Mechanics for the Fitness Professional
Introduction of Sports Mechanics
So you may be wondering what sports biomechanist or movement analysts actually do? Well they study and analyse human movement patterns in sport to help individuals perform and enhance their specific sporting activity and to reduce the potential risk of injury. They also do this type of job because it is a very interesting area within sports science field. It is also intellectually challenging and can be personally gratifying. just imagine how would you feel when you can contribute to reducing an athlete’s injury risk or improving their sporting performance.
Understanding Movement Patterns
Most sports biomechanics textbooks that I have read (well tried too), have included the mathematical, engineering or physics backgrounds of their corresponding authors and their predominant research culture. I remember as a young student sitting in my biomechanics college class lost and confused due to the large volume of mathematical calculations and strange terminologies. It wasn’t until some years later within the gymnasiums teaching Olympic lifting that I started to understand this area.
Now you know why this area historically was so difficult with a strong mechanical focus, particularly in earlier texts, as well as an emphasis on quantitative analysis in sports biomechanics. Fortunately, over the last decade, the ‘real world’ of sport and exercise outside of academia has generated – from coaches, athletes and other practitioners – increasing demand for good qualitative movement analysts. Indeed, I often use the term ‘movement analyst’ instead of ‘sports biomechanist’ to reflect this shift from quantitative to qualitative analysis.
Defining Human Movement
In this segment, we discuss briefly how we can describe human movements. To specify explicitly the movements of the human body in sport, exercise and other activities, we need to use applicable sports science terminology. Terminology such as ‘bending elbows’ and ‘raising legs’ are suitable in everyday language, including when communicating with sports coaches, but ‘raising legs’ is vague and we should endeavour for correct terms. ‘Bending elbows’ is often thought to be unacceptable in sport science– a view with which I deeply disagree with as I consider that simplicity is always preferable, particularly in communications with clients in the gym or amateur level athletes. To help you understand human movement we need to start by establishing the planes in which these movements occur and the axes about which they take place, along with the body postures from which we define these movements.
Planes & Axes of Movement
There are various terms used to describe the three mutually perpendicular intersecting planes in which many, although not all, joint movements occur. The common point of intersection of these three planes is most conveniently defined as either the centre of the joint being studied or the centre of mass of the whole human body. In the latter case, the planes are known as cardinal planes – the sagittal, frontal and horizontal planes – as seen in Figure 1 and described underneath.
Various terms are used to describe the three vertical intersecting planes in which many, (although not all) joint movements occur. The common point of the intersection of these three planes is defined as either the centre of the joint being examined or the centre of mass (CoM) of the whole human body. In the CoM case, the planes are known as cardinal planes – the sagittal, frontal and horizontal planes – as depicted in Figure 1 and described below.
Figure 1. Cardinal planes& axes of movement: (left) frontal plane; (centre) sagittal plane; (right) horizontal plane.
Movements at the joints of the human musculoskeletal system are primarily rotational and take place about a line perpendicular to the plane in which they occur. This line is known as an axis of rotation. Three axes – the sagittal, frontal and vertical (longitudinal) – can be defined by the intersection of pairs of the planes of movement, as seen in Figure 1. The main movements about these three axes for a particular joint are flexion and extension about the frontal axis, abduction and adduction about the sagittal axis, and medial and lateral (internal and external) rotation about the vertical (longitudinal) axes.
The sagittal plane is a vertical plane passing from the rear (posterior) to the front (anterior), dividing the body into left and right halves, as in Figure 1 (centre). Most sport and exercise movements that are (almost) two-dimensional (i.e. running and long jumping) take place in this plane. The frontal axis passes horizontally from left to right and is formed by the intersection of the frontal and horizontal planes.
The frontal plane Figure 1 (left) is also vertical and passes from left to right, dividing the body into posterior and anterior segments, as in . It is also often known as the coronal or the medio-lateral plane.
The horizontal plane Figure 1 (right) separates the body into top (superior) and bottom (inferior) segments. It is also known as the transverse plane. The sagittal axis passes horizontally from posterior to anterior and is formed by the intersection of the sagittal and horizontal planes. The vertical or longitudinal axis passes vertically from inferior to superior and is formed by the intersection of the sagittal and frontal planes.
The movements of body segments are typically defined from the anatomical (Figure 2) reference postures or positions. Note that the anatomical position has the palms facing forwards.
Figure 2. Anatomical reference position.
This introduction section has focused on movements in the sagittal plane about the frontal axis of rotation (Figure 1). No what we have to do is imagine viewing a person side-on (stand next to a mirror side ways like Figure 3) he bends his elbow and then straightens it (like performing a bicep curl). These movements are called flexion and extension, respectively, and they take place in the sagittal plane around the frontal axis of rotation. Flexion is mainly a bending movement, with the body segment (i.e the elbow, the forearm moving forwards.) Another example is when the knee flexes, the calf moves backwards (stand on one leg and try it). The movements at the ankle joint are called plantar flexion when the foot moves downwards towards the rear of the calf, and dorsiflexion when the foot moves upwards towards the front of the calf. The movement of the whole arm about the shoulder joint from the anatomical reference position is called flexion, and its return to that position is called extension; the continuation of extension beyond the anatomical reference position is called hyperextension.
Figure 3. Movement of the foreman about the elbow joint in the sagittal plane.
Main Movements in Other Planes
The same scientific terminology is used to define movements in the sagittal plane for the thigh about the hip joint. These arm and thigh movements are usually defined with respect to the trunk. Sports scientists typically use the convention that the fully extended position of most joints is 180° (when most joints flex the angle decreases). Clinical biomechanist tend to use an alternative convention in which a fully extended joint is 0°, so that flexion increases the joint angle. We will use the former convention throughout because the examples of movement patterns that we will study in this section are mainly in the sagittal plane, we will leave formal consideration of movements in the other two planes until later.
Movements in the frontal plane about a sagittal axis are usually called abduction (away from the body) and adduction (back towards the body), as seen in Figure 4a. For some joints, such as the elbow and knee, these movements are not possible, or are very restricted. Movements in the horizontal plane about a vertical axis are called medial (or internal) and lateral (or external) rotation of the limbs, as in Figure 4b, and rotation to the right or to the left for the trunk.
Figure 4. (left) Abduction and adduction of the arm about the shoulder joint and the thigh about the hip joint; (right) Medical(internal) and lateral (external) rotation of the arm about the shoulder joint.
Video & Flash Cards on Planes of Motion
When analysing any human movement, ask yourself, ‘What are the “constraints” on this movement?’ The constraints can be related to the sports task, the environment or the organism. This logical method aids in developing your analytical skills and also establishing a greater appreciation of why we observe specific movement patterns.
In the video and practical examples and the sequences (Figure 5) and videos below, an environmental constraint might be walking ‘over ground’ or on the ‘treadmill’. Jumping vertically to achieve maximum height in basketball is evidently a task constraint. Organismic constraints are, essentially, biomechanical; they relate to a certain body characteristics, which affect their movement responses to the task and environmental constraints. These biomechanical restraints will be affected, by genetic make-up, age, biological sex, fitness, injury record and stage of rehabilitation, and pathological conditions. Predictably, the movement patterns observed when one person executes a specific sports task will seldom be equal to those of another person; indeed, the movement patterns from repetitions of that task by the same individual will also vary.
These inconstant movement patterns (often known as movement variability), can affect the method that movement analysts examine sports movements. The qualitative descriptions in the following sections will not, therefore, apply to every individual. The fundamental developmental patterns of maturing children up to a certain age display significant differences from those of a developed adult.
The first stage in the examination of a complex motor skill is often to establish the different phases into which the movement can be divided for analysis. For example, the division of a throwing movement into separate, but linked, phases are beneficial because of the complexity of various throwing practices. The phases of the movement should be identified so that they have a biomechanically distinct role in the overall movement, which is different from that of the initial and subsequent phases. Each phase then has a defined biomechanical function with identified phase boundaries, often termed key events. Although phase analysis can help the understanding of movement patterns, the essential feature of all sports movements is their completeness; this should always be considered when undertaking any phase analysis of a movement pattern.
Figure 5. Common temporal divisions of the gait cycle. [Figure from; Bonnefoy, A. & Armand, S., 2015. Normal gait. Orthopedic Management of Children With Cerebral Palsy: A Comprehensive Approach, p.200.
Understanding the Walking Gait Cycle
Walking is a cyclic activity in which one stride follows another in a continuous pattern. We define a walking stride as being from touchdown of one foot to the next touchdown of the same foot, or from toe-off to toe-off. In walking, there is a single-support phase, when one foot is on the ground, and a double-support phase, when both are. The single-support phase starts with toe-off of one foot and the double-support phase starts with touchdown of the same foot. The duration of the single-support phase is about four times that of the double-support phase. Alternatively, we can consider each leg separately. Each leg then has a stance and support phase, with similar functions to those in running. In normal walking at a person’s preferred speed, the stance phase for one leg occupies about 60% of the whole cycle and the swing phase around 40% (see, for example, Figure 1.7). In normal walking, the average durations of stance and swing will be very similar for the left and right sides. In pathological gait, there may be a pronounced difference between the two sides, leading to arrhythmic gait patterns.
We can also observe each leg independently. Each leg has a stance and support phase, with similar functions to those in running. In a normal walking cycle at an individuals desired speed, the stance phase for one leg is approximately 60% of the complete cycle and the swing phase approximately 40% (watch the video). In normal walking, the average durations of stance and swing will be comparable for the left and right sides. In extreme gait, there may be an evident difference between the two sides, leading to arrhythmic gait patterns. Observing the videos in this section, you should in general be able to follow the patterns of flexion and extension of the hip, knee and ankle. Further, you should also be able to differentiate from the video sequences and become familiar with identifying these movements on video.
Please observe from the videos that the hip flexes during the swing phase and then begins to extend just before touchdown; extension continues until the heel rises just before toe-off. The hip then starts to flex for the next swing phase, roughly when the other foot touches down. The knee is normally slightly flexed at touchdown and this flexion continues, although not necessarily in slow walking. Some, but not much, extension follows before the knee starts to flex sharply immediately after the heel rises; this flexion continues through toe-off until about halfway through the swing, when the knee extends again, before flexing slightly just before touchdown. The ankle is approximately in a neutral position at landing. The ankle then plantar flexes until the whole foot is on the ground, when dorsiflexion starts; this continues until the other leg touches down. Plantar flexion then follows almost to toe-off, just before which the ankle dorsi-flexes quickly to allow the foot to clear the ground as it swings forwards.
Running Gait & Gait Analysis
Running, similar to walking, is a cyclic action with one running stride following another in a continuous sequence. Running stride is often defined as being from the landing of one foot to the next landing of the same foot, or from toe-off to toe-off.
Running unlike walking can fundamentally be separated into a support phase, when one foot is on the ground, and a recovery phase, in which both feet are off the ground. The runner can only apply force to the ground for propulsion during the support phase, which defines that phase’s central function and provides the key events that signify the start of the phase, foot strike, and its end, toe-off. The support phase starts at toe-off and ends at landing; at this stage, it is considered that its function is to prepare the leg for the next landing. In jogging, the recovery phase will be limited and it will then increase with running speed.
The various videos demonstrates the gait cycle and the gait analysis on the treadmill. Observing and listen to the video, you should note, the following patterns of flexion and extension of the hip, knee and ankle. The hip continues to extend early in the swing phase, roughly until maximum knee flexion, after which it flexes then begins to extend just before landing; extension continues until toe-off. The knee is normally slightly flexed at landing and this flexion continues, depending on running speed, to absorb shock, until the hip is roughly over the ankle. Knee extension then proceeds until toe-off, soon after which the knee flexes as the hip continues to extend. The knee starts to extend while the hip is flexing and continues to extend almost until touchdown, just before which the knee might flex slightly. The ankle movements vary depending on whether the runner lands on the forefoot or rear foot.
The ankle is roughly in a neutral position at landing, as in the reference positions of For a rear foot runner, in particular, the ankle then plantar flexes slightly until the whole foot is on the ground; dorsiflexion then occurs until mid-stance. The ankle plantar flexes from mid-stance until toe-off, as the whole support leg lengthens. The ankle then dorsiflexes to a neutral position in the swing phase and plantar flexes slightly just before touchdown. As you should note from the video clips and practical activities within labs, this sequence of movements varies somewhat from person to person with the shoes worn, with running speed, and between over ground and treadmill running.
A Biomechanical Analysis of the Squat: A Brief Overview
The squat is a commonly performed exercise that is frequently used to develop lower body strength and muscular hypertrophy (Schoenfeld, 2010). The movements of a squat mimic the functional characteristics of fundamental lower body physical tasks that are performed on a daily basis. These movements require neurological activation and motor unit recruitment in a coordinated and synchronised manner in which to accomplish particular lower body movements. The squats have been regarded by some in the fitness and rehabilitation field as an important exercise to help promote and enhance the standard of active daily living. Fry et al. (2003) states that squat exercises are capable of activating multiple muscle groups in a single movement. The squatting action and motion can therefore be simulated in efforts including the lifting of objects from the floor (Schoenfeld, 2010).
The squat has also been used as a key exercise in resistance training programmes designed for athletes to enhance lower body power and strength. Escamilla et al., (2001a) states that the squat shares comparable activation of nerves, muscles, and movements which are influenced by particular mechanical principles. The (back) squat therefore is an important element in sports including powerlifting and Olympic weightlifting, and is considered by some as the ultimate exercise for strength development of the lower extremities (Escamilla, 2001).
The purpose of this brief review is to critically evaluate existing evidence on the performance of the squat towards the develop of optimal squat performance in the recreationally trained. whilst considering biomechanical principles that can. In addition, considerations of basic biomechanical principles that explore angular kinetics and kinematic of the spine, hip, knee and ankle with regards to the performance of the squat will be discussed. This review is a useful read for anyone interested in sport and exercise science and biomechanical principles.
Joint Kinematic and Kinetics during the Squat
The Spine Complex
The spinal column is made of 24 vertebral segments, with each segment having approximately three degrees of freedom. The movement actions permitted individually or collectively include lateral flexion in the frontal plane, with rotation in the transverse plane, and extension and flexion produced in the sagittal plane (Signorile, Kwiatkowski, Caruso, and Robertson (1995), as cited in Schoenfeld, 2010). The vertebral segments are thinner at the top of the spinal column and progressively increase in width toward the bottom of the column at the lumbar region in which loading of forces can withstand compression forces. Between each segment, an array of intervertebral discs helps dissipate compression forces when performing the squat exercise. These specialised joints are thick pads that sandwich between two corresponding vertebral bodies.
A study by Figura and Gazzani (1985) evaluated the effects of compression based loading on the key section of the spine (Lumbar three-four) when performing the half squat exercises with a barbell placed on subject’s shoulders. The results of the half squat exercise demonstrated that 0.8 to 1.6 times (88-160% body weight) were placed on L3-L4 segments. Adams and Dolan (1995) (as cited in Schoenfeld, 2010) estimates a maximal strength of 7,800N in athletes 40 years or younger, suggesting that they are more susceptible to spinal injuries as they surpass their absolute. These findings underpin the need for correct biomechanical alignment in terms of preventing additional stress to the vertebrae that would not allow for dissipation of external forces that have been placed on the spine. However, only limited evidence is available that details the full impact of squats. The study by Figura and Gazzani (1985) had a small population group that could have magnified the findings and the movement performed was only on a half squat rather than a full squat.
Conversely, studies have shown that compressive forces when held in unnecessary lumbar extension. The work by Adams and Dolan (1995) (as cited in Schoenfeld, 2010) stated that by just a two-degree increase in extension from the neutral position can as a result increase compressive forces by as much as 16%. It has been recommended therefore that individuals should seek to maintain a neutral spine throughout the full movement of when performing the squat exercise. This is to limit any additional spinal flexion or extension. Also, Hay, Andrews, Vaughan, and Ueya (1983) (as cited in Schoenfeld, 2010) noted that athletes lifting 40% to 80% using 4RM demonstrated the body tilting forwards. This evidence indicates the significance of having the correct spinal adjustment when performing heavy external loads to avoid strain on the lower spine (Schoenfeld, 2010). If the external force is exerted quickly, peak compressive force doubles at the spine (Vakos, Nitz, Threlkeld, Shapiro, & Horn, 1994, as cited in Schoenfeld, 2010).
The Hip Complex
The hip complex is a ball and socket joint, which provides motions in all three planes of motion- extension and flexion in the sagittal plane, external and internal, and horizontal adduction and abduction in the transverse plane, and adduction and abduction in the frontal plane (Signorile, Kwiatkowski, Caruso, & Robertson, 1995). Hemmerich, Brown, Smith, Marthandam, and Wyss (2006) described the range of movement at the hip to have flexion of 95 +/- 27 degrees, suggesting that athletes would need to make improvements to their flexibility before considering deeper squats.
Nagura, Dyrby, Alexander, and Andriacchi (2002) states that a positive correlation between hip flexion and hip torque is demonstrated at the lowest point of the squat, where they will both increase. Fry, Smith, and Schilling (2003) conducted a study investigating joint kinetics during unrestricted and restricted movements. The study used seven males with weight training experience to perform 2 different types of barbell squats. The unrestricted condition allowed the knees to anteriorly surpass the toes, and the restricted did not. There was a significant difference between the unrestricted and restricted conditions for hip torque (p < 0.05, unrestricted = 302.7 +/- 71.2 Nm, restricted = 150.1 +/- 50.8 Nm). The unrestricted movement allowed the upper body and shank to move forward more, and provide a superior internal angle and the knees. This resulted in the production of a greater moment around the hips.
Caterisano et al. (2002) conducted a randomised controlled study that examined three different squat depths on trained athletes and measured how the four thigh and hip muscles influenced performance. The ten subjects performed full, parallel and partial depth squats, using an external force of 100-125% of subject’s body weight. Measurements were taken using electromyographic surface electrodes to record the muscle activity of the gluteus maximus, biceps femoris, vastus lateralis, and vastus medialis. Caterisano and colleagues (2002) reported that vastus medialis, vastus lateralis, bicep femoris did not significantly contribute during the concentric phase (full depth- 35.4%, parallel- 28%, and partial- 16.9%; p <0.001) when performing different squatting depths. There were however greater activation gluteus maximus muscle groups when the depth of squat was greater.
There were no statistical (P = .05) differences between full, front, and parallel squats in any of the tested muscles. However, given the results of previous research, it is recommended that individuals use a full range of motion when squatting, assuming full range can be safely achieved, to promote more favourable training adaptations. Furthermore, despite requiring lighter loads, the front squat may provide a similar training stimulus to the back squat.
Escamilla et al. (2001b) established that performing a narrower stance that the gastrocnemius’ activation was greater by 21%. Also, McCraw and Melrose (1999) found that adductor longus and gluteus maximus had greater activation when performing a wider stance, where optimal activation was when the feet were 140% of the shoulder width. It had also been stated that squats performed using a wider stance could generate more muscular torque from the adductors and hip extensors (Ninos, Irrgang, Burdett & Weiss, 1997; Paoli, Marcolin & Petrone, 2009).
Figure Above Hip Flexor Complex
The Knee Complex
The knee complex includes the tibiofemoral joint which has a range of motion of roughly zero to 160 degrees, moving in the sagittal plane (Signorile et al., 1995; van Eijden et al., 1987, as cited in Schoenfeld, 2010). This joint is referred to as a hinge joint, which is positioned between the femur and tibia (Schoenfeld, 2010). As the body moves, axial rotation is present when the femur rotates outward from the tibia during flexion, and in towards the tibia during extension (Schoenfeld, 2010). This effects the knees centre of rotation to change a little during the squat. (Schoenfeld, 2010). The patellofemoral is another joint which makes up the knee complex and is referred to as a plantar joint which slides over the surface of the femur as the knee flexes and extends. (Schoenfeld, 2010).
Escamilla et al. (2001a) investigated the impact variations in squat and leg press techniques has on knee forces and muscle activity. The study had ten trained male trainees perform a high foot placement, low foot placement, wide stance, narrow stance and two-foot placements (feet straight versus feet turned out at a 30-degree angle. Escamilla and colleagues (2001a) reported that no difference was found in muscle activity or on knee forces between the two-foot placements. However, there was greater muscle activity when performing the squat with the quadriceps and hamstring than other techniques. In relation to compressive and tensile forces to the knee complex, the squat exercise generated greater tibiofemoral, posterior cruciate ligament and patellofemoral compressive forces than other squat variations. The performance of squat based exercises may aid in enhanced muscular strength due to greater muscle activity and knee forces compared to different squat variations. However, the different population groups (untrained versus trained) should evaluate individuals with patellofemoral or posterior cruciate disorders due to greater knee flexion angles. That said, the lack of anterior cruciate ligament forces applied during the squat may be an effective exercise for anterior cruciate ligament rehabilitation.
Sahli et al. (2008) established that peak compression was present and that shear stress escalates as the size of the load increases. The peak compressive force equal to 58% of the bodyweight (BW) with no external load applied, but was 149% of the BW with a load that was equivalent to 120% was applied. Correspondingly, peak anteroposterior and mediolateral shear force had gone from 8% BW to 11% BW, and from 46% to 67% BW as the external load had increased from 50% to 100%. Similarly, Markolf, Slauterbeck, Armstrong, Shapiro, and Finerman (1996) (as cited in Shoenfeld, 2010) discovered that regardless of the flexion angle at the knee, which the ACL force increased because of tendon load.
Athletes who perform lift of double their BW had shown tensile forces of the ACL to be around 50% and the 25% from the ACL when using their maximal strength capacity (Escamilla et al, 2001a; Walsh, Quinlan, Stapleton, Fitzpatrick, & McCormack, 2007). Escamilla et al. (2001b) recorded that the tibiofemoral force increased by 16% and 15% for the patellofemoral when athletes performed a wider stance when squatting in comparison to those with a smaller stance. The descent of the squat demonstrated great compression forces in comparison to the ascension at greater knee angles, whereas greater compression forces were present at lower flexion angles of the knee when ascending compared to descending. Arguably, using a smaller stance can produce greater shear force when the knees are positioned 4 to 6 centimetres further forward, so using a wider stance may be advisable when ensuring minimal stress at the knees (Escamilla et al., 2001b).
The Ankle Complex
During the performance of the squat based exercise the ankle complex aids in the production of movement using both plantar and dorsiflexion. This complex is made up of a series of joints namely the talocrural and subtalar joints. The movement normally permitted at the ankle complex are inversion, eversion, plantar, dorsiflexion, including limited adduction and abduction (Signorile et al., 1995). Hung and Gross (1999) stated that the ankle complex is an important factor regarding stability, and assists in generating power for squat performance. Unfortunately, a limited body of scientific evidence is available that focuses on ankle kinetics during the squat with more research being conducted on other body segments (knee, spine, hip).
The evidence collated from previous research which investigates the kinematics and kinetics of squats indicate that the types of squats performed and the type of external load applied determines how much musculature activation, range of motion, the torque produced, and acting stress. The type of squat determines how narrow or wide a stance is used, and helps to increase muscle activation of the gluteus. Supporting evidence proposes that lumber alignment be achieved to minimise stress. Gluteus maximus muscle group activation was greatest at greater squat depths, with the vastus medialis, vastus lateralis, bicep femoris did not significantly contribute during the concentric phase. The lack of evidence on the ankle complex suggests that future research be conducted, to identify how it can influence the squat performance.
Caterisano, A., Moss, R. E., Pellinger, T. K., Woodruff, K., Lewis, V. C., Booth, W., & Khadra, T. (2002). The effect of back squat depth on the EMG activity of 4 superficial hip and thigh muscles. The Journal of Strength & Conditioning Research, 16, 428-432.
Escamilla, R. F. (2001). Knee biomechanics of the dynamic squat exercise. Medicine & Science in Sports & Exercise, 33, 127-141.
Escamilla, R. F., Fleisig, G. S., Lowry, T. M., Barrentine, S. W., & Andrews, J. R. (2001b). A three-dimensional biomechanical analysis of the squat during varying stance widths. Medicine & Science in Sports & Exercise, 33, 984-998.
Escamilla, R. F., Fleisig, G. S., Zheng, N., Lander, J. E., Barrentine, S. W., Andrews, J. R., & Moorman III, C. T. (2001a). Effects of technique variations on knee biomechanics during the squat and leg press. Medicine & Science in Sports & Exercise, 33, 1552-1566.
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McCraw, S. T., & Melrose, D. R. (1999). Stance width and bar load effects on leg muscle activity during the parallel squat. Medicine & Science in Sports & Exercise, 31, 428-436.
Nagura, T., Dyrby, C. O., Alexander, E. J., & Andriacchi, T. P. (2002). Mechanical loads at the knee joint during deep flexion. Journal of Orthopaedic Research, 20, 881-886.
Ninos, J. C., Irrgang, J. J., Burdett, R., & Weiss, J. R. (1997). 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. Journal of Orthopaedic & Sports Physical Therapy, 25, 307-315.
Paoli, A., Marcolin, G., & Petrone, N. (2009). The effect of stance width on the electromyographical activity of eight superficial thigh muscles during back squat with different bar loads. The Journal of Strength & Conditioning Research, 23, 246-250.
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Signorile, J. F., Kwiatkowski, K., Caruso, J. F., & Robertson, B. (1995). Effect of foot position on the electromyographical activity of the superficial quadriceps muscles during the parallel squat and knee extension. The Journal of Strength & Conditioning Research, 9, 182-187.
Walsh, J. C., Quinlan, J. F., Stapleton, R., FitzPatrick, D. P., & McCormack, D. (2007). Three-dimensional motion analysis of the lumbar spine during “free squat” weight lift training. The American Journal of Sports Medicine, 35, 927-932.