The past 15 years has shown increasing interest into studying different aspects of squat biomechanics (1,6,12,21,34). However, the research evaluating the influence of supportive equipment, in particular footwear, is limited. Variation in footwear design has been shown to impact performance from an athletic and therapeutic perspective in multiple sports that typically involve running. Regarding squatting, three styles of footwear are typically used: running shoes, barefoot (often simulated) or canvas style, or weightlifting shoes. Weightlifting shoes (WS) are a common purchase for weightlifters and powerlifters in addition to the every-day gym users and are marketed to help with power production, protect the athlete’s feet as they offer a stable incompressible base with a raised heel between 1.5-2.5cm and a rigid mid sole and a mid-tarsal strap to assist with stability (14,28,34).
The purpose of this review was to determine  the effect that different ankle-foot positions have on the squat performance, including squat derivatives (e.g. front squat, back squat, overhead squat, Zercher squat, split squat and single leg squat) and  the effect that any changes to the ankle-foot complex, including footwear and orthotics, have on the biomechanics of squat performance. The focal hypothesis of this review is that modification to the ankle-foot position effects the biomechanical profile of squat performance.
Research included in this review extends to all peer-reviewed studies researching the effect of foot position has on lower-limb or trunk biomechanics during squat performance with or without additional load. The following search terms were used: ‘weightlifting shoes’, ‘barefoot and squat, ‘heel raise and squat’, ‘squat kinematics and kinetics’, ‘squat biomechanics’, ‘ankle and foot biomechanics’, ‘orthotics and squat’. These terms were entered into EBSCOhost, Google Scholar, and PubMed databases (Figure 1). In addition, citations in identified studies were explored using Google Scholar, EBSCOhost and PubMed. Authors were contacted by email for further information about their method if aspect were not included that were felt to be of value .
Research studies that considered the impact of kinematic changes to the ankle-foot complex on static lower limb strengthening exercises were the focus of this review. All quantitative randomised control studies, systematic reviews and articles published in the English language that researched biomechanical effects of the lower limb and trunk in static strengthening exercises, squat or squat derivatives, were selected. This resulted in the identification of 193 studies from the original search results. Journals that were not peer-reviewed were excluded, reducing the total to 185. The next step was to exclude any research article that did not involve biomechanical analysis of squat performance with applied changes to the ankle-foot position. After critical analysis of the initial collection 19 studies were identified for this review.
The ankle-foot complex is a complicated structure that enables the synchronised movement and stability of twenty six bones and thirty three joints (23). A flattened arch or over-pronation is the main concern regarding the foot when assessing lifting technique and in particular whether or not the arch can be maintained or stable in closed chain load bearing activities (21). Over-pronation is a triple planar motion of the foot and ankle involving calcaneus eversion, mid tarsal joint pronation, talus plantar flexion and tibial internal rotation (17,31), which can cause 5-6° of hip adduction and internal rotation (31). Adding a mid-foot or arch support has been shown to reduce knee valgus, increase knee flexion and reduce ankle dorsiflexion (DF) (17,31). However, studies have either limited squat depth to 60° knee flexion (31) or allowed subjects to self-select squat depth (24), highlighting that these effects occur in a partial squat movement but make it difficult to ascertain if these effects are maintained throughout the squat.
There is limited analysis of changes in lower-limb kinematics due to heel height variation (14,24,27,28,30,32,34). Studies have compared WS to running shoes (RS) (14,30,32,34), barefoot to RS (28) and WS to simulated barefoot shoes (30,32). In the majority of studies, subjects squatted with loads between 40-90% of their one repetition maximum (1RM), maintaining a shoulder width squat stance, mid-trap bar position and parallel foot position (Table 1) (12,27,28). However, technique is not always specified clearly and terms like: ‘were allowed to use their normal squat position’ are often used to explain squat technique instead (32,34). Research in this area demonstrated that WS reduce dorsiflexion angle, enabling a more vertical shank position which increases knee flexion and reduces hip flexion and trunk inclination. Although these results show WS impact whole body kinematics, the restricted information regarding squat style, technique, and shoe design limits the ability to interpret and compare these results across studies.
In all WS studies, lower limb joint range of movement (ROM) was not assessed. This has been demonstrated to impact single leg squat (SLS), overhead squat and traditional squat (3,4,8,19,24). In particular, restricted DF between 11° (8,19) and 17° (4) reduces knee flexion by 7-15o (8,19) , increases knee valgus by 1o (8,19) knee internal and external rotation by 3-5o (9,20) , hip adduction by 6.5o and reduced internal rotation by 6.1o (27). Research showed that a small heel raise of between 1-5 cm (3,4,24) can significantly improve these kinematics variations. Justification for use of these heights was not reasoned and why varying heel heights were not measured limits the impact of these research studies.
An aspect not considered, is the impact of varying heel height on lower-limb biomechanics. In the majority of case it is unclear if the results are due to the heel height or the composition of the sole as RS have reduced lateral heel support to assist with running gait. It has been suggested a raised heel will reduce lateral ankle muscular tightness and restore the length-tension relationship between medial and lateral talo-crural musculature, there-by optimising lower limb biomechanics (17,24). Barrance et al. (2) demonstrated a heel incline of 5° causes both medial and lateral femoral condyles to move posteriorly on the tibial plateau in 20° flexion. This would improve knee flexion potentially demonstrating that reduced knee flexion angles in barefoot conditions are secondary to an anatomical block at the knee. This would result in increased hip flexion to achieve parallel squat depth but information regarding what occurs throughout a full squat range is still speculative.
The majority of squat kinematic studies have used a shoulder/narrow width stance with feet straight/ parallel, which is in keeping with original guidance (7). However there have been investigations into different stance positions revealing increased width between medium (121-153% of shoulder width) to wide stance (158-196% of shoulder width) enables the lifter to have a more vertical shank position, reduce knee flexion, increase hip flexion and reduces trunk inclination; these kinematics were consistent for loads ranging from 30% to 100% 1RM (13,34). This suggesting biomechanics can be impacted with adjustment of the participant’s stance width however in these articles (12,13) , joint ROM, foot position and footwear was not investigated or presented which would add clarity to the impacts of varying stance width on squat biomechanics.
Issues with investigations using the traditional style squat are that the majority standardise the exercise with shoulder width stance and feet parallel (3,4,5,7,8,14,16,18,25,26,27,28,31), however several studies allowed the participants to set up in their natural squat style (11,12,13,22,32,33,34) making it harder to cross analyse the methods (Table 1 presents analysis of methods used in these investigations). In addition evidence regarding the kinematic effect of tibial torsion or out-turned feet is limited to one study (1) where body weight squats with 30o foot rotation in shoulder or wide stance width significantly increased knee flexion moment magnitude, reduced thigh internal rotation in wide stance but not shoulder width. Wider stance position showed to reduce both internal rotation and adduction relative to shoulder stance. However, foot wear wasn’t specified and squat technique restricted knees over toes there by limiting its potential to fully study the effects of the foot variation on a non-restrictive squat style.
KINETIC EFFECT OF CHANGES TO ANKLE-FOOT COMPLEX
Research into changes to the ankle-foot complex have on squat performance kinetics are mostly limited to the traditional style squat with fewer studies analysing the impacts when the ankle-foot complex is altered. In an unrestricted squat the hip force arm of the gluteal maximus (GM) is at its lowest at 90o, however extensor forces paradoxically are recorded to peak with a hip angle flexed more than 90o, suggesting full squat depth is required for gluteal activation (29). Quadriceps electromyographical (EMG) activity is at its lowest at 30-44° of knee flexion, remaining in status quo 60-104° and peaks between 105-130° (6,29). Hamstring activity remains constant throughout the squat and is thought to work purely as a antagonist to the quadriceps regardless of load (29). The tricep surae muscles (i.e. gastrocnemius and soleus) are the primary plantar flexor muscles for the ankle joint. Together they exhibit moderate levels of activation throughout the squat, peaking with knees flexion and reduce with knee extension (29,9) in addition with the soleus muscle EMG having higher activity than gastrocnemius throughout the squat (19).
The effect that WS have on squat performance kinetics have not been studied. However, individuals with reduced DF have shown increase soleus activity and reduced quadriceps activity, which may result in high GM activity – this unfortunately was not measured in any study analysing these effects (16,19). Knee torque is significantly reduced in individuals with restricted DF, while hip torque has shown to be significantly higher (16). This information suggested a small raise heel can impact muscle activity in the squat.
Rehabilitation focused studies that use a heel raise of 25° (which equates to a 15 cm heel raise depending on shoe size) have found, compared to RS, the heel raise condition demonstrated higher quadriceps EMG activity (18,25,26) , higher patella tendon forces of 25-30% (18), reduced hip torque (15,18) and higher gastrocnemius activity (15,18,25,26). However the squat performance range of motion in these studies was limited to 90° of knee flexion and this evidence was directed towards patella tendinopathy rehabilitation, so relevance to WS is limited. Although these studies investigated the effects of an extreme heel height they are the only investigations to provide insight into the effect that heel height might have on squat performance.
The effect that foot position has on squat performance kinetics is limited to 4 studies (1,5,20,22) and demonstrate increased relative impulse with 30o external rotation in both narrow and wide stance relative to parallel. However, parallel feet showed significant increase in impulse relative to rotated position in the wide stance (1). Conversely, there is evidence to suggest there is no change in EMG activity between neutral or out-turned foot position during leg press exercise (20), partial range of motion (22) and parallel squat performance (1,5). The leg press machine had a 35° incline footplate (20) suggesting kinetic variables are impacted by the direction of applied force and the amount of bodyweight support. Furthermore, it is worth noting that some of this research (22) prevented knee displacement over the toes, which influences the kinematics (16), whereas other research were limited to a relatively small sample size (n = 6) (5). Finally, the technique enforced in the squat wasn’t described, bringing into question the generalizability of this research.
Relating to the impact of the shoe sole, research (30) compared barefoot and simulated barefoot shoes to traditional trainers (with a 3.5 mm raised heel rubber sole) during squatting. They demonstrated that at 80% 1RM squat, power generated, centre of mass peak velocity and excursion were highest in the trainer condition. This demonstrates that a firmer sole is an important factor with regards to foot anterior-posterior and medial-lateral excursion making the individual more stable; however, power and velocity decreased. Whether these effects were due to the softer elastic sole or the 3.5 mm heel raise position is difficult to say. The main limitations were that the squat stance was not defined and a partial squat depth was used. This affects the ability to associate results to parallel or full squat position with different loads.
To conclude, increased heel height can assist with limiting any over-pronatory mechanism by reducing length-tension relationships of the talo crural musculature and improving plantar flexor activity, although a specific height has yet to be identified (3,17,24). A heel raise of 10mm has demonstrated to help lifters with restricted DF ROM of 17° or less (4,8,19) by enabling them to maintain an upright trunk position, a more vertical shank position, increase knee flexion and knee torque, reduce knee valgus, hip flexion and hip torque in a traditional squat technique (14,16,27,28,32,34). Regarding the kinetic effects, although the studies looked at extreme ranges there are inclinations it can increase quadriceps activity (15,18,25,26) and in individuals with restricted DF can reduce soleus and increase gastrocnemius (19) activity. The properties of the incompressible sole has been shown to help limit foot excursion (30) which may make the lifter more stable, when combined with a heel raise reducing any over-pronatory mechanisms. As demonstrated, the information surrounding WS is limited, showing a need for further research in this area and in particular to kinematics, the effects of different foot stance widths, foot rotation and heel heights with WS relative to participant lower limb joint ROM and moreover analysing the impact of kinetic variables.
The WS itself offers an incompressible sole providing stability and safety by potentially limiting excursion for the lifter. A raised heel of 10mm or more can assist individuals that have restricted ankle dorsiflexion, reducing knee valgus, forefoot pronation and maintain an upright trunk position in a traditional style squat. For lifters that find it difficult to access depth a 5o heel raise can increase knee flexion by altering normal kinematics, which may assist lifters that utilise a powerlifter style or hip dominant squat that struggle to get below parallel, however this will increase knee torque. From a kinetic perspective it suggests a raised heel can increase quadriceps EMG activity in addition with potentially increasing gluteal activity due to enabling the lifter to access greater depth. It is still questionable as to whether WS improve power and velocity output and a purchase should not be based on this alone. Overall research shows the squat can be influenced by changes to the ankle-foot complex so caution should be taken when altering or changing the technique or footwear.
- Almosnino, S, Kingston, D, and Graham, RB. Three-Dimensional knee joint moments during performance of the bodyweight squat: Effects of stance width and foot rotation. J App Biomech 29:33-43, 2013.
- Barrance, PJ, Gade, V, Allen, J, and Cole, JL. American society of biomechanics clinical biomechanics award 2013: Tibiofemoral contact location changes associated with lateral heel wedging- A weightbearing MRI study. Clin Biomech 29:997-1002, 2014.
- Bell, DR, Padua, DA, and Clark, MA. Muscle strength and flexibility characteristics of people displaying excessive medial knee displacement. Arch Phys Med Rehabil 89:1323-28, 2008.
- Bell-Jenje, T, Olivier, B, Wood, W, Rogers, S, Green, A, and Mckinon, W. The association between loss of ankle dorsiflexion range of movement, and hip adduction and internal rotation during a step down test. Manual Ther 21:256-61, 2015.
- Boyden, G, Kingman, J, and Dyson, R. A comparison of quadriceps electromyographic activity with the position of the foot during parallel squat. J Strength Cond Res 14:379-82, 2000.
- Bryant, MA, Kennedy, MD, Carey, JP, and Chiu, LZF. Effect of squat depth and barbell load on relative muscular effort in squatting. J Strength Cond Res 26:2820-28, 2012.
- Chandler, TF, and Stone, MH. The squat exercise in athletic conditioning: A position statement and review of literature. J Strength Cond Res 13:51-8, 1991.
- Dill, KE, Begalle, RL, Frank, BS, Zinder, SM, and Pardua, DA. Altered knee and ankle kinematics during squatting in those with limited weight-bearing-lunge ankle-dorsiflexion range of motion. J Athl Train 49:723-32, 2014.
- Donnelly, DV, Berg, WP, and Fiske, DM. The effect of the direction of gaze on the kinematics of the squat exercise. J Strength Cond Res 20:145–50, 2006.
- Eitner, JD, Lefavi, RG, and Rieman, BL. Kinematic and kinetic analysis of the squat with and without knee wraps. J Strength Cond Res 25:S41, 2011.
- Escamilla, RF. Knee biomechanics of the dynamic squat exercise. Med Sci Sports Exerc 33:127-41, 2001.
- Escamilla, RF, Fleisig, GS, Lowry, TM, Barrentine, SW, and Andrews, JR. A three-dimensional biomechanical analysis of the squat during varying stance widths. Med Sci Sports Exerc 33:984-98, 2001.
- Escamilla, RF, Fleisig, GS, Zheng, N, Lander, JE, Barrentine, SW, Andrews, JR, Bergman, BW, and Moorman, CT. Effects of technique variations on knee biomechanics during the squat and leg press. Med Sci Sports Exerc 33:1552-66, 2001.
- Fortenbaugh, D, Sato, K, and Kyle-Hitt J. The effects of weightlifting shoes on squat kinematics. 28 International Conference on biomechanics in sport 28:1-4, 2010.
- Frohm, A, Halvorsen, K, and Thorstensson, A. Patella tendon load in different types of eccentric squats. Clin Biomech 22:704-11, 2007.
- Fry, AC, Chadwick Smith, J, and Schilling, BK. Effect of knee position on hip and knee torques during the barbell squat. J Strength Cond Res. 2003;17:629-33.
- Hirth, CJ. Clinical Movement analysis to identify muscle imbalances and guide exercise. J Hum Kinet 12:10-14, 2007.
- Kongsgaard, M, Aagaard, P, Roikjaer, S, Olsen, D, Jenson, M, Landberg, H, and Magnusson, SP. Decline eccentric squats increases patella tendon loading compared to standard eccentric squats. Clin Biomech 21:748-54, 2006.
- Macrum, E, Bell, DR, Boling, M, Lewek, M, and Padua, D. Effect of limited ankle-dorsiflexion range of motion on lower extremity kinematics and muscle-activation patterns during a squat. J Sport Rehabil 21:144-50, 2012.
- Murray, N, Cipriani, D, O’Rand, D, and Reed-Jones, R. Effects of foot position during squatting on the quadriceps femoris: An electromyographic study. Int J Exer Sci 6:114-25, 2013.
- Myer, GD, Kushner, AM, Brent, JL, Schoenfeld, BJ, Hugentobler, J, Lloyd, RS, Vermeil, A, Chu, DA, Harbin, J and McGill, S. The back squat: a proposed assessment of functional defecits and technical factors that limit performance. Strength and conditioning journal 36: 4-27, 2014.
- Ninos, JC, Irrgang, JJ, Burdett, R, and Weiss, JR. Electromyographic analysis of the squat performed in self-selected lower extremity neutral rotation and 30° of lower extremity turn-out from the self selected neutral position. J Orthop Sports Phys 25:307-15, 1997.
- Palastanga, N, Field, D, and Soames R. Anatomy and human movement structure and function. 5th ed. London:Butterworth Heinemann, 2006.
- Power, V, and Clifford, AM. The effects of rearfoot position and lower limb kinematics during bilateral squatting in asymptomatic individuals with a pronated foot type. J Hum Kinet 31:5-15, 2012.
- Purdam, CR, Cook, JL, Hopper, DM, and Khan, KM. VIS tendon study group. Discriminative ability of functional tests for adolescent jumper’s knee. Phys Ther Sport 4:3-9, 2003.
- Purdam, CR, Johnsson, P, Alfredson, H, Lorentzon, R, Cook, JL, and Khan, KM. A pilot study of the eccentric decline squat in the management of painful chronic patellar tendinopathy. Brit J Sport Med 38:395-97, 2004.
- Sato, K, Fortenbaugh, D, and Hydock, DS. Kinematic changes using weightlifting shoes on barbell back squat. J Strength Cond Res 26:28-33, 2012.
- Sato, K, Fortenbaugh, D, Hydock, DS, and Heise GD. Comparison of back squat kinematics between barefoot and shoe condition. Int J Sports Sci Coach 8:571-8, 2013.
- Schoenfeld, BJ. Squatting Kinematics and Kinetics and their application to exercise performance. J Strength Cond Res 24:3497-3506, 2010.
- Shorter, K, Lake, J, Smith, N, and Lauder M. Influence of the foot-floor interface on squatting performance. Portugal J Sport Sci 11:385-88, 2011.
- Silva, RS, Maciel, CD, and Serrao, FV. The effects of forefoot varus on hip and knee kinematics during single leg squat. Manual Ther 10:1-5, 2015.
- Sinclair, J, McCarthy, D, Bentley, I, Hurst, HT, and Atkins, S. The influence of different footwear on 3-D kinematics and muscle activation during the barbell back squat in males. Eur J Sport Sci 15:583-90, 2014.
- Swinton, PA, Lloyd, R, Keogh, JWL, Agouris, I, and Stewart, AD. A biomechanical comparison of the traditional squat, powerlifting squat, and box squat. J Strength Cond Res 26:1805-16, 2012.
- Whitting, J, Meir, R, Crowley-McHatton, Z, and Holding, R. The influence of footwear type on barbell back squat using 50,70 and 90% of 1RM: A biomechanical analysis. J Strength Cond Res 30:1085-92, 2015.
- Wretenburg, P, Feng, Y and Arboelius, UP. High and low-bar squatting techniques during weight-training. Medicine and science in sports and exercise 28: 218-224, 1996.