THE SHOCKING TRUTH ABOUT POWER STRAPS


When used appropriately, power straps can be very effective in decelerating forward movement of the shank when transient perturbations in snow reaction force exceed the limits of the balance system. But Power straps are typically used to provide a very snug fit of the leg with the rear spine of the boot shaft by reducing space between the calf muscle and the rear spine. As shaft buckles are increasingly tensioned, volume and fore-aft space within the confines of the shaft is proportionally reduced. But by acting directly on the leading edge and wrapping around the sides of the shank, a securely tightened power strap can severely limit ankle dorsiflexion by fixing the forward most position of the shank and eliminating any free space between the calf muscle and the spine of the boot shaft. By binding the shank to the structurally stiffest element of the shaft, the spine acts to rigidly splint the shank while impinging on the soft tissue that is normally effective in absorbing energy from transient shock loads from perturbations in snow reaction force.  The unavoidable consequence of a securely tightened  power strap is that flexion of the ankle joint is greatly constricted or substantially eliminated.

The two photos below use a skeleton leg to graphically simulate the effect of a single lap power strap on shank position without the shaft buckles being operated. In the left photo, neither the shaft buckles or the power strap are operated. In the right photo, only the power strap is operated. Operating the power strap with moderately light force had the effect of reducing the angle dorsiflexion of the shaft by 11 degrees.

Pwr Diff

Rigidly connecting the leg to the ski has its origins in the widely held view that the leg should be used as a lever with which to apply force to the ski. Power straps became ubiquitous in race boots when self-turning, fixed-radius skis spawned the technique of skiing on two skis and holding the skis on edge with the legs and later, the short-lived slip-catch technique that placed high loads on the lower limbs. But interfering with ankle flexion and especially shank position in relation to the proximate center of the head of the first metatarsal, can have serious implications.

A skier in motion across the surface of the snow is standing on a moving platform that is simultaneously being perturbed in two planes (saggital and frontal). The situation is similar to those that exist in balance studies conducted in laboratories where a subject is standing on a platform that is suddenly tilted without warning, perturbing the subject’s balance. The difference is that in skiing the COM of a skier has momentum that tends to smooth gross perturbations of COM. In the management of perturbations in skiing, the ankle is the primary joint at which perturbations in GRF are modulated by dorsiflexion/plantarflexion primarily through changes in the magnitude of contraction of the soleus muscle.  This is the balance strategy used to maintain upright postures. The pull of gravity on COM disturbs balance by causing the ankle to dorsiflex. The CNS modulates forward sway by regulating contraction of the triceps surae in an ankle plantarflexion  strategy that maintains balance by opposing ankle dorsiflexion. Shaft resistance to the shank movement associated with ankle dorsiflexion can greatly diminish muscle contraction and degrade the mechanism that maintains balance.

In a similar manner, perturbing forces travelling along the length of the outside ski of a turn are modulated primarily by the soleus muscle. But this is only possible when the ankle joint is in the Resistive Shank Angle and has a range of motion sufficient to allow the soleus to modulate perturbations in GRF. Without the ability to move, the shank becomes a vertical shock transmitter. In addition to modulating perturbing forces, the soleus acts as a powerful shock absorber in dissipating perturbations in GRF.

Shock Absorbers

Securing the shank of the user to the rear spine by drawing it rearward suppresses the 3 degrees of freedom in the ankle/foot complex. Depending on how linear the alignment of the shank with the femur is, transient shocks from peak perturbations in GRF may bypass knee and go straight to pelvis and lower back where they can cause gross disturbances in skier equilibrium compromising pressure control of the skis. Limiting flexion of the ankle joint limits the suspension travel from coordinated ankle, knee and hip flexion that maintains contact of the skis with the snow over terrain changes and also the control of pressure exerted on the snow by the skis.

For reasons I will explain in a future post called, STANCE BASICS 101: RESISTIVE SHANK ANGLE, the boot shaft angle should allow the shank angle that occurs in late stance. This shank angle allows the load from the central load-bearing axis to be transferred to the heads of the first and second metatarsals. Power straps can be used to advantage by adjusting them so they help decelerate forward movement of the shank beyond the limits of eccentric gastrocnemius-soleus muscle contraction. But the margin for error is narrow.

Long before the introduction of power straps, the importance of ankle flexion was stressed in the chapter on The Ski Boot in the book, The Shoe in Sport (1989), published in German in 1987 as Der Schuh Im Sport– ISNB 0-8151-7814-X

“If flexion resistance stays the same over the entire range of flexion of the ski boot, the resulting flexion on the tibia will be decreased. With respect to the safety of the knee, however, this is a very poor solution. The increasing stiffness of the flexion joint of the boot decreases the ability of the ankle to compensate for the load and places the entire load on the knee”. – Biomechanical Considerations of the Ski Boot (Alpine) – Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland

“The shaft of the boot should provide the leg with good support, but not with great resistance for about two thirds of the possible arc, i.e., (14 degrees) 20 to 22 degrees. Up to that point, the normal, physiologic function of the ankle should not be impeded”.

“Previous misconceptions concerning its role in absorbing energy must be replaced by the realization that shaft pressure generates impulses affecting the motion patterns of the upper body, which in turn profoundly affect acceleration and balance.

“When the lateral stability of the shaft (the leg) is properly maintained, the forces acting in the sagittal direction should not be merely passive but should be the result of active muscle participation and tonic muscular tension. If muscular function is inhibited in the ankle area, greater loads will be placed on the knee”. – Kinematics of the Foot in the Ski Boot – Professor  Dr. M. Pfeiffer – Institute for the Athletic Science, University of Salzburg, Salzburg, Austria

“Many alpine skiers have insufficient mobility in their knees and ankle. The range of motion, particularly in the ankles, is much too small. The lack of proper technique seem so often is not due to a lack of ability, but to an unsatisfactory functional configuration of the shaft in so many ski boots”. – Ski-Specific Injuries and Overload Problems – Orthopedic Design of the Ski Boot –  Dr. med. H.W. Bar, Orthopedics-Sportsmedicine, member of GOTS, Murnau, West Germany

 

BIO-MEDICAL PERSPECTIVE

Biomechanics in sports, can be stated as the muscular, joint and skeletal actions of the body during the execution of a given task, skill and/or technique.  Athletic performance is governed by these actions. Coordinated, uninhibited, fluid execution of these actions leads to efficient superior performance. Interference or inhibition of these synchronized body mechanics leads to poor performance and injury. These interferences may be caused by inherent structural limitations in our own bodies, injury induced, training deficiencies or equipment related.

In an attempt to enhance your understanding of David’s piece on power straps, let’s review the functional anatomy and mechanics in the skier’s foot/ankle complex in a ski turn.  Ankle dorsiflexion is critical to stance and balance on a stable turning foot in a ski boot. Dorsiflexion enhances pronation and leg rotation. This combination of forces controls the edge angle. Edge angle is increased by increasing pressure on the inside (medial) aspect of the foot by pronation.  As pronation increases, an obligatory 1:1 internal rotation of the lower leg (tibia) occurs. Whole leg internal rotation with hip joint stabilization completes the rotary response.

3 degrees of freedom r1

We can see that interfering with ankle flexion and especially lower leg (shank) position in relation to the center of the head of the first metatarsal of the foot can have serious implications. Compression of the foot in normal pronation stretches the plantar aponeurosis (plantar fascia) which is a primary source of sensory feedback.  The ankle joint is also a source of sensory feedback modulated by dorsiflexion/plantarflexion through stretch receptors in the soleus muscle of the calf.  As the soleus muscle contracts or relaxes, its combined Achilles tendon insertion to the calcaneus (heel bone) lowers or elevates the rear foot in association with ankle dorsiflexion or plantarflexion. The other calf muscle, the gastrocnemius, reflexly flexes the knee joint during ankle dorsiflexion, since it crosses the knee joint. TRY FLEXING YOUR ANKLE WITHOUT FLEXING YOUR KNEE!

Active muscle contraction does not flex a ski boot,  Leg pressure from COM driven tibial flexion is used to create ankle dorsiflexion in a ski boot. Here are some variables that interfere with this mechanism:

  • The power strap is a 5th buckle. It increases the height of the boot cuff anteriorly in some boots by as much as 45mm. In short-legged individuals, the lever of the high cuff is most instrumental in preventing ankle flexion. This is exacerbated in the vertical cuffs of more contemporary boots. This makes lower leg (shank) length a major factor in overcoming a higher boot cuff. In women with shorter legs and larger lower calf diameters fitted in a higher volume boot shell, the lever of the higher cuff inhibits ankle flexion.
  • Power straps inhibit knee flexion. As a result lower leg flexion, lower leg rotation and ankle    flexion are restricted and pronation is impaired.
  • Boot flex (stiffness) is another ankle motion modifying factor that varies greatly from each boot manufacturer and model.  All boot flex indices should be standardized to accurately inform boot fitters and boot buyers.

The natural reflex interaction among the foot, ankle and knee joint muscles should expose the misconception of adding structural supports that interfere with normal anatomic function.  When a power strap inhibits knee flexion, lower leg flexion, lower leg rotation and ankle flexion are restricted and pronation is impaired.

CONCLUSION:  Synergistic reflex responses and muscle co-contractions cannot occur when their sources of neural sensory input such as stretch or positional proprioception are blocked by mechanical interference.  This is especially true in the foot, the body’s base of support. Interference in sensory input leads to poor skeletal alignment and loss of balance. Good balance minimizes the stress on the body while maximizing the efficiency of movement. In skiing, loss of alignment and balance leads to poor performance and at times, severe falls and injury.

Dr. Kim Hewson is an Orthopaedic Surgeon and former Director of Orthopaedic Sports Medicine  at the University of Arizona.  He is currently a veteran Telluride Ski School Alpine Instructor and Staff Trainer in the Biomechanics of Alpine Skiing.