DR. DIBYENDUNARAYAN BID [PT]T H E S A R VA J A N I K C O L L E G E O F P H Y S I O T H E R A P Y,

R A M P U R A , S U R AT

Biomechanics of the

Hip Complex: 4

Reduction of Muscle Forces in Unilateral Stance

If the hip joint undergoes osteoarthritic changes that lead to pain on weight-bearing, the joint reaction force must be reduced to avoid pain.

If total joint compression in unilateral stance is approximately three times body weight, a loss of 1 N (~4.5 lb) of body weight will reduce the joint reaction force by 3 N (13.5 lb).

For most painful hip joints, however, the reductions in compression generally required are greater than can be realistically achieved through weight loss.

The solution must be in a reduction of abductor muscle force requirements.

If less muscular countertorque is needed to off-set the effects of gravity,

there will be a decrease in the amount of muscular compression across the joint,

although the body weight compression will remain unchanged.

The need to diminish abductor force requirements also occurs when the abductor muscles are weakened through: paralysis,

through structural changes in the femur that reduce biomechanical efficiency of the muscles, or

through degenerative changes producing tears at the greater trochanter.

Hip abductor muscle weakness will inevitably affect gait, whereas paralysis of other hip joint muscles in the presence of intact abductors will permit someone to walk or even run with relatively little disability.

Several options are available when there is a need to decrease abductor muscle force requirements.

Some compression reduction strategies occur automatically, but at a cost of extra energy expenditure and structural stress.

Other strategies require intervention such as assistive devices but minimize the energy cost.

Compensatory Lateral Lean of the Trunk

Gravitational torque at the pelvis is the product of body weight and the distance that the LoG lies from the hip joint axis (MA).

If there is a need to reduce the torque of gravity in unilateral stance and if body weight cannot be reduced, the MA of the gravitational force can be reduced by laterally leaning the trunk over the pelvis toward the side of pain or weakness when in unilateral stance on the painful limb.

Although leaning toward the side of pain might appear counterintuitive [Contrary to what common sense

would suggest],

the compensatory lateral lean of the trunk toward the painful stance limb will swing the LoG closer to the hip joint,

thereby reducing the gravitational MA.

Because the weight of HATLL must pass through the weight-bearing hip joint regardless of trunk position, leaning toward the painful or weak supporting hip does not increase the joint compression caused by body weight.

However, it does reduce the gravitational torque. If there is a smaller gravitational adduction torque, there will be a proportional reduction in the need for an abductor countertorque.

Although it is theoretically possible: To laterally lean the trunk enough to bring the LoG through the

supporting hip (reducing the torque to zero) or

To the opposite side of the supporting hip (reversing the direction of the gravitational torque),

these are relatively extreme motions that require high energy expenditure and would result in excessive wear and tear on the lumbar spine.

More energy efficient and less structurally stressful compensations can still yield dramatic reductions in the hip abductor force.

Example 10-6Calculating Hip Joint Compression with Lateral Lean

Returning to our hypothetical subject weighing 825 N, let us assume that he can laterally lean to the right enough to bring the LoG within 2.5 cm (0.025 m) of the right hip joint axis (Fig. 10-32).

The gravitational adduction torque would now be:

HATLL torque adduction = [5/6 (825 N)] x 0.025 m

HATLL torque adduction = 17.2 Nm

If only 17.2 Nm of adduction torque were produced by the superimposed weight, the abductor force needed would be as follows:

Torque abduction: 17.2 Nm = Fms x 0.05 m

Fms: 17.2 Nm ÷ 0.05 m = 343.75 N

If only ~344 N (~77 lb) of abductor force were required, the total hip joint compression in unilateral stance using the compensatory lateral lean wouldnow be:

343.75 N abductor joint compression+ 687.5 N body weight (HATLL) compression-------------------------------------------------------=1031.25 N total joint compression

The 1031.25N joint reaction force estimated in Example 10-6 is half the 2062.5 N of hip joint compression previously calculated for our hypothetical subject in single-limb support.

This 50% reduction in joint compression is enough to relieve some of the pain symptoms experienced by a person with arthritic changes in the hip joint or to provide some relief to a weak or painful set of abductors.

The compensatory lean is instinctive and commonly seen in people with hip joint disability.

Whether a lateral trunk lean is due to muscular weakness or pain, a lateral lean of the trunk during walking still uses more energy than ordinarily required for single-limb support and may result in stress changes within the lumbar spine if used over an extended time period.

Use of a cane or some other assistive device offers a realistic alternative to the person with hip pain or weakness.

Use of a Cane Ipsilaterally

Pushing downward on a cane held in the hand on the side of pain or weakness should reduce the superimposed body weight by the amount of downward thrust;

that is, some of the weight of HATLL would follow the arm to the cane, rather than arriving on the sacrum and the weight-bearing hip joint.

Inman et al. suggested that it is realistic to expect that someone can push down on a cane with approximately 15% of his body weight.

The proportion of body weight that passes through the cane will not pass through the hip joint and will not create an adduction torque around the supporting hip joint.

Example 10-7Calculating Hip Joint Compression with a Cane

Ipsilaterally

If our 825N subject can push down on the cane with 15% of his body weight, 123.75 N of body weight (825 N 0.15) will pass through the cane.

The magnitude of HATLL is thereby reduced to 563.75 N (687.5 N – 123.75 N).

If the gravitational force of HATLL works through our estimated MA of 10 cm or 0.10 m (remember, the cane is intended to prevent the trunk lean), the torque of gravity is reduced to 56.38 Nm (563.75 N 0.10 m).

With a gravitational adduction torque of 56.38 Nm, the required force of the abductors acting through the usual 5 cm (0.05 m) MA is reduced to 1127.6 N (56.38 Nm ÷ 0.05 m).

The new hip joint reaction force using a cane ipsilaterally would then be:

Total hip joint compression of 1691.35 N calculated in Example 10-7 when a cane is used ipsilaterally provides some relief over the total hip joint compression of 2062.5 N ordinarily experienced in unilateral stance.

The total hip joint compression when the cane is used ipsilaterally is still greater, however, than the total joint compression of 1031.25 N found with a compensatory lateral trunk lean.

Although a cane used ipsilaterally provides some benefits in energy expenditure and structural stress reduction, it is not as effective in reducing hip joint compression as the undesirable lateral lean of the trunk.

Moving the cane to the opposite hand produces substantially different and better results.

Use of a Cane Contralaterally

When the cane is moved to the side opposite the painful or weak hip joint, the reduction in HATLL is the same as it is when the cane is used on the same side as the painful hip joint;

that is, the superimposed body weight passing through the weight-bearing hip joint is reduced by approximately 15% of body weight.

However, the cane is now substantially farther from the painful supporting hip joint (Fig. 10-33) than it would be if the cane is used on the same side;

that is, in addition to relieving some of the superimposed body weight, the cane is now in a position to assist the abductor muscles in providing a countertorque to the torque of gravity.

A classic description of the benefit of using a cane in the hand opposite to the hip impairment presumes that the downward force on the cane acts through the full distance between the hand and the stance (impaired) hip joint.

We will first look at an example using the

classic analysis and then determine how this analysis might be misleading.

Example 10-8Classic Calculation of Hip Joint Compression with a

Cane Contralaterally

Our sample 825-N patient has a superimposed body weight (HATLL) of 687.5 N, of which 123.75 N (W 0.15) passes through the cane.

Consequently, 563.75 N of body weight will pass through the right stance hip joint and the gravitation adduction torque will be:

HATLL torque adduction: 563.75 N x 0.10 m = 56.38 Nm.

The downward force on the cane of 123.75 N acts through an estimated MA of 50 cm (0.5 m) between the cane in the right hand and the right weight-bearing hip joint (see Fig. 10-31).

The cane, therefore, would generate an opposing abduction torque as follows:

Cane torque abduction: 123.75 N x 0.5 m = 61.88 Nm

The torque around the right stance hip produced by a cane in the left hand (61.88 Nm) exceeds the torque produced by the remaining weight of HATLL (56.38 Nm).

Because the gravitational torque (HATLL) may be underestimated, let us assume that the gravitational adduction torque and the countertorque provided by the cane offset each other.

If the cane completely offset the effect of gravity, there would be no need for hip abductor muscle force.

The total hip joint compression in unilateral stance when a cane is used in the opposite hand would be:

According to the classic analysis of the value of a cane in the opposite hand in Example 10-8, the hip joint reaction force would be due exclusively to body weight (563.75 N).

This is, of course, an improvement over our calculated total hip compression with a lateral lean (1031.25 N) and a greater improvement yet over joint compression in normal unilateral stance (2062.5 N) for a person weighing 825 N.

Unfortunately, the classic treatment of biomechanics of cane use appears to substantially overestimate the effects of the cane.

Krebs and colleagues (monitoring the patient with an instrumented hip prosthesis) found reductions in peak pressure magnitudes of 28% to 40% during cane-assisted gait.

Although they reported pressures rather than forces, these values do not match the nearly 75% reduction in force that the classic calculation would indicate.

Furthermore, Krebs and colleagues found a 45% reduction in gluteus medius EMG, not an elimination of activity.

The discrepancy in the classic analysis and laboratory and modeling data can be resolved by examining how the force applied to the cane by a person provides a countertorque to gravity.

Hip Joint Compression with ContralateralCane Use: A Hypothesis

The classic description of how using a cane in the hand opposite to a painful or weak hip affects forces across that joint can be found in numerous texts and journal articles.

However, few publications address the question of how the downward thrust of the arm on the cane actually acts on the pelvis.

The explanation for the effect of the cane is not logical unless we can explain how the force on the cane translates to a force applied to the pelvis.

Although this is conjectural, we propose that the force of the downward thrust on the cane arrives on the pelvis through a contraction of the latissimus dorsi muscle.

It is well established that the latissimus dorsi is a depressor of the humerus through both its humeral attachment and its more variable scapular attachment and has been classically defined as the “crutch-walking muscle.”

Because the downward thrust on the cane is accomplished through shoulder depression just as crutch walking is, it is logical to assume that the latissimus dorsi is active when a cane is used.

The latissimus dorsi attaches to the iliac crest of the pelvis.

A contraction of the latissimus dorsi would result in an upward pull on the iliac crest on the side of the cane (opposite the weak or painful weight-bearing hip), as shown in Figure 10-31.

An upward pull on the side of the pelvis opposite the supporting hip joint axis (hip hiking force) creates an abduction torque around the supporting hip joint.

This abduction torque can offset the gravitational adduction torque around the same hip joint.

It is reasonable to estimate that the magnitude of a latissimus dorsi muscle contraction should be approximately the same as the downward thrust on the cane on the same side (123.75 N in our examples) under the supposition that this muscle initiates the thrust.

Measures of the MA of the pull of the latissimus dorsi muscle on the pelvis are not readily available. However, the latissimus dorsi muscle has an attachment to the pelvis on the posterior iliac crest, lateral to the erector spinae.

Given this attachment site, the line of pull of the muscle can be approximated to have a point of application on the pelvis above the ipsilateral acetabulum.

In our sample MA for HAT of 10 cm between the LoG and the hip joint axis, the line of pull of, for example, the left latissimus dorsi muscle (presuming the subject is using a cane in the left hand) should lie twice that distance (about 20 cm, or 0.20 m) from the right weight-bearing and impaired hip joint (see Fig. 10-31).

Now let us use the estimated upward pull of the latissimus dorsi muscle and its estimated MA to calculate the total hip joint compression for our hypothetical hip patient using a cane in the contralateral hand.

Example 10-9Hypothesized Calculation of Hip Joint Compression

with a Cane Contralaterally

We have already established in Example 10-8 that the adduction torque of the body weight when a cane is used (HATLL – cane) is 56.38 N (563.75 N 0.10 m).

The countertorque (abduction around the stance right hip) produced by a contraction of the left latissimus dorsi is given as follows:

If the gravitational adduction torque at the right hip is 56.38 Nm and the abduction torque produced by the left latissimus dorsi at the right hip is 24.75 Nm, there is still an unopposed adduction torque around the stance right hip of 31.63 Nm. Consequently, a contraction of the right hip abductors is still needed.

The magnitude of required abductor force (continuing to use the estimated abductor MA of 5 cm (0.05 m) will be as follows:

In Example 10-9, body weight compression and abductor muscle compression were used to compute total joint compression on the right stance hip without taking into consideration any compression from the contraction of the contralateral latissimus dorsi.

The latissimus dorsi, unlike the hip abductor muscles, does not cross the hip and cannot create compression across the hip joint.

The estimated total hip joint compression in right stance when a cane is used in the left hand and with an assumed contraction of the left latissimus dorsi (Example 10-9) was 1196.35 N.

The estimate is a 42% reduction from the estimated joint compression of 2062.5 N for unaided unilateral stance (Example 10-5).

This reduction is well in line with the findings of Krebs and colleagues that use of a cane opposite a painful hip can relieve the affected hip of as much as 40% of its load and reduce gluteus medius activity by 45%.

Adjustment of a Carried Load

When someone with hip joint pain or weakness carries a load in the hand or on the trunk (as with a backpack or purse), there is a potential for increasing the demands on the hip abductors and increasing the hip joint compression.

The added external load will increase the superimposed weight acting through the affected sup-porting hip in unilateral stance.

Concomitantly, the gravitational torque may increase, resulting in an increased demand on the supporting hip abductors to prevent drop of the pelvis.

Although the increase in superimposed weight when a load is carried cannot be avoided, it is possible to minimize the demand on the abductor muscles on the side of a painful or weak hip.

If the external load is carried in the arm or on the side of the trunk ipsilateral to the painful or weak hip, the asymmetrical external load will cause a shift in the combined force of HAT/external load center of mass (CoM) toward the painful hip.

Any shift of the combined CoM (or resulting LoG) toward the painful hip will reduce the MA of the HAT/external load.

If the external load is not too great, the reduction in MA of the HAT/external load can result in a reduction in adduction torque not only of the combined load but also of HAT alone around the stance hip joint.

With a reduction in adduction torque, the demand on the hip abductors is reduced.

Of course, the reverse effect will occur if the load is carried on the side opposite to the weak or painful hip.

In that scenario, the external load both increases superimposed body weight and increases the gravitational MA around the weak or painful hip when in unilateral stance on that hip.

Neumann and Cook measured EMG activity in the gluteus medius during varying load-carrying conditions.

They found that a load of 10% of body weight carried on the right reduced the need for hip abductor activity in right unilateral stance; that is, the increase in superimposed body weight was more than offset by the decrease in the MA of HAT/external load, which resulted in a diminished adduction torque and a reduced need for abductor muscle contraction.

When the load on the right was increased to 20% of body weight, the right abductor activity was statistically similar to the activity before the load was added.

That is, the reduction in the MA of HAT/external load resulted in the same adduction torque as was found in the no-load condition; the abductor activity did not change from the no-load condition because the adduction torque did not change.

When the load was carried in the left hand, there was a substantial increase in right abductor activity during right stance.

This load condition increased the magnitude of HAT/external load and displaced the combined CoM (and LoG) away from the stance hip joint, increasing the gravitational torque and increasing the need for hip abductor activity.

Neumann and Cook looked at gluteus medius activity as a measure of the impact of a carried load on the stance hip joint.

Bergmann and colleagues estimated hip joint reaction forces in several subjects and measured actual forces in one subject with an instrumented femoral head prosthesis.

They found that most of their subjects could carry loads of up to 25% of body weight in the right hand and still show a slight reduction in hip joint compression over the no-load condition when in right unilateral stance.

They pointed out, however, that a typical compensatory shift of the trunk away from the load should be avoided if the goal is to reduce hip joint compression.

End of Part - 4