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1. Foot pronates beyond midstance--> internal rotation moment to leg; the opposite leg is swinging forward and rotating the pelvis --> externally rotation moment to leg on ground --> conflict between proximal external rotation and distal internal rotation moments -- initially the pronated foot causing the internal rotation moment wins the battle and foot does not resupinate to accommodate that proximal external rotation moment ..... eventually as heel comes off ground, friction between the ground and foot can no longer no longer resist the external rotation moment coming from above --> abductory twist
2. As heel starts to come off the ground a functional hallux limitus kicks in (for whatever reason); as the body has to move forward over the first MPJ, it can do so by a number of mechanisms; one of these is to abduct the foot to roll off the medial side of the blocked first MPJ --> abductory twist
__________________
Craig Payne
Department of Podiatry
La Trobe University
Melbourne, Australia http://www.latrobe.edu.au/podiatry
__________________________________________________ ___________________________________ God put me on this earth to accomplish a certain number of things - right now I am so far behind, I will never die.
The views expressed above are those of the author and not that of La Trobe University This is where I am, where are you?
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Here is the lecture I gave on "Biomechanics of Abductory Twist" at the 2004 PFOLA International Foot Biomechanics and Orthotic Therapy Seminar in Boston. Hope this helps.
Quote:
Biomechanics of Abductory Twist
By Kevin A. Kirby, DPM
Abductory twist is a relatively common gait abnormality seen during walking
Seen as a rapid medial movement of the heel at the instant of heel-off (i.e. abduction of the foot)
Involves the rapid pivoting of the foot and lower extremity around the metatarsal heads in an external rotation direction
Often seen in conjunction with late midstance pronation
Rapid medial movement of the heel at the instant of heel-off that causes pivoting of foot and lower extremity on plantar forefoot
Abductory twist involves a rapid medial movement of the heel at the instant of heel off, where the foot and lower extremity externally rotate, pivoting on plantar forefoot
Abductory twist involves a rapid medial movement of the heel at the instant of heel-off, where the foot and lower extremity externally rotate, pivoting on plantar forefoot
In order to accomplish the locomotor task of walking, the lower extremities must rotate in opposite directions within the sagittal plane. In other words, when the left leg moves forward, the right leg moves backward and vice versa.
Counter-rotating sagittal plane motions of lower extremities are coordinated with transverse plane rotations within pelvis in order to allow for longer stride lengths and to allow for more smooth and efficient bipedal locomotion
(Rose J, Gamble JG (eds.): Human Walking, 2nd ed. Williams and Wilkins, Baltimore, 1994, p. 8)
Transverse plane rotations of tibia, femur and pelvis are synchronized with each other during gait
Pelvis reaches transverse plane maxima at the points of forefoot contact and at toe-off during gait cycle (Rose J, Gamble JG (eds.): Human Walking, 2nd ed. Williams and Wilkins, Baltimore, 1994, pp. 13-21)
As left lower extremity (swing phase limb) moves anteriorly from toe-off to forefoot contact, the left pelvis moves anteriorly relative to right pelvis
At same time, right lower extremity (stance phase limb) moves posteriorly from forefoot contact to toe off, causing right pelvis to move posteriorly relative to left pelvis
Sagittal plane lower extremity movement causes cyclical transverse plane rotations of pelvis so that during late midstance, the swing phase pelvis is rotating anteriorly relative to the stance phase pelvis
Effect of Stationary Stance Phase Foot on Constantly Rotating Pelvis
During stance phase, the foot is held stationary by frictional forces from the ground
Therefore, unless the foot slides against the ground, the foot cannot rotate within the transverse plane along with the pelvis to match the constant rotations of the pelvis within the transverse plane during walking
The foot and lower extremity must have a mechanism to allow femur and tibia to be able to rotate externally during late stance phase in order to match the transverse plane movements of the pelvis above it and allow more efficient gait
During stance phase, the foot is on the floor and external rotation of the leg occurs because mechanisms exist in the ankle and foot that permit the leg to rotate externally while the foot remains stationary. If such mechanisms did not exist, the foot would have to slip as shown [above]. (Rose J, Gamble JG (eds.): Human Walking, 2nd ed. Williams and Wilkins, Baltimore, 1994, pp. 15-16)
What mechanism is available in human foot and lower extremity to allow required transverse plane rotations of pelvis to match transverse plane rotations of tibia during stance phase?
Triplanar arrangement of subtalar joint axis allows tibia to externally rotate when subtalar joint supinates, thus allowing the tibia to externally rotate relative to the foot during the latter half of stance phase
Talus, tibia and subtalar joint axis all externally rotate relative to the ground with subtalar joint supination
Subtalar joint supination allows tibia and femur to externally rotate so that they may match transverse plane rotational movements of pelvis above
Mechanical Effect of Abnormal STJ Pronation in Late Midstance
Late midstance STJ pronation causes internal rotation of the tibia and femur at a time in gait when the STJ should be supinating and the tibia and femur should be externally rotating
The internal rotation of tibia and femur caused by STJ pronation will cause the tibia and femur to rotate in opposite directions relative to pelvis
Different rotational directions and/or rotational speeds between tibia and femur and the pelvis cause a mismatching of transverse plane rotations of between pelvis and lower extremity
(Kirby, KA: The Biomechanics of Abductory Twist. Precision Intricast Newsletter. Precision Intricast, Inc., Payson, Arizona, July 2003)
Mismatching of Pelvic to Tibial-Femoral Transverse Plane Rotations
Mismatching of rate of transverse plane rotation of pelvis relative to transverse plane rotations of tibia and femur will create abnormal external rotation moments within the soft tissue structures of the hip and knee
Mismatching of tibial-femoral and pelvic transverse plane rotations can also occur when STJ does not supinate rapidly enough in late midstance to match the transverse plane rotations of pelvis above it
Frictional forces between foot and ground prevent the foot from abducting on the ground to match the transverse plane rotational speed of the pelvis during late midstance
Elastic Strain Energy of Soft Tissue Structures Produces Abductory Twist
External rotation moment that is caused by mismatch between speed of pelvic rotation and lack of speed of external tibial rotation (i.e. lack of STJ supination) is stored as a form of potential energy, elastic strain energy, within the soft tissue structures of the hip and knee
Elastic strain energy is the energy that is stored in an elastic structure when it is either stretched or compressed (Alexander, R. McNeill: Principles of Animal Locomotion. Princeton University Press, Princeton, New Jersey, 2003, p. 40)
Elastic strain energy is stored within soft tissue structures surrounding hip joint so that rapid external rotation of foot will occur when elastic strain energy is released as kinetic energy
Elastic strain energy is a potential energy that occurs in any structure when it either stretched or compressed
Release of the potential energy as kinetic energy will occur when the structure is allowed to return to its normal length, thus allowing the stored elastic strain energy to be released
Can Tendons Store Energy?
Storage and release of elastic strain energy within the long tendons of the limbs of animals is a common energy conservation mechanism during running, jumping and galloping activities in many members of the animal kingdom
In horses, the plantaris tendon is 900 mm long and stretches 50 mm during galloping
The oxygen consumption of red kangaroos remains nearly the same as their hopping speed is increased due to storage of elastic strain energy in their plantaris and gastrocnemius tendons
Before heel lift, any external rotation moments on femur and tibia (caused by rotation of pelvis) that have built up during late midstance will be resolved into a rapid external rotation motion of the foot just after heel lift
The rapid medial movement of the heel that occurs from the sudden external rotation of the lower extremity and foot at heel lift is due to release of elastic strain energy into an abductory twist
Abnormal external rotation moments acting on femur and tibia from transverse plane rotations of pelvis will not cause foot to externally rotate as long as there is adequate frictional force preventing the heel from rotating medially on the ground
At the instant of heel lift, the sudden reduction in frictional force on the heel allows the external rotation moment acting on the lower extremity to be resolved into a rapid medial rotation of the heel relative to the forefoot, or an abductory twist
Factors that Affect Abductory Twist
Walking speed and stride length
Increased walking speed and increased stride length increases amount and speed of pelvic transverse plane rotations that will tend to increase abductory twist
Friction between ground and foot
Abductory twist is enhanced when walking barefoot on tile or linoleum due to increased coefficient of friction between foot and ground with these flooring surfaces
Angle of foot at contact phase
More adducted placement of foot during contact phase will increase elastic strain energy in hip and knee at late midstance causing increased tendency for abductory twist to occur
Late Midstance Pronation is Common Cause of Abductory Twist
Abnormal magnitude of STJ pronation moments during late midstance is the cause of late midstance pronation
STJ supination must occur at sufficient speeds during late midstance to produce adequate tibial external rotation to match pelvic transverse plane rotations, or abductory twist is more likely to occur
Increased STJ pronation moment in late midstance may cause one of three kinematic effects on STJ:
STJ pronation motion
No STJ motion
Inadequate velocity of STJ supination
Association of Abductory Twist with Functional Hallux Limitus
Abductory twist is commonly seen along with functional hallux limitus since increased STJ pronation moments during late midstance are responsible for both abductory twist and functional hallux limitus
Increase in STJ pronation moment in late midstance causes increased first ray dorsiflexion moment and increased tensile forces in plantar fascia
Increased first ray dorsiflexion moment (right) from increased STJ pronation moment in late midstance causes increased 1st MPJ plantarflexion moment that, in turn, decreases tendency for normal hallux dorsiflexion to occur during propulsion
Lack of normal first ray plantarflexion and concomitant lack of hallux dorsiflexion during propulsion causes functional hallux limitus
Conclusion
Abductory twist is a common gait finding caused by mismatching of transverse plane rotational speeds of the pelvis and lower extremity during late midstance
Insufficient STJ supination occurring during late midstance will lead to abnormal external rotation moments and abnormal build-up of elastic strain energy within the hip and knee
Abductory twist occurs at the instant of heel lift due to the sudden reduction in frictional force on plantar heel and the sudden release of stored elastic strain energy
Abductory twist can be used as an indicator of abnormal gait mechanics that may otherwise be difficult to detect
__________________
Sincerely,
Kevin
**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
Don't know if these diagrams will help. (terms torque and moments are interchangeable) Probably best to print off the diagrams first.
1) Resting stance = no torque in femur and tibia, which I will call the shank.
2) To progress fron position diagram 2 to position diagram 3 it is neccessary to rotate the pelvis about the right hip joint.
3) During this transition and in normal function there must be a position, which is during left swing thru, that the foot, ankle joint, shank and hip are aligned perpendicular to the pelvis (for the sake of evaluation). At this point muscle M (diag 2a) has a certain tensile force that produces a torque about the hip joint. This is equalised by the torque of the body mass rotating about the hip in the opposite direction and which produces torque P. (diag 2). These torques cannot exist in space on their own and so this torque is transmitted down the shank and is equalised by horizontal frictional forces on the ground (FE+ & Fi- Diag 6). This is shank torque x.
4) Imagine now that at the same point in the left swing thru the right foot is internally rotated. This internal rotation distance K will increase the muscle tension M and so increase the torque acting on the shank ie P1 = x+KM (in terms of moments about the hip). You will notice that the angle between hip and pelvis in diag 2a is 90dgs and the same angle in diag 4 is less than 90dgs.
4a) You will now see that the diplacement K is achieved by STJ pronation and internal tibial rotation but the foot is straight ahead. In this way the muscle M increases its tension and again we see P2 = x+KM in terms of moments about the hip.
Now as the pelvis rotates about the right hip it is restricted by the increased muscle tension and this torque is transmitted down the shank. If the tibia / shank was free to externally rotate this is what would happen. How ever the Vertical ground reaction force VGRF (diag 5) causes pronation and so by the mechanism of the ankle/stj complex also internal rotation.
5) To get from the pronated position 5a)to the vertical position 5c) there must be a supinatory force. This can be produced by a combination of -Fz and Tp (shank torque)
6) These forces and torques are balanced by equal and opposite force * lever arm acting about point c below the ankle. IE (Fe + Fi-) = TP.
If we now remove force FE+ as happens at heel lift, we are only left with Fi-, which cannot resist the torque TP since we require two forces for a force couple to produce an opposing torque.
The rotation point c now moves to the forefoot (P2), since this is the only point of contact and still in fact forces there exists forces Fi- and FE+ but their lever arms are too short and or the frictional force is too low to produce enough enough torque to resist the external rotational torque of the shank TP.
Therefore the shank, and with it the foot, externally rotate and we see an abductory twist at heel off.
Is that clear Tried to keep it as simple as possible, dunno if I did tho.
All the best Dave
__________________
Descartes seems to consider here that beliefs formed by pure reasoning are less doubtful than those formed through perception.
The Following User Says Thank You to David Smith For This Useful Post:
Could someone explain to me the biomechanics of abductory twist?
I think that Craig and Kevin have it, but I'll try a short explanation as well.
In gait with abductory twist, the trunk is applying an external rotation moment to the leg. The bottom of the leg can externally rotate at two locations: the STJ and the foot versus ground interface. The external rotation moment applied to the leg will attempt to externally rotate the talus on top of the foot and rotate the foot relative to the ground. A pronation moment (e.g. from ground) at the STJ will resist the supination moment (external rotation moment of talus) so the leg wont externally rotate relative to the foot. When the heel is on the ground there is more friction preventing external roation of the foot relative to the ground. As the heel lifts off of the ground there is less friction preventing external rotation of the foot relative to the ground and the whole leg will rotate and that is what you see with an abductory twist. Anecdotally, you will see more adductory twist with leather shoes on linoleum as compared to rubber soled shoes on carpet.
Cheers,
Eric Fuller
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Here's a video of abductory twist, now with narration.
__________________
Sincerely,
Kevin
**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
I have been trying to capture this phenomenon to show patients and thus far failed. I need to invest in better equipment...
Thanks Kevin for the vid and Eric for the explanation.
Regards,
__________________
"If we all worked on the assumption that what is accepted as true is really true, there would be little hope of advance." - Orville Wright
Very thorough explanations by all. (That is one lecture I would love to hear Kevin.)
I would like to add that any sagittal plane block can sometimes lead to an abductory or adductory twist in compensation. I am not convinced that there is always a straight-forward reason why one occurs over the other. It may be a reflection of individual structure, the position of the limbs or even neuromechanics and movement patterns. Abductory twist is certainly more commonly found.
Other possible theories for a mechanism of action:
• Greater stiffness in the forefoot of the shoe relative to the midfoot.
• Elevatus or short first ray causing late phase pronation and asymmetrical ground reaction force after heel off.
• Correction of external or internal rotation of the stance phase limb to guarantee clearance of the foot when it passes the opposing limb during float/swing phase.
• Excessive or Inadequate Base of Stance correction.
• Forefoot Valgus or Varus deformity.
I think that the direction of propulsive force may be a factor in some patients. One intriguing question is why everyone with a Functional Hallux Limitus or a Hallux rigidus do not all have a transverse plane twist after heel off. Some may have other transverse plane compensations but others seem to have sufficient muscular control to dampen the forces internally. It is fun to see patients who you predict will have a certain biomechanics which just does not occur during function.
Perhaps others may be able to comment on why running vs walking seems to exacerbate the twist in different patients.
Biomechanics of abductory twist
Tried to keep it as simple as possible, dunno if I did tho.
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