the back squat ( bs ) is a fundamental exercise prescribed for both athletes and non - athletes for developing lower - body strength .
the resulting leg , hip , and back strength from the prescription of systematic squat resistance training reportedly improves athletic performance when included in a training program ( 20 , 23 ) .
recently , it has been suggested in the lay media that the rear leg elevated split squat ( rless ) places less compressive force on the back , while placing higher stress on the legs , hips and stabilizer muscles ( 6 ) . since bs may be contraindicated in persons with lower back pain
, it may be beneficial to examine different variations of squat exercises to determine the benefits of each , as it may have implications for athletic populations .
for instance , there appears to be differences between variations of the squat for lifting heavy loads ( 3 ) , and how trunk position affects the joints and muscles involved ( 18 ) .
additionally , gender differences in mechanics have been demonstrated when performing squats ( 2 , 8 , 25 ) .
electromyographic ( emg ) analyses are commonly conducted to quantify electrical activity of muscles during weight training .
while the majority of research examining emg activity and unilateral squats has focused on rehabilitation ( i.e. no external load ) and general populations ( 1 , 4 , 5 , 17 , 26 ) , one study ( 21 ) compared emg activity levels of the biceps femoris , rectus femoris , and gluteus medius in elite female athletes while performing both loaded back squats and loaded rless .
they concluded that the rless produced greater biceps femoris and gluteus medius emg activity when compared to the traditional bilateral squat .
in addition , rless produced a greater knee valgus angle , which may produce greater hamstring activity in an attempt to better stabilize the knee ( 21 ) .
however , a direct comparison with other types of unilateral squat was not conducted in that study .
in addition to kinematics and emg , the ground reaction forces of bilateral squatting movements have been examined ( 7 , 9 , 10 , 16 , 19 ) , in both loaded ( 10 , 16 , 19 ) , and unloaded conditions ( 7 , 9 ) .
similar variables have also been examined during unilateral squatting motions ( 9 , 11 , 18 , 22 , 27 ) , however , these studies focused on unloaded single - leg squats for rehabilitation purposes .
additionally , only two studies used a unilateral squat with a knee range of motion similar to a bilateral squat ( 9 , 11 ) . to our knowledge , this was also the only study that compared the kinetics of a unilateral and bilateral squat , but it focused on patellofemoral force differences and did not report any comparisons of ground reaction forces ( 9 ) . in one investigation that compared different squat techniques , peak force and peak power appeared to be similar ( 19 ) , however , this study compared the box squat and traditional bs , not unilateral and bilateral squats . in an effort to further understand the biomechanical aspects of bilateral and unilateral squat exercises , specifically bs , rless , and split squat ( ss )
, this investigation was designed to examine the vertical displacement , muscular activity and unilateral ground reaction forces of these three exercises .
we hypothesized that the vertical displacement would be similar in the three types of squat .
additionally , it was expected that most of the thigh and hip musculature would be similarly active in all three exercises with exception being biceps femoris , which was expected to be more active during the unilateral exercises as suggested previously ( 21 ) .
further , the ankle and knee stabilizer muscles were expected to be more active in the rless and ss than the bs , and that the vertical grf will be similar between the exercises , suggesting similar demands are placed upon the prime mover musculature .
nine healthy men ( ages 24 to 36 ; 26.1  3.8 years ) were recruited to participate .
all participants had been participating in a heavy - resistance training program that included squatting exercises for at least the previous six months .
all participants completed a health history questionnaire to screen for any pre - existing health conditions and injuries that would prevent participation .
the study s purpose , procedures , and possible risks and benefits were explained to participants both orally and in written form , followed by the signing of informed consent documents .
the study procedures were approved by the local university institutional review board prior to beginning research .
this study used a repeated measures design to compare the biomechanical differences between a bs , ss , and rless .
all three exercises were performed with one leg on the force platform and video recorded while muscle activity was monitored via emg .
the independent and dependent variables were selected based on the existing literature , and we have attempted to increase the internal validity by carefully assigning loads based on bs 1-rm .
all participants reported for two sessions : one informed consent and practice session , and one data collection session .
after paperwork was completed during the first session , the participants completed a five - minute , self - paced general warm - up on a cycle ergometer , followed by the instruction and the practice of all three squat exercises .
in addition , at the end of this session , bs one - repetition maximum ( 1-rm ) testing was completed following protocol described by harman et al .
, participants completed the same cycle ergometer warm - up followed by seven warm - up sets of bilateral squats .
the warm - up sets were performed as follows : six repetitions at 10% , 20% , and 30% 1-rm , three repetitions at 40% and 50% 1-rm , and one repetition at 60% and 70% 1-rm , with a rest period of exactly two minutes between sets .
subsequent to warm - up , single - repetition bs , ss and rless were completed in stratified random order .
for all lifts , the participant removed the weighted barbell from a rack with a high bar position on the upper back .
bs were performed at 85% of the participants 1-rm with their feet shoulder - width apart and only the left foot on the force plate .
this load was used because it represents a normal training load for the participants in our study .
participants were instructed to squat to the lowest level possible and then complete the lift by returning to the starting position .
this load was chosen for internal validity reasons , and for the fact that the participants were less trained in using rless and ss and thus 1-rm testing for those exercises was not feasible . for the rless , the participant was positioned with their left foot on the force platform under their hips , with their right foot elevated behind them with the anterior portion of the ankle on a 40-cm high stand designed for single - leg squats .
the participants descended to a position where the knee of the right leg ( elevated ) touched the ground , and then returned to the starting position .
the distance between the center of the stand and the tip of the 1 phalange of left foot was determined for each participant by calculating 85% of leg length ( from asis to floor ) . for the ss
, each participant was positioned with their left foot in the center of the force platform and right foot behind with a stance at the same length as rless ( 1 phalange to 1 phalange ) .
participants then descended until the right knee touched the ground , and then returned to the starting position .
a rest period of two minutes between the different squat exercises was provided . a single video camera ( panasonic digital video camcorder , pv - dv203 ) captured the two dimensional ( 2-d ) motion for analysis .
the camera was interfaced with a pc and analyzed with datapac 5 software ( run technologies ; mission viejo , ca ) .
the shutter speed was set to 1/125 sec and the iris was set to + 18 db .
data were sampled at 60 hz and filtered with a 4th order , low - pass butterworth filter at 20 hz .
analog and kinematic data was synchronized with an analog spike ( light - emitting diode ( led ) placed in camera s view ) to serve as a signal for acquisition . a frame ( 1.8 m ) with active led markers on each corner
markers ( active led ) were placed on the left end of the barbell and on the left midaxial line at the level of the anterior superior iliac spine ( asis ) .
vertical displacement was determined from the barbell marker , and eccentric and concentric phases of the lifts were determined using the vertical displacement of the asis marker .
muscle activity was measured for 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 .
this leg was the non - dominant leg for all participants , and was the front leg for the unilateral squat variations in this study .
the non - dominant leg as chosen because generally , balance is better in the non - dominant leg .
this was thought to improve the likelihood that the participants would not lose their balance during unilateral squats .
prior to electrode application , the area was shaved to remove any hair ; the skin was then gently abraded with fine sandpaper to remove any other debris and the area was cleansed with alcohol .
the electrodes were placed parallel to the estimated resting pennation angle so that the same muscle fibers intersected both electrodes .
electrodes ( ambu inc . ; glen burnie , md ) were 2-cm round ag / agcl with an inter - electrode distance of two cm , and the ground electrode was placed on the anterior aspect of the patella for signal noise reduction .
( run technologies ; mission viejo , ca ) with eight dual - lead channels .
the system has a common mode rejection of 90db , a band pass filter ( 10450hz ) , and input impedance of 10m. gain was set at 1000 .
synchronized data were collected at 2khz ( datapac 5 ; run technologies ; mission viejo , ca ) and channeled through a 12-bit analog - to - digital converter ( das1200jr ; measurement computing , middleboro , ma ) .
data were quantified by computing a root mean square ( rms ) , 125ms time constant running average of the raw signal over the eccentric and concentric rom . vertical ground reaction force data ( n )
were acquired with an amti bp600900 ( watertown , ma ) force platform amplified with an amti msa-6 mini amp ( watertown , ma ) at a sampling rate of 2400hz using a das1200jr 12-bit analog to digital converter board ( measurement computing ; norton , ma ) and analyzed using datapac 5 ( run technologies ; mission viejo , ca ) .
data was filtered with a 4 order low pass butterworth digital filter at 20hz . means and standard deviations were calculated for all variables of interest .
each dependent variable was compared with a 1  3 repeated measures anova ( p < 0.05 ) to determine if any significant differences existed between squatting modalities .
bonferroni post - hoc adjustments to dependent t - tests were used to determine where pairwise differences existed and cohen s d effect sizes ( es ) were calculated to quantify the magnitude of those differences ( 15 ) , with corrections for repeated measures .
anova and bonferroni adjustments were calculated with spss v. 20 ( ibm ; armonk , ny ) .
nine healthy men ( ages 24 to 36 ; 26.1  3.8 years ) were recruited to participate .
all participants had been participating in a heavy - resistance training program that included squatting exercises for at least the previous six months .
all participants completed a health history questionnaire to screen for any pre - existing health conditions and injuries that would prevent participation .
the study s purpose , procedures , and possible risks and benefits were explained to participants both orally and in written form , followed by the signing of informed consent documents .
the study procedures were approved by the local university institutional review board prior to beginning research .
this study used a repeated measures design to compare the biomechanical differences between a bs , ss , and rless .
all three exercises were performed with one leg on the force platform and video recorded while muscle activity was monitored via emg .
the independent and dependent variables were selected based on the existing literature , and we have attempted to increase the internal validity by carefully assigning loads based on bs 1-rm .
all participants reported for two sessions : one informed consent and practice session , and one data collection session .
after paperwork was completed during the first session , the participants completed a five - minute , self - paced general warm - up on a cycle ergometer , followed by the instruction and the practice of all three squat exercises .
in addition , at the end of this session , bs one - repetition maximum ( 1-rm ) testing was completed following protocol described by harman et al .
, participants completed the same cycle ergometer warm - up followed by seven warm - up sets of bilateral squats .
the warm - up sets were performed as follows : six repetitions at 10% , 20% , and 30% 1-rm , three repetitions at 40% and 50% 1-rm , and one repetition at 60% and 70% 1-rm , with a rest period of exactly two minutes between sets .
subsequent to warm - up , single - repetition bs , ss and rless were completed in stratified random order .
for all lifts , the participant removed the weighted barbell from a rack with a high bar position on the upper back .
bs were performed at 85% of the participants 1-rm with their feet shoulder - width apart and only the left foot on the force plate .
this load was used because it represents a normal training load for the participants in our study .
participants were instructed to squat to the lowest level possible and then complete the lift by returning to the starting position .
this load was chosen for internal validity reasons , and for the fact that the participants were less trained in using rless and ss and thus 1-rm testing for those exercises was not feasible . for the rless , the participant was positioned with their left foot on the force platform under their hips , with their right foot elevated behind them with the anterior portion of the ankle on a 40-cm high stand designed for single - leg squats .
the participants descended to a position where the knee of the right leg ( elevated ) touched the ground , and then returned to the starting position .
the distance between the center of the stand and the tip of the 1 phalange of left foot was determined for each participant by calculating 85% of leg length ( from asis to floor ) .
for the ss , each participant was positioned with their left foot in the center of the force platform and right foot behind with a stance at the same length as rless ( 1 phalange to 1 phalange ) .
participants then descended until the right knee touched the ground , and then returned to the starting position .
a rest period of two minutes between the different squat exercises was provided . a single video camera ( panasonic digital video camcorder , pv - dv203 ) captured the two dimensional ( 2-d ) motion for analysis .
the camera was interfaced with a pc and analyzed with datapac 5 software ( run technologies ; mission viejo , ca ) .
the shutter speed was set to 1/125 sec and the iris was set to + 18 db .
data were sampled at 60 hz and filtered with a 4th order , low - pass butterworth filter at 20 hz .
analog and kinematic data was synchronized with an analog spike ( light - emitting diode ( led ) placed in camera s view ) to serve as a signal for acquisition .
a frame ( 1.8 m ) with active led markers on each corner was used to calibrate the space for the motion analysis .
markers ( active led ) were placed on the left end of the barbell and on the left midaxial line at the level of the anterior superior iliac spine ( asis ) .
vertical displacement was determined from the barbell marker , and eccentric and concentric phases of the lifts were determined using the vertical displacement of the asis marker .
muscle activity was measured for 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 . this leg was the non - dominant leg for all participants , and was the front leg for the unilateral squat variations in this study .
the non - dominant leg as chosen because generally , balance is better in the non - dominant leg .
this was thought to improve the likelihood that the participants would not lose their balance during unilateral squats .
prior to electrode application , the area was shaved to remove any hair ; the skin was then gently abraded with fine sandpaper to remove any other debris and the area was cleansed with alcohol .
the electrodes were placed parallel to the estimated resting pennation angle so that the same muscle fibers intersected both electrodes .
glen burnie , md ) were 2-cm round ag / agcl with an inter - electrode distance of two cm , and the ground electrode was placed on the anterior aspect of the patella for signal noise reduction .
( run technologies ; mission viejo , ca ) with eight dual - lead channels .
the system has a common mode rejection of 90db , a band pass filter ( 10450hz ) , and input impedance of 10m. gain was set at 1000 .
synchronized data were collected at 2khz ( datapac 5 ; run technologies ; mission viejo , ca ) and channeled through a 12-bit analog - to - digital converter ( das1200jr ; measurement computing , middleboro , ma ) .
data were quantified by computing a root mean square ( rms ) , 125ms time constant running average of the raw signal over the eccentric and concentric rom . vertical ground reaction force data ( n ) were acquired with an amti bp600900 ( watertown , ma ) force platform amplified with an amti msa-6 mini amp ( watertown , ma ) at a sampling rate of 2400hz using a das1200jr 12-bit analog to digital converter board ( measurement computing ; norton , ma ) and analyzed using datapac 5 ( run technologies ; mission viejo , ca ) .
was compared with a 1  3 repeated measures anova ( p < 0.05 ) to determine if any significant differences existed between squatting modalities .
bonferroni post - hoc adjustments to dependent t - tests were used to determine where pairwise differences existed and cohen s d effect sizes ( es ) were calculated to quantify the magnitude of those differences ( 15 ) , with corrections for repeated measures .
anova and bonferroni adjustments were calculated with spss v. 20 ( ibm ; armonk , ny ) .
all nine participants completed the study and their descriptive statistics are presented as mean  sd ( table 1 ) .
muscle activity was only significantly greater for the bf during rless and bs than ss during the concentric phase ( rless / ss - es= 2.11 , p=0.008 ; bs / ss - es=1.78 , p=0.029 ) , and significantly greater during rless than ss during the eccentric phase ( es= 2.13 , p=0.012 ; figures 1 and 2 ) .
maximum vertical force ( n ) was also significantly greater during rless than ss ( es= 3.03 , p=0.001 ; figure 3 ) and tended to be greater during bs than ss but the trend did not reach significance ( es = 1.42 ; p=0.058 ) .
this study was designed to compare two different single leg squat techniques and bilateral back squats with respect to vertical range of motion , muscle activity , and vertical ground reaction force .
our data show significantly greater biceps femoris activity during rless and bs than ss , similarities in vertical displacement between the three lifts , and some differences in grf between the different variations of the squat .
our data support contentions that similar lower body muscle activity can be achieved using the rless with half the load of bs .
this likely would result in less compressive force on the back , however compressive force on the back is beyond the scope of the present study .
our emg data support the contention that similar stimulus can be achieved with single leg squats of several types and bs .
the only exception was bf activity , where rless and bs had a significantly higher activity than the split squat ( es = 2.11 and 1.78 respectively ) during the concentric phase of the lifts .
rless also had significantly higher bf activity during the eccentric phase than split squats ( es = 2.19 ) .
contrary to previous findings ( 21 , 24 ) , neither the rless nor the split squat had greater bf or rf activity than the bilateral squat
. this may in part be due to differences in load calculation or sample population .
one previous study ( 21 ) used 85% of the participants three - repetition maximum ( 3rm ) for each lift , where we used 85% of bs 1-rm for the bs and half that load for the single - leg lifts .
they also used a sample of female athletes and we used a sample of resistance - trained men .
the other study ( 24 ) used a 50 lb barbell for both bilateral and single leg squats , and a sample of healthy men .
it would be expected that muscle activity would be higher during a single leg squat if the same load were used . however , there are always limitations to calculating relative loads between different weight training exercises .
peak vertical grf were similar between the bs and rless , suggesting that we adjusted the load sufficiently for loading the leg unilaterally .
the rless had significantly larger peak ground reaction force than the ss ( es=3.03 ) at the same load .
peak vertical forces during the bs ( 1414.81  250.98n ) were larger than the ss ( 1198.56  187.88n ) , however failed to reach significance ( es = 1.42 ; p=0.058 ) .
previous researchers reported that the rear leg supported between 25% and 45% of the load during a split squat ( 13 ) suggesting that the smaller peak forces during the split squat are likely due to a larger portion of the load being supported by the rear leg .
however , this is beyond the scope of the present study , as we were only able to collect data from the lead leg within the limitations of our experimental setup .
as was expected , vertical bar displacement was similar between squats , suggesting similar depth of squat .
this variable has not been compared between these lifts previously , therefore comparisons with other studies is impossible .
we acknowledge that similar bar displacement does not equate to similar joint ranges of motion .
one study ( 21 ) reported larger trunk inclination during bilateral squats than during rless , which likely would translate to greater hip flexion and rom . since joint rom
was not examined herein , it is unclear if this was true in the current study ; however , for trunk inclination to be different , lower extremity joint rom would likely be different between squats to maintain the overall vertical displacement .
the findings of the current study combined with those previously reported ( 21 ) suggest further examination of joint rom in these three squat types . due to the inherent limitations in using 2d video analysis for joint motion ,
in addition , we acknowledge the limitation that both legs were not monitored for emg activity and grf . while adding 3d analysis and additional emg channels may be feasible , instrumenting multiple force platforms so that force information can be collected from both legs may be challenging , especially instrumenting a stand for the rear leg in the rless .
in spite of these limitations , our data are an important contribution to the relative scarcity of data on these types of squat .
additionally , future studies may consider basing the prescribed loads off of the 1-rm for each individual squat type .
performing multiple repetition sets , including those designed to induce considerable fatigue , may also be of interest to the practitioner .
rear leg elevated split squats ( rless ) activate the lower body musculature similar to bilateral back squats while using half the load , but increased bf activity was seen for rless .
therefore , if additional bf activity is desired , rless may be more appropriate than ss or bs .
future research should consider additional measures of force and time such as impulse to clarify the potential differences in bilateral and unilateral squatting .
additionally , comparisons of the dominant and non - dominant leg in unilateral squats may be of interest .
 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 , displacment 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 . 
INTRODUCTION
METHODS
Participants
Protocol
Statistical Analyses
RESULTS
DISCUSSION