Body height in the rugby scrum: the value
of equal hip and knee joint angles
by Bruce
Ross, CEO, MyoQuip Pty Ltd (November 2006)
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. Figure 1 summarises his views
on body position:
Figure 1
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.
Figure 1 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."
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 Figure 2:
Figure 2
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
Figure 3:
"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."
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 as Figure 4 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.
"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."
"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 as Figure 5. where
the player with correct technique appears to
be in the 90-90 position:
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:
Author
Hip
Angle (°)
Knee
Angle (°)
Jim
Greenwood (1978)
90
90
Graham
Smith (2000)
90
90 (120 for props)
Phil
Vickery
90
120
Brian
O'Shea (2004)
90
110-115
New
South Wales Rugby Union
90
100-110
Argentinian
'Bajada'method
90
90
Recommended
Joint Angles in the Scrum
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.
Figure 6
Figure 6 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, Figure
7 shows how the body position of the player
would change if he retained the 90° hip angle
but increased his knee angle to 110°.
Figure 7
As can be seen in Figure 7 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.
Figure 8
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.'
References
Jim Greenwood, Total Rugby: 15-Man Rugby for Coach and Player, London: Lepus Books, 1978
