activity is desired.

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Alex’s Notes: We know strength influences performance in ways outside of lifting heavy things. But there are many roads to Rome, and this was demonstrated in our previous analysis of back squats versus weight sleds. Now we are going to turn our attention to single-legged squats. There are countless reasons to implement unilateral exercises into a training routine, not the least of which is to correct muscle imbalances or prevent injury (e.g. the back squat is contraindicated in persons with low back problems). Regarding the back squat (BS), the rear-leg elevated split squat (RLESS; also called the Bulgarian split squat) and standard split squat (SS) are common variations that allow for less load to be used while still providing a great workout. So how do they compare?

Nine healthy 24 to 36 year old men (avg. 26-years) who had been regularly performing squat-based heavy resistance training for the previous six months and familiar with all three exercises being tested were recruited for two sessions. The first was a practice session where performance during the exercises was practiced with instruction and the BS 1-RM was determined. The second was the data collection session where, following a warm-up, single-repetition BS, SS, and RLESS were completed in random order. The BS was performed with 85% of the 1RM, and the SS/RLESS used half that load. A rest period of two minutes was provided between exercises.

With regard to muscle activation via EMG, the only significant differences were seen in the biceps femoris (i.e. hamstring). While there were no differences between the BS and RLESS, the SS had 40% lower EMG activity during the concentric phase of the lift, and 37% lower activity during the eccentric portion. Other muscles that had no significant difference included the gluteus maximus, quadriceps, semitendinosus (another hamstring muscle), tibialis anterior, and gastrocnemius.

While the above prompts the immediate belief that the SS is inferior, it was also shown that SS had a significant 15% less vertical force production. The plates that measured this were placed under the front foot of the unilateral lifts, and previous research has demonstrated that the rear leg in a SS supports 25-45% of the load during the exercise, which would not only explain the lower peak force, but also why the EMG activity was lower. Moreover, the peak vertical force was similar between the BS and RLESS, suggesting that the load was adjusted appropriately between the squat and its unilateral alternatives. It is unfortunate that the peak vertical force was not measured for both legs.

So what, they are all equal?

Fundamentally, perhaps. The EMG activity was overall not significantly different between the lifts, suggesting that a unilateral squat with half the load of what would be used for the bilateral back squat is a suitable substitution. However, I am a bit skeptical of this conclusion and would propose that the small sample size of nine men limited the statistical power of the analysis.

Annoyingly, the researchers don’t provide the probability data for every muscle’s EMG activity and only indicate which are significant. Thus, we cannot see which approached significance and which were complete random. Regardless, it is worth noting that the bilateral back squat had the greatest EMG values during the concentric lift phase for the gluteus maximus and quadriceps – likely due to the greater load utilized, while the RLESS had the greatest values for the hamstrings, tibialis anterior, and gastrocnemius – likely due to the increased need for stability. During the eccentric portion of the lift, the RLESS had the greatest activation for all muscles.

Bottom line

All three squat variations are excellent exercises that most definitely have a place in any training routine depending on the goals and needs of the athlete. Although the unilateral variations are suitable substitutes for the bilateral back squat, I would be hesitant to make the switch unless need be (such as during times of injury). The back squat – although not statistically significant – activates the quads and butt more than its variations, and these are exactly the muscles that this exercise is designed to target. Moreover, the back squat hits the core and lower back hard, making for a killer (nearly) full-body exercise. I would rather prefer to work in the RLESS and SS as assistance movements to aid in lower body development and hammer out any imbalances between legs. But that’s just me, and I would love to hear your thoughts as well.

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