Squatting and the Implications of Technique on Muscle Function

Training Articles
1

[b]Squatting and the Implications of Technique on Muscle Function
©Copywrite 2003 David Woodhouse

Introduction[/b]
The squat has been described as the ‘King of Exercises’ since it activates the largest, most powerful muscles in the body and is the greatest test of lower body strength (4, 6, 12). The major muscles that are activated are the ankle, knee and hip extensors, the spinal erectors and the abdominals. As a result the squat is one of the most popular exercises for development of lower body strength and power. It constitutes one of the three competitive lifts in the sport of Powerlifting and the front squat variation is also a component of the ‘Clean’ lift in weightlifting.

The ‘Sticking Point’ Phenomena

When load is near maximal, squat technique may be adjusted to permit a successful completion of the lift. This results in an asymmetric lift when descending and ascending components are compared (6). The point of minimal bar velocity during the ascent is often described as the ‘sticking point’.

The sticking point is thought to result from the force-length properties of muscles and the torque produced by the load (6). The quadriceps’ ability to produce tension decreases as they extend and hence so does their net extensor moment. At the sticking point, they are no longer able to produce sufficient force to continue extending the knees (6). Hip flexion at this point occurs to shorten the load’s moment arm at the knee joints and enables the quadriceps to extend them (17) (and also lengthen the hamstrings). The vasti muscles of the quadriceps group all show similar peak EMG activity during ascent and descent (12). The rectus femoris is the only bi-articular muscle in the quadriceps group, it creates a hip flexor moment and shows ~30% greater activation during the ascent versus the descent, but still significantly less than the vasti (12). The vasti each have specific length tension relationships, the vastus medialis is most active during the latter stages of extension hence it may be a weakness in the vastus lateralis that most contributes to the sticking point (15).

The load’s moment arm at the hip increases with flexion at the sticking point but lengthening of the gluteal and hamstring muscles is advantageous for producing force since it improves their length tension relationships and hence increases net torque (6). The hamstrings are a bi-articular muscle, crossing both knee and hip joints and during the ascent they shorten at the hip and lengthen at the knees. However, the shortening through hip extension is disproportionate to the lengthening through knee extension, and therefore their ability to produce tension is improved by hip flexion (6).

Hip flexion increases the moment arm of the load and therefore requires an increase in the isometric tension produced in the spinal erectors. At the beginning of the ascent some lifters (particularly in the front squat) hyperextend the spine to shorten the hip’s moment arm and also to help keep the load-body centre of gravity over the feet (4). Hyperextension of the lumbar spine places greater stress on the facet joints and may increase the risk of chronic lower back pain (8). Repeated training in this hyperextended position may cause an exaggeration of the lumbar curve - lordosis (8). At the sticking point however the spine generally flattens and in some cases may partially flex. This allows the knees to extend fractionally without any increase in bar height (4).

As the spine flexes, the spinal erector’s length tension relationship moves closer to optimum but the net extensor torque decreases because there is a significant decrease in the angle the logissimus thoracis and iliocostalis lumborum muscle fibres make with the spine. This compromises their ability to support shear forces and hence, at full flexion, those forces are transferred to passive tissues, (i.e. ligaments and spinal disks) significantly increasing the risk of injury (13). Powerlifters may allow their spine to flex to within two or three degrees of full flexion hence preventing injury yet maximising the ability to negotiate the ‘sticking region’ (14). Squatting typically stresses the spinal erectors isometrically in an extended position. Due to the specificity of this mode of training there is little cross over to the longer muscle fibre lengths involved in spinal flexion (4). Therefore, if a lifter has not been conditioned to partially flex the spine, the muscles may not be able to maintain sufficient tension and it may ‘buckle’ (14).

When squatting lifters employ the Vasalva manoeuvre, that is a voluntary increase in pressurisation of the abdominal cavity achieved by closing the epiglottis and activating trunk and abdominal muscles (11). The effect of increasing intra abdominal pressure is increased stability of the spine though the mechanism is not fully understood (10). An early theory was that a hydrostatic force within the abdominal cavity induced an extensor moment by pushing down on the pelvic floor and up on the diaphragm (10). However contraction of rectus abdominis, and the internal/external oblique muscles causes a flexor moment that offsets the extensor moment caused by intra-abdominal pressure (3). It is now believed that increased co-activation of spinal flexor and extensor muscles increases spinal stiffness and hence spinal stability (3). This means that increased intra-abdominal pressure is simply a useful by-product that negates the flexor moment caused by the abdominal and oblique muscles as discussed above (3).

The changes in technique at the sticking point highlight the influence of load on technique. This is important when designing protocols to examine the kinematics of the lift. Also the sticking point phenomena has implications for training to improve squatting strength, for example lifters might train isometrically at the sticking point to improve strength specifically in that posture (4).

Width of Stance

A wide stance is typical of Powerlifters whilst weightlifters typically use a narrower stance (although not narrow as defined by the research) Typically in the research a narrow stance was defined to be ~75% of shoulder width whilst wide stance was defined to be ~140% shoulder width. Commonly lifters show greater lateral rotation of their feet as stance width increases but this variable has not been shown to influence any muscle activities (6).

Research involving EMG has shown no significant difference in quadriceps activation between narrow and wide stances (12). Adductor activation has been shown to increase with a wide stance (12). This is because the thigh shows increased abduction and lateral rotation during the descent with a wide stance and, during the ascent, the adductors are therefore activated to draw the thigh back to the midline of the body and also medially rotate it back to a neutral position (12).

Gluteus maximus and hamstring activation also increases with a wide stance (6). It has been suggested, for the former, that this is due to the positioning of its’ distal attachment, which causes the gluteus to lengthen with thigh abduction (12). This lengthening shifts it away from its’ optimum position on the length-tension curve and hence greater activation is required to create the same tension as the narrower stance (12). Greater activation of hip extensor muscles may also occur because width stance effects torso inclination (see later).

Narrow stance causes greater forward knee movement and hence greater plantar flexion at the ankle as the shank inclined (6). This caused an increase in activity of the gastrocnemius during the ascent phase and since the gastrocnemius is a bi-articular muscle crossing both ankle and knee joints there is an increased knee flexion moment (6). Intuitively greater quadriceps activity is expected to counteract this but as discussed above this is not the case. This implies that either this moment is negligible or that the kinematics are altered. The increased forward knee movement also increases knee shear force due to more acute angle formed by thigh and shank (6).

These findings, have implications for training for example, for greater development of the adductors and to minimise shear forces, a wide stance may be preferable. In contrast, a narrower stance may be more beneficial to increase activation of the gastrocnemius. Powerlifters have found that a wider stance is one factor that permits them to lift greater loads, however they need only squat to a position where the thighs break parallel and a wide stance may be less efficient for deeper variations of the lift.

Bar Position

There are three major methods of supporting the barbell when performing a squat lift. ‘Low bar’ where the bar lies across the spine of the scapula. ‘High bar’, the traditional technique, where the bar rests on top of the posterior deltoids and trapezius muscles, and ‘Front bar’ where the barbell rests on the anterior deltoids and clavicals. The load is posterior to the body’s line of gravity with low and high bar but anterior with front bar. Low bar is the technique utilised by powerlifters, whilst front bar is used directly in weightlifting during the clean. High bar is the most common technique in strength training for sport since it has the lowest flexibility and skill requirement.

Whilst there are correlations between bar position and the kinematics of the lift these are subject to individual differences in technique (16). The major difference is the degree of trunk inclination. Increased trunk inclination, increases the moment arm at the hip but decreases it at the knee and (as discussed previously) the greater the moment arm, the greater the muscle activation required to extend the joint. Bar position also directly influences the moment arm since, at any hip angle, the distance of the bar to the hip increases from low bar to high bar and again from high bar to front bar (16).

Typically the front bar causes the most erect trunk posture since over inclination would cause the bar to fall forward off the chest (4, 17). Since the spine is more resistant to compressive forces (directed axially) than to shear forces, an erect trunk posture reduces risk of lower back injury (16). In the high bar the load is typically positioned centrally between knee and hip joints (17). Low bar allows the greatest hip flexion and hence the shortest moment arm at the knee. This latter bar position is therefore the most mechanically efficient and hence permits the greatest loads (17).

At the present time there is no research to show whether bar position affects other factors such as intra-abdominal pressure. There may also be some interaction between other variables such as stance and depth.

Cadence

Increased lifting speed causes higher maximum, and greater variation in, shear and compressive force in the knee and spine (9). This is because a faster decent requires greater deceleration forces from the knee and hip extensors in order to slow and stop the weight at the bottom of the descent (6). Slower cadences during the descent may therefore be preferred since they decrease risk of injury and also increase the time under tension hence increasing the training effect (4). Cadence has not however been shown to significantly effect intra-abdominal pressure (7).

Other factors

There are many other factors, outside the scope of this paper, that influence squat kinematics. For future work however these include the depth of squat, segmental length, fatigue and equipment such as shoes, lifting belts and suits.

References

  1. Ariel, B. G. Biomechanical analysis of the knee joint during deep knee bends with heavy loads. In: Biomechanics IV, R. Nelson and C. Morehouse (Eds.). Baltimore: University Park Press, 1974, pp. 44-52

  2. Bird, M. and J. Hudson. Mearsurement of elastic like behaviour in the power squat. Journal of Science and Medicine in Sport. 1(2):89-99, 1998

  3. Cholewicki, J., J. Kishna and and S. M. McGill. Intra Abdominal pressure mechanism for stabilizing the lumbar spine. Journal of Biomechanics. 32: 13-17, 1999

  4. Dreshler, A. In The Weightlifting Encyclopedia, S. Heath (Ed.). A is A Communications, 1998

  5. Escamilla, R. F., N. Zheng, G. S. Fleisig, et al. The effects of technique variations on knee biomechanics during the squat and leg press. Med. Sci. Sports Exerc. 29:S156, 1997

  6. Escamilla, R. F., G. S. Fleisig, T. M. Lowry, S. W. Barrentine and J. R. Andrews. A three-dimensional biomechanical analysis of the squat during vaying stance widths. Med. Sci. Sports Exerc. 33: 984-998, 2000

  7. Hall, S. J. Effect of attempted lifting speed on forces and torque exerted in the lumbar spine. Med. Sci. Sports Exerc. 17:440-444, 1985

  8. Hall, S. J. In Basic Biomechanics IV, V. Malinee (Ed.). McGrraw-Hill, 2002 pp. 275–305

  9. Hattin, H. C., M. R. Pierrynowski, and K. A. Ball. Effect of load, cadence and fatigue on tibio-femoral joint force during a half squat. Med. Sci. Sports Exerc. 21:613-618, 1989

  10. Hodges, P. W., A. G. Cresswell., K. Daggfeldt. and A. Thostensson. In vivo measurement of the effect of intra-abdominal pressure on the human spine. Journal of Biomechanics 34: 347-353, 2001

  11. Enoka, R. M. In Neuromechanical Basis of Kinesiology II, L. Galasyn-Wright (Ed.). Human Kinetics, 1994, pp. 56-59

  12. McCaw, S. T., and D. R. Melrose. Stance width and bar load effects on leg muscle activity during the parallel squat. Med. Sci. Sports Eerc. 31(3):428-436, 1999

  13. McGill, S. M, R. L. Hughson, and K. Parks. Changes in lumbar lordosis modify the role of the extensor muscles, Clin. Biomech. 15:777, 2000

  14. McGill, S. M. Low. Back stability: From formal descriptions to issues for performance and rehabilitation. Exerc. And Sport Sci. 29:26, 2001

  15. Palastanga, N., D. Field, and R. Soames. In Anatomy and Human Movement

  16. Russell, P. J., and S. J. Phillips. A preliminary comparison of front and back squat exercises. Res. Q. 60:201-208, 1989

  17. Wretenberg, P., Y. Feng, and U. P. Arborelius. High and low bar squatting techniques during weight training. Med. Sci. Sports Exerc. 28:218-224, 1996

  18. Wright, G. a., T. H. Delong, and G. Gehlsen. Electromyographic activity of the hamstrings during performance of the leg curls, stiff-leg deadlifts, and back squat movements. J. Strength Cond. Res. 13:168-174, 1999