Background: Unstable training surfaces and loads are often used to enhance neuromuscular activation, but their comparative effects on core and lower limb muscle activity during unilateral exercises remain unclear. Objective: To compare the effects of Bulgarian split squats performed under unstable surface conditions (USC) and unstable load conditions (ULC) on core and lower extremity muscle activation. Methods: Twenty-one physically active adults performed Bulgarian split squats under three conditions: stable condition (SC), USC using a balance pad, and ULC using a water-filled aqua bag. Surface electromyography (EMG) measured muscle activation during both descending and ascending phases. Data were normalized to %MVIC and analyzed using repeated measures ANOVA or non-parametric tests depending on data normality. Results: USC significantly increased core (rectus abdominis, external oblique, internal oblique, erector spinae) and selected lower limb muscle (gluteus maximus, gluteus medius, biceps femoris) activation compared to SC (p < .05). ULC elicited the greatest activation across both phases (p < .001). No significant differences were found in vastus lateralis and vastus medialis among conditions. Conclusions: Bulgarian split squats with unstable loads result in greater activation of core and selected lower limb muscles than unstable surfaces or stable conditions. Unstable load training may be an effective method to enhance muscular activation and stability in unilateral lower body exercises.
TseMAMcManusAMMastersRS. Development and validation of a core endurance intervention program: implications for performance in college-age rowers. J Strength Cond Res2005; 19: 547–552.
2.
FreemanMDWoodhamMAWoodhamAW. The role of the lumbar multifidus in chronic low back pain: a review. Pm&r2010; 2: 142–146.
3.
GranacherUGollhoferAHortobágyiT, et al.The importance of trunk muscle strength for balance, functional performance, and fall prevention in seniors: a systematic review. Sports Med2013; 43: 627–641.
4.
HuxelKCAndersonBE. Core stability training for injury prevention. Sports Health2013; 5: 514–522.
5.
StuberKJBrunoPSajkoS, et al.Core stability exercises for low back pain in athletes: a systematic review of the literature. Clin J Sport Med2014; 24: 448–456.
6.
OliverGDDwellyPMSarantisND, et al.Muscle activation of different core exercises. J Strength Cond Res2010; 24: 3069–3074.
7.
MartuscelloJMNuzzoJLAshleyCD, et al.Systematic review of core muscle activity during physical fitness exercises. J Strength Cond Res2013; 27: 1684–1698.
8.
Cortell-TormoJMGarcía-JaénMChulvi-MedranoI, et al.Influence of scapular position on the core musculature activation in the prone plank exercise. J Strength Cond Res2017; 31: 2255–2262.
9.
LyonsBCMayoJJTuckerWS, et al.Electromyographical comparison of muscle activation patterns across three commonly performed kettlebell exercises. J Strength Cond Res2017; 31: 2363–2370.
10.
SchwanbeckSChilibeckPDBinstedG. A comparison of weight squat to Smith machine squat using electromyography. J Strength Cond Res2009; 23: 2588–2591.
11.
AndersenVFimlandMSBrennsetØ, et al.Muscle activation and strength in squat and Bulgarian squat on stable and unstable surface. Int J Sports Exerc Med2014; 35: 1196–1202.
12.
MackeyERRiemannBL. Biomechanical differences between the Bulgarian split-squat and back squat. Int J Exerc Sci2021; 14: 533–543.
13.
JonesMTAmbegaonkarJPNindlBC, et al.Effects of unilateral and bilateral lower-body heavy resistance exercise on muscle activity and testosterone responses. J Strength Cond Res2012; 26: 1094–1100.
14.
McCurdyKO’KelleyEKutzM, et al.Comparison of lower extremity EMG between the 2-leg squat and modified single-leg squat in female athletes. J Sport Rehabil2010; 19: 57–70.
15.
SaeinAMKahriziSBoozariS. Effects of unstable load and unstable surface ontrunk muscles activation and postural control in healthy subjects. J Biomech2024; 173: 112257.
16.
MasonBRLeaRMcKuneAJ, et al.Comparison of stable load, unstable load and variable resistance training interventions. Int J Sports Sci Coach2025; 20: 1661–1672.
17.
Chulvi-MedranoIMartínez-BallesterEMasiá-TortosaL. Comparison of the effects of an eight-week push-up program using stable versus unstable surfaces. Int J Sports Phys Ther2012; 7: 586.
18.
MarshallPWMurphyBA. Core stability exercises on and off a Swiss ball. Arch Phys Med Rehabil2005; 86: 242–249.
19.
LawrenceMACarlsonLA. Effects of an unstable load on force and muscle activation during a parallel back squat. J Strength Cond Res2015; 29: 2949–2953.
20.
SeoGEHaDWYuIY, et al.Effects of different types of unstable loads on core and lower extremity muscle activity during squatting in young adult women. PNF and Movement2024; 22: 233–242.
21.
Rodríguez-SanzJLlurda-AlmuzaraLLópez-de-CelisC, et al.Peroneal muscle activity during stable and unstable load exercises. A cross-sectional study. Phys Ther Sport2023; 60: 84–90.
22.
GlassSCWisneskiKA. Effect of instability training on compensatory muscle activation during perturbation challenge in young adults. J Funct Morphol Kinesiol2023; 8: 136.
23.
AndersonKBehmDG. Trunk muscle activity increases with unstable squat movements. Can J Appl Physiol2005; 30: 33–45.
24.
LockieRGRissoFGLazarA, et al.Between-leg mechanical differences as measured by the Bulgarian split-squat: exploring asymmetries and relationships with sprint acceleration. Sports2017; 5: 65.
25.
Aguilera-CastellsJBuscàBMoralesJ, et al.Muscle activity of Bulgarian squat. Effects of additional vibration, suspension and unstable surface. PLoS One2019; 14: e0221710.
26.
MeylanCMcMasterTCroninJ, et al.Single-leg lateral, horizontal, and vertical jump assessment: reliability, interrelationships, and ability to predict sprint and change-of-direction performance. J Strength Cond Res2009; 23: 1140–1147.
27.
SlaterLVHartJM. Muscle activation patterns during different squat techniques. J Strength Cond Res2017; 31: 667–676.
28.
LiYCaoCChenX. Similar electromyographic activities of lower limbs between squatting on a reebok core board and ground. J Strength Cond Res2013; 27: 1349–1353.
29.
DinisRVazJRSilvaL, et al.Electromyographic and kinematic analysis of females with excessive medial knee displacement in the overhead squat. J Electromyogr Kinesiol2021; 57: 102530.
30.
WezenbeekEVerhaegheLLaveyneK, et al.The effect of aquabag use on muscle activation in functional strength training. J Sport Rehabil2022; 31: 420–427.
31.
DitroiloMO’SullivanRHarnanB, et al.Water-filled training tubes increase core muscle activation and somatosensory control of balance during squat. J Sports Sci2018; 36: 2002–2008.
32.
DanneelsLAVanderstraetenGGCambierDC, et al.A functional subdivision of hip, abdominal, and back muscles during asymmetric lifting. Spine2001; 26: E114–E121.
33.
FreriksBHermensHJ. SENIAM 9: European Recommendations for Surface ElectroMyoGraphy, Results of the SENIAM Project. Enschede, Netherlands. Roessingh Research and Development BV, 1999.
34.
MuyorJMMartín-FuentesIRodríguez-RidaoD, et al.Electromyographic activity in the gluteus medius, gluteus maximus, biceps femoris, vastus lateralis, vastus medialis and rectus femoris during the Monopodal Squat, Forward Lunge and Lateral Step-Up exercises. PLoS One2020; 15: e0230841.
35.
CollinsKSKlawitterLAWalderaRW, et al.Differences in muscle activity and kinetics between the goblet squat and landmine squat in men and women. J Strength Cond Res2021; 35: 2661–2668.
36.
LehmanGJMcGillSM. The importance of normalization in the interpretation of surface electromyography: a proof of principle. J Manipulative Physiol Ther1999; 22: 444–446.
37.
SchoenfeldBJContrerasBTiryaki-SonmezG, et al.Regional differences in muscle activation during hamstrings exercise. J Strength Cond Res2015; 29: 159–164.
38.
KimSYKangMHKimER, et al.Comparison of EMG activity on abdominal muscles during plank exercise with unilateral and bilateral additional isometric hip adduction. J Electromyogr Kinesiol2016; 30: 9–14.
39.
WorrellTWKarstGAdamczykD, et al.Influence of joint position on electromyographic and torque generation during maximal voluntary isometric contractions of the hamstrings and gluteus maximus muscles. J Orthop Sports Phys Ther2001; 31: 730–740.
40.
BehmDGColado SanchezJC. Instability resistance training across the exercise continuum. Sports Health2013; 5: 500–503.
41.
NairnBCSutherlandCADrakeJD. Motion and muscle activity are affected by instability location during a squat exercise. J Strength Cond Res2017; 31: 677–685.
42.
PandyMGLaiAKSchacheAG, et al.How muscles maximize performance in accelerated sprinting. Scand J Med Sci Sports2021; 31: 1882–1896.
43.
MeindersEPizzolatoCGonçalvesBA, et al.The deep hip muscles are unlikely to stabilize the hip in the sagittal plane during walking: a joint stiffness approach. IEEE Trans Biomed Eng2021; 69: 1133–1140.
44.
López-de-CelisCSánchez-AlfonsoNRodríguez-SanzJ, et al.Quadriceps and gluteus medius activity during stable and unstable loading exercises in athletes. a cross-sectional study. J Orthop Res Ther2024; 42: 317–325.
45.
LehmanGJHodaWOliverS. Trunk muscle activity during bridging exercises on and off a swissball. Chiropr Osteopat2005; 13: 1–8.
46.
ArokoskiJPValtaTAiraksinenO, et al.Back and abdominal muscle function during stabilization exercises. Arch Phys Med Rehabil2001; 82: 1089–1098.
47.
López-de-CelisCLabata-LezaunNRomaní-SánchezS, et al.Effect of load distribution on trunk muscle activity with lunge exercises in amateur athletes: cross-sectional study. In Healthcare2023; 11: 916.
48.
ZebaZTanwarTIrshadN, et al.Correlation between lumbopelvic stability and hamstring strain recurrence in sprinters. Physio Ther2023; 31: 80–85.
49.
LeeNGYouJSHKimTH, et al.Intensive abdominal drawing-in maneuver after unipedal postural stability in nonathletes with core instability. J Athl Train2015; 50: 147–155.
