Myoquip Myotruk Resistance Training Machine

by Tom Carter

(Article adapted from an assignment submitted in the course, Applied Biomechanics in Strength and Conditioning, in the Masters of Exercise Science (Strength and Conditioning) program at Edith Cowan University)

Abstract

The MyoTruk accommodating resistance machine promotes strength and power training development for the muscle groups responsible for triple extension. The MyoTruk, one of MyoQuip's “Myo-” range of strength equipment, embodies direct-linkage force transmission and replaces the ScrumTruk. Since its introduction in 2004 the ScrumTruk has been routinely used for enhancing the basic strength, muscle mass and explosiveness of rugby union players of all levels and ages. This paper examines the MyoTruk and its capacity to facilitate strength, power and speed development, whilst concurrently comparing it to traditional weight training exercises such as variations of the squat and Olympic type lifts. It examines the MyoTruk’s capacity to enhance the physiological capacities of athletes within the game of rugby union.

Kinematic features and performance characteristics of the MyoTruk

MyoQuip’s mission is to develop fundamentally innovative resistance training equipment with a particular focus on lower body power and core stability development. Each individual machine embodies Broad Biomechanical Correspondence (BBC) technology with a specific focus upon:

1. Strength development exercises that promote the kinematics of triple extension

2. The activation and strengthening of specific muscle groups throughout the entire range of motion

3. The development of speed and power capacities specific to particular sports.

Figure 1: The MyoQuip MyoTruk triple extension strength/power training machine

The two most critically distinguishing features of the MyoTruk are the horizontal pushing position of the athlete and the use of MyoQuip's BBC technology, ensuring constantly increasing resistance throughout the range of the exercise movement.

The MyoTruk is a very effective strength and power training alternative and/or complement to the barbell squat in building strength in the gluteal, hamstring and quadriceps muscle groups. In addition to its unique consistent resistance training throughout range the MyoTruk also reduces the compressional forces on lower back that can be attained at times by traditional squatting exercises.(4) Figure 2 below illustrates the biomechanical starting position of the MyoTruk. The hip and knee joint starting angles are able to be adjusted to below 90° if a greater range of movement is desired, but as discussed the integral component of the machine is the ability of the back and spine to maintain normal curvature. (24)

Figure 2: Kinematic and biomechanical features of the MyoTruk - Tom Carter demonstrating starting position for the MyoQuip MyoTruk - note back and shins parallel to ground - hip and knee joint angles at 90º (25)

Figure 3 below illustrates the functional benefits of the MyoTruk as the extension position facilitates the demands of many sports, in particular football codes where extensor strength plays an integral role in the development of speed, power and force characteristics. The triple extension position is able to be attained without the same risks associated with conventional resistance exercise. This perspective is elaborated upon further on within the article.

Figure 3: Tom Carter demonstrating full extension on the MyoQuip MyoTruk - hip and knee joint angles change at same rate. No adverse consequences from attempting to use excessive weight - athlete cannot be trapped under heavy load unlike barbell squat or 45° leg press (25)

The horizontal trunk position stimulates co-contraction of the stabilising muscles of the pelvic and abdominal regions whilst simultaneously providing full-range effective activation of leg extensors from start to complete lock-out.(25) Further, the synchronicity of hip and knee joint angles ensures appropriate distribution of effort between gluteus maximus and quadriceps muscles through extension phases and gluteus and hamstrings during eccentric re-loading phases. The final functional characteristic of the MyoTruk resistance machine is strong activation of the gastrocnemius and soleus muscles of the calf during significant dorsiflexion and plantar flexion of the ankle joint.(25)

MyoTruk’s influence on speed and power development

From a training safety and injury prevention perspective the main mechanical advantage of the MyoTruk compared to squat variations lies in its ability to facilitate below parallel (<90º) resistant training throughout range of motion. Compared to the “sticking point” encountered in squat movements, which often occurs at periods when the lifter is moving from a horizontal thigh position through the sticking point (approximately 30º above horizontal).(19,20) The tendency with the squat for excessive trunk lean predominantly occurs at the lowest point of the squat, when the angle at the hip joint is significantly less than at the knee joint.(20) The great difficulty often facing athletes and their coaches is the ability to develop strength and power through range in the lower body with minimal potential of injury occurring.(10)

Triple extension movements in resistance training can be defined as those that involve the extension of the three major joints: hip, knee, and ankle. These three joints, when moved from the flexed to extended position create the explosiveness needed to apply force with the feet against the ground.(8) Extension movements facilitate the main factor involved in the generation of explosive strength. As such it is widely believed that the triple extension is the most important physiological component for enhanced speed-strength training and development. Speed-strength training is a combination of maximum speed and maximum strength, which combined can produce a tremendous amount of force. This force is what we want on the playing field when the foot hits the ground.(8) This has practical applications to sports specific elements such as running the ball into contact in the game of rugby union.

Traditionally it has been proposed that Olympic lifting exercises such as the snatch and clean and jerk variations facilitated the greatest development of triple extension.

Roman and Shakirzyanov (1978) proposed that:

The explosion during an Olympic lifting exercise is executed by the simultaneous action of the muscles of the legs and torso… From this position, the athlete extends his legs and torso and rises up onto his toes and…the shoulders are elevated…Such a position is the most advantageous condition for maximal utilization of the participating muscle groups and the subsequent transfer to the barbell upward.(21)

However the MyoTruk resistance-training machine provides a significant and contemporary alternative to the traditional Olympic lifting methods which have been proposed to enhance triple extension. Not only does the MyoTruk enhance triple extension with increased efficiency of movement; it involves a significantly less intensive time period spent learning complex movement patterns that occur in Olympic weightlifting exercises. The MyoTruk facilitates triple extension development both through traditional strength training paradigms and also dynamically without the same impacts on time limitations of the athlete and coach, the central nervous system and the musculosketal system.

When developing speed and power the muscles of the hip extensors are of the most critical importance because they are usually the weak links in the large majority of athletes.(3) These muscle groups, in particular the glutes, hamstrings, and those of the lower back, are specifically targeted and developed by the MyoTruk. The primary goal of maximal strength exercises is to increase the force or strength producing capabilities of muscles. Through developing strength with various speeds throughout range of motion, the athlete has a subsequent increase in the contractual force producing capabilities of the muscles that are involved in the movement and consequent sporting performance.(3) The MyoTruk allows consistent resistance to be moved throughout range and as such has significant sports specific and functional training connotations. Heavy resistance training results in increases in the contractile rate of force development (RFD), impulse and efferent neuromuscular drive of human skeletal muscle allowing for subsequent transformation into enhanced sports specific speed and power characteristics.(1)

A distinguishing characteristic of the MyoTruk in terms of strength training lies in its ability to be eccentrically controlled so efficiently throughout range whilst the athlete maintains normal lumbar curvature. This has further positive implications for the ability of the MyoTruk to generate strength and power development. Eccentric deceleration is integral in absorbing a load as well as enhancing the elastic potential of the muscle.(14) The elastic energy stored in the series elastic elements (which includes the tendons, the aponeuroses, cross-bridges, actin, myosin filaments and the giant protein Titin) in the eccentric phase is re-used during the concentric phase.(22)


Sports specific connotations: the MyoTruk and the game of rugby union

Dynamic strength is defined as the maximal ability of a muscle to exert a force or torque at a specified velocity.(20) Explosive muscle strength can be defined as the rate of rise in contractile force at the onset of contraction.(1) Rugby union is a dynamic and explosive strength-based sport involving a significant number of collisions both in attack and defence. Successful performance in rugby union is significantly influenced by the physiological capabilities of the athlete; therefore performance can be significantly improved through the implementation of an effective resistance training program. An effective resistance training program should be run in conjunction with dynamic sport specific training. The ability to be able to produce force throughout time (impulse), possess dynamic strength capacities at contact situations and utilise a vast array of different speed attributes are all critical features within the game of rugby union. Figure 4 illustrates the manner in which heavy resistance strength training and ballistic plyometrics have the ability to positively interact with the force time curve and further enhance the strength attributes defined above:

Figure 4: Isometric force: time curve indicating maximal strength, maximal rate of force development, and force at 200 ms for untrained, heavy-resistance strength-trained, and explosive-strength-trained subjects.(11,12)


The MyoTruk resistance-training machine facilitates the development of maximal rate of force development (MFRD) through enhancing the dynamic and explosive strength capacities of the athlete. Through using the MyoTruk both ballistically and through a normal range of speed a variety of aspects along the force/time curve are able to be enhanced. The MyoTruk has the capacity to be used as a vehicle that can extend the force/time curve both vertically and horizontally improving a variety of capacities simultaneously and/or individually. Specifically regarding the game of rugby union, the greater the capacity of the athlete to produce force within the initial <150ms, the greater the ability to create advantageous situations. An enhanced ability to generate speed, power and forceful movements repeatedly over time provides a significant advantage both in attack and defence within a game.(23) This perspective is further enhanced with the continued development of such strong defensive systems and patterns within the modern game leading to the dominance of such orientated teams. However the ability to break these systems down through enhanced physiological capabilities provides a significant opportunity to greatly influence the nature of how the game of rugby union is actually played.(6)

The development of maximal strength of both the agonist and antagonist muscle groups, particularly in the lower limbs is important within the game of rugby union.(26) The role of eccentric strength training in power development was mentioned previously but obviously the strength development of antagonist muscles should not be neglected for athletes who require rapid limb movements, as research suggests enhanced strengthening of the agonist muscles increases both limb speed and accuracy of movement as well as further enhancing positive alterations in the neural firing patterns.(14) This in conjunction with maximal strength training significantly enhances the capacity of the stretch-shorten cycle (SSC).(22)


The MyoTruk effectively enhances power and speed development, and in particular the SSC, through the kinematic structure and motion of the machine not only through eccentric motion but more specifically for the ability to develop sport specific ballistic and explosive extensor strength development as previously discussed after a pre load effect and through a variety of different ranges (above or below 90º in thigh angle). This has practical applications for not only ball carrying and scrummaging facets within the game of rugby union but also at breakdown contests as well.

• Players covered on average 6,953 m during play (83 minutes). Of this distance, 37% (2,800 m) was spent standing and walking, 27% (1,900 m) jogging, 10% (700 m) cruising, 14% (990 m) striding, 5% (320 m) high-intensity running, and 6% (420 m) sprinting. • Greater running distances were observed for both players (6.7% backs; 10% forwards) in the second half of the game. • Positional data revealed that the backs performed a greater number of sprints (>20 km•h-1) than the forwards (34 vs. 19) during the game. Conversely, the forwards entered the lower speed zone (6-12 km•h-1) on a greater number of occasions than the backs (315 vs. 229) but spent less time standing and walking (66.5 vs. 77.8%) • Players were found to perform 87 moderate-intensity runs (>14 km•h-1) covering an average distance of 19.7 m (SD = 14.6). Average distances of 15.3 m (backs) and 17.3 m (forwards) were recorded for each sprint burst (>20 km•h-1), respectively. • Players exercised at <80 to 85% Vo2max during the course of the game with a mean heart rate of 172 b•min-1 (<88% HRmax)

Table 1: The physiological demands of the game of Rugby Union (5)

Further, the kinematic motion of the MyoTruk has positive implications on the functional demands of sprinting within the game of rugby union (See Table 1 above) for detail on the physiological demands of the game. The speed at which a player begins to sprint can affect the body position with the game at certain times. Data shows that forwards perform 41% of all accelerations from a standing start, 21% from walking and only 6% from striding.(6) In order to maximise acceleration from a standing start, a low body position is needed. Therefore the ability to generate strength and force in a horizontal fashion throughout range of the exercise provides distinct functional advantages of forwards using the MyoTruk as a resistance-training machine in their particular athletic development. Backs have also been shown to perform 29% of all sprints from a standing start; however they also perform an average of 8 sprints more than forwards do from a striding start.(6)

The ability to generate force off the mark and extensor strength is still critical and thus maximal strength properties obtained through use of the MyoTruk would benefit performance greatly. The type of start initiating the sprints can also affect different muscular recruitment patterns. Short sprints from a standing start involve the quadriceps muscles more and require high relative strength, whereas when a player approaches top speed the hamstrings are strongly recruited.(10) The nature of the game can also affect body positions in preparation to sprint. For example, using a blitz-like defence requires a low body position to maximise speed over 5-10 m. Reactive support play, however, requires a more vertical body position as the player is probably already maximally accelerating to keep pace with the attacking ball carrier.(6)


References

1. Aagaard, P and Andersen, J.L (1998) Correlation between contractile strength and myosin heavy chain isoform composition in human skeletal muscle. Med Sci Sports Exerc 30:1217-1222


2. Astorino, T. and Kravitz, L. (2001) Glycogen and Resistance Training. IDEA Personal Trainer No.4

3. Baggett, K (2007). The Vertical Jump Development Bible. Higher Faster Sports, 3rd Edition.

4. Campbell C. and Muncer, S.J (2005). The cause of low back pain: a network analysis. Social Science and Medicine 60:409–419.

5. Cunniffe, B., Hore, A.J., Whitcombe, M.J., Jones, K.P., Baker, J.S and Davies, B. (2009). Time course of changes in immuneoendocrine markers following an international rugby game. European Journal of Physiology

6. Duthie, G.M., Pyne, D.B., Marsh, D.J. and Hooper, S.L. (2006). Sprint patterns in rugby union during competition. J Strength Cond Res. 20(1):208-14.

7. El-Abd, J. (2005). An objective time-motion analysis of elite rugby union. Sports Medicine, 33(13):973-991.

8. Escamilla, R.F. and Garhammer, J. (2002). “Biomechanics of Powerlifting and Weightlifting Exercises.” Exercise and Sports Science. Eds. Garrett and Kirkendale. Lippincott, Williams and Wilkins. p 585-615.

9. Fitts, R.H, Mc Donald, K.S., and Schluter, J.M. (1991). The determinants of skeletal muscle force and power; their adaptability with changes in activation pattern. Journal of Biomechanics 24,1:111-122

10. Francis, C (2002). Charlie Francis 2002 Forum Review, (e-book) available from CharlieFrancis.com

11. Hakkinen, K. and P.V. Komi, 1985a. Changes in electrical and mechanical behaviour of leg extensor muscles during heavy resistance strength training. Scand. J. Sports Sci 7:55-64.

12. Hakkinen, K. and P.V. Komi, 1985b. The effect of explosive type strength training on electromyography and force production characteristics of leg extensor muscles during concentric and various stretch-shortening cycle exercises. Scand. J. Sports Sci 7:65-76.

13. Hutton, R. S (1992). Neuromuscular basis for stretching exercises in Komi ed. Strength and Power Training for Sport, Blackwell, and London.

14. Jaric, S., Rupert, M. Kuok, and D.B.Ilic. (1995) Role of agonist and antagonist muscle strength in rapid movement performances. European Journal of Applied Physiology. 71:464-468

15. Kraemer, W.J and Hakkinen, K (2002). Strength training for sport.

16. Kraemer, W. J and Newton, R. U. (1994). Training for improved vertical jump. Sports Science Exchange, 7(6):1-12

17. McLaughlin, T.M. (1975). A kinematic analysis of the parallel squat as performed in competition by national and world-class powerlifters. Microform Publications. Eugene: University of Oregon, College of Health, Physical Education and Recreation.

18. McLaughlin, T.M., Dillman, C.J. & Lardner, T.J. (1977). A kinematic model of performance in the parallel squat by champion powerlifters. Medicine and Science in Sports, 9:128-33.

19. McLaughlin, T.M., Lardner, T.J. & Dillman, C.J. (1978). Kinetics of the parallel squat. The Research Quarterly, 49:173-89.

20. Moore, R.L and Sully, J.T (1984) Myosin light chain phosphorylation in fast and slow skeletal muscle in situ. Am. Journal of Physiology. 143:257-262.

21. Nindl, B.C., Kraemer, W.J., Marx, J.O., Arciero, P.J., Dohi, K., Kellogg, M.D. and Loomis, G.A. (2001) Overnight responses of the circulating IGF-1 system after acute, heavy resistance training. Journal of Applied Physiology 90:1319-1326.

22. Rimmer, E and Sleivert, G (2000). Effects of Plyometric Intervention Program on Sprint Performance. J Strength Cond Res14(3):295-301

23. Roman, R.A. and M.S. Shakirzyanov. (1978) The Snatch, The Clean and Jerk. Moscow: Fizkultura I Sport, English translation Andrew Charniga Jr. Livonia: Sportivny Press.

24. Ross, B. (2004). Squat or ScrumTruk: which is best for leg extensor training for athletes? http://myoquip.com.au/Squat_or_ScrumTruk.htm

25. Ross, B (2006). A biomechanical model for estimating moments of force at hip and knee joints in the barbell squat. http://www.myoquip.com.au/Biomechanical_model_squat_article.htm

26. White, C (2006). Charlie Francis.Report from 1-1 Internship with Craig White (2nd April – 11th April)

27. Worrell, T.W. (1994). Factors associated with hamstring injuries. Sports Medicine 17:338-345.

28. Wolfe, R.R. (2001). Control of Muscle protein breakdown: effects of activity and nutritional states. International Journal of Sports Nutrition and Exercise Metabolism 11:164-169
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Body height in the rugby scrum: the value of equal hip and knee joint angles

Introduction

Despite the undoubted importance of efficient force delivery in the scrum, there is very limited published material addressing the actual dynamics of force delivery.

Powerful scrummaging is dependent on appropriate body position and limb alignment, not just in the relatively static situation immediately after engagement but throughout the entire contest of the scrum. Much of what passes for best practice in scrum formation reflects a failure to critically examine the actual geometry and mechanics of body position and how these change during the scrum contest.

I believe that an optimal configuration of body position and limb alignment on engagement involves hip and knee angles each set at 90° with both trunk and shank being parallel to the ground. During the scrum, hip and knee joints should move synchronously so that their angles remain equal. The hips may sink slightly relative to the shoulders but trunk and shank should remain parallel.

Body height and joint angles – what the experts advocate

Modern thinking on scrummaging usually advocates consistency of body shape for all participants regardless of position, with the feet approximately shoulder width apart and toes level. There also seems to be general agreement on the need for the trunk to be horizontal or for the shoulders to be slightly higher than the hips. (Greenwood, 1978; Smith, 2000; NSWRU, 2004; Vickery; O'Shea, 2004: Argentinian Bajada method)

However, when joint angles are discussed there is substantial divergence of opinion on the appropriate angle at the knee joint:

Jim Greenwood, Total Rugby, 1978

More than three decades on Greenwood's book, though overtaken by a succession of Law changes, remains a rugby classic. Its underlying logic is compelling. The figure below summarises his views on body position:

Greenwood argued that the optimal pushing position required hips below shoulders, 90° joint angles at hip and knee, and "knees near the deck." It can be seen from his drawing 1c above that the trunk and shank are parallel.

The figure also considers the effect of different joint angles on force delivery, and this is further discussed elsewhere in the book:

"Thighs approximately vertical. It's obvious that the more acute the angle of the knee the greater the potential range of the drive, but the more strength is required to initiate it. … [Y]ou only have to go into the full-flexed position to realise that a drive from that position is very much slower and more difficult than a drive from a half-squat. Players tend to assume the position in which they feel most capable of a snap drive. On the other hand, the smaller their degree of flexion the smaller the range of drive. For a six-foot player, a flexion of 90° at the knee produces a potential forward movement of about a foot, which allows for a snap drive, and the necessary continuation shove. That is more than enough for all practical purposes, and may well be seen as a maximum."

Greenwood also emphasises pack height:

"Shoulder height in the front row determines how low the pack can get. From every point of view, the lower the pack gets the better - provided the hooker is capable of striking. … Against the head, it's better to get even lower than usual. What this comes to is that the props get closer and closer to the basic driving position, with their feet further back and wider, their hips correspondingly lower, and their upper bodies close to horizontal. This has two advantages: it restricts the opposing hooker's strike, and may even prevent it, and it ensures a more powerful and effective drive. It's worth pointing out that most scrum-machines are set too high to allow effective low scrumming practice."

Graham Smith, RFU Technical Journal, Autumn 2000

Smith emphasises body position. "Each player must take up a position by which the force generated by the large muscles of the lower body, the quadriceps and gluteals particularly, can be transmitted effectively and SAFELY through the spine, the shoulders and the neck."

"The power the legs can produce or resist is conditioned by the angle at the knee. With the thigh vertical, or near vertical, this angle should be maintained between 90° and 120°. The greater angle will be required by the props who need to be more upright in stance in order to provide a base on which, the locks can push. The other forwards can however, adjust their positions to achieve 90° at knee."

Smith examines the consequences of a prop being experienced enough and strong enough "to alter the height of the scrummage quite legally." and "produce a significant disruption of the opposition scrummage. A prop can thus legally force his opponent to scrummage lower, at a height he finds uncomfortable, and which is mechanically inefficient."

An opponent who is unequal to this pressure will normally react in one of two ways. Firstly, he can move his feet further and further back to relieve the discomfort, as in the figure below:

He may be forced to "take his feet so far back that he goes to ground flat on his face … Even if he doesn't go to ground the position he is forced to adopt allows less and less of the power generated behind him to be transmitted though on to the opposition."

Alternatively, the prop that is being forced to scrummage too low may "bend forward at the hip, his head gradually getting well below the line of the hip," as in the figure at right.

"Because of the pressure from behind by his own lock the prop can be put into a seriously uncomfortable position. He's caught in a vice, and his position becomes even more unpleasant should his superior opponent drive forward at him."

New South Wales Rugby 2004 Coach Education Series - Effective Scrummaging

This document states that "almost 99% of all scrimmaging problems can be related directly to the body shape of the participant(s)." Amongst its prescriptions for "correct body shape" are:

"Knee bend (100 - 110° approx) directly beneath hips will assist in generating and transferring weight.

"High, steady hips will allow those players behind to apply force through a near vertical surface. The hips should NOT at anytime be higher than the shoulders."

Further on there is a series of images showing the sequence of scrum formation. The final image, reproduced at left shows the engagement. My rough scaling indicates that the loose head prop's hip and knee angles are around 90° and 120° respectively. However it appears that his shoulders are about 15°lower than his hips. This would not only be illegal but would place him and his fellow front rowers in an inherently unstable situation. He is not in a position either to support his own bodyweight or to generate a horizontal shove.

Phil Vickery, Scrummaging Masterclass

"The knee must be bent to generate the explosive power of the legs. If only slightly bent, there will only be a small, but quick, motion forwards. If a deep bend, the forward movement will be slow but be farther. Straight legs prohibit players going backwards but there is little forward momentum. The ideal is a vertical thigh with an angle of about 120 degrees between the thigh and the calf which should provide the required thrust."

Brian O'Shea, Scrum Presentation, England July 2004

In discussing body shape O'Shea specifies:

"A bend at the knees which provides an angle of approximately 110-115°, which permits power generation by the legs. This position is a 'trade-off' between the generation of dynamic power and the length of push that can be achieved. If the bend at the knees is not adequate the distance gained by the push is hardly worthwhile. If the bend at the knees is too great the loss of mechanical advantage makes it difficult to be dynamic."

He does not deal directly with the hip angle but calls for a "straight, flat back" and "high hips" that "should not be higher than the shoulders." Significantly, when illustrating individual common faults, he uses a diagram, reproduced above. where the player with correct technique appears to be in the 90-90 position.

Argentinian 'Bajada' scrum method

Argentinian teams are renowned for the effectiveness of their scrummaging and the central importance of the scrum to their game. From an early age, Argentinian forwards are schooled in the 'Bajada' or 'Bajadita,' a radically different scrum method invented in the late 'Sixties by the legendary Francisco Ocampo.

A defining characteristic is the 'Empuje Coordinado' or 'Coordinated Push.' "The scrumhalf gives a three part call after the "engage". On "pressure" all members of the pack tighten their binds and fill their lungs with air. On the call "one" everyone sinks; the legs at this point should be at 90 degrees. On "two" the pack comes straight forward while violently expelling the air from their lungs. A key note is that nobody moves their feet until forward momentum is established. If the first drive is insufficient the scrumhalf begins the call again and the opposing pack is usually caught off guard and pushed back." Rugby Union from the Virtual Library of Sport

Sergio Espector, a Level 3 Argentinian coach, recently summarised the main features of the Bajada. After the engagement he stipulates that "all eight players must flex their knees to 90 degrees ... [and] players must never move their feet off the ground until they overcome their opponents and have positive inertia."

The Bajada is recognised as an extremely effective and powerful form of scrummaging.

Summarising the views of these authors:

Analysing joint angles in the scrum

We can visualise the body in scrummaging mode as a system of skeletal levers articulated primarily at the hip and knee joints. The levers are activated by muscular contraction of the relevant extensor and flexor muscle groups. The task is to determine optimal ways to operate those levers to achieve the desired goal of delivering force in the horizontal plane, given that the primary objective of any pack is to effectively resist and, if possible, overcome the horizontal weight force generated from the opposing pack.

The figure above depicts the limb configurations of a player packed into a scrum with his hip and knee angles both at 90°. (For the sake of illustration I have assumed that the player is 1850mm tall with trunk, thigh and shank lengths of 650mm, 460mm and 480mm respectively.) In order to compare the 90-90 configuration with that advocated by some of the experts listed above, the figure below shows how the body position of the player would change if he retained the 90° hip angle but increased his knee angle to 110°.

As can be seen in the figure a knee angle of 110° requires the shank to slope upward 20° above the horizontal. This results in the height of the trunk above ground level rising by 160mm, a quite substantial difference when packs are preparing to engage.

A pack using the 90-110 configuration and therefore accustomed to training and playing with an obtuse knee angle will be disadvantaged if forced lower on engagement. The front row will have no choice but to reduce their knee angle if they are to avoid packing illegally, i.e., with hips above shoulders, and the rest of the pack will have to similarly adjust. Quite apart from the illegality, a failure to adjust the knee angle places the front row in an essentially unstable body position with the risk of the shoulders being driven even further below hip height.

As with the squat exercise, when players under severe load go into a deeper joint contraction than they are accustomed to, they have to operate in a 'zone of discomfort.' The cohesion of the pack is threatened; players may be forced to give ground and at the very least are not in a position to generate a powerful forward shove.

By contrast, a pack accustomed to function with a 90° knee angle can quite comfortably cope if the engagement takes them higher than they would prefer, as they are still operating in the range of joint angles they are familiar with.

Effective scrummaging requires coordinated and synchronised activity by all eight members of a pack. It is also essential that throughout the whole scrum engagement the pack remains in a position to initiate or effectively repel considerable force. Adoption of a 90-90 joint configuration facilitates both objectives.

Coordinated action can be readily achieved if players are trained to start from a common orientation of the joints whatever their playing position, and then to keep their shanks and trunk parallel at all times. This means that the joint angles at hip and knee remain equal as the pack drives forward. Each player is effectively contributing to the collective transmission of force along the line of their backs.

Muscles generate most force in the mid range between full extension and full flexion. From a starting point of 90-90 the leg extensors typically remain operating within that efficient range even when the pack achieves a significant shunt forward. Figure 8 illustrates how joint angles change following a push forward of 300mm. As Greenwood suggests, a "forward movement of about a foot ... may well be seen as a maximum" without repositioning of the feet. As can be seen both joint angles have extended to 138°, but this still leaves the players in a position to continue their forward momentum if necessary. Note that both the trunk and shanks have dropped 6° below the horizontal.

The 90-90 joint alignment provides the optimal platform for horizontal force delivery which can be sustained through a considerable range of movement forward, while simultaneously tending to force the opposing pack to function within a 'zone of discomfort.'

Reference

Jim Greenwood, Total Rugby: 15-Man Rugby for Coach and Player, London: Lepus Books, 1978

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