BPT lectures

1. DR. DIBYENDUNARAYAN BID [PT]
T H E S A R V A 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 A T
Biomechanics
of the
Hip Complex: 4

2. 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).

3.  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.

4.  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.

5.  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.

6.  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.

7.  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.

8. 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.

9.  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.

10.  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.

11.  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.

12. Example 10-6
Calculating 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

14.  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

15. If only ~344 N (~77 lb) of abductor force were
required, the total hip joint compression in unilateral
stance using the compensatory lateral lean would
now be:
343.75 N abductor joint compression
+ 687.5 N body weight (HATLL) compression
-------------------------------------------------------
=1031.25 N total joint compression

16.  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.

17.  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.

18. 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.

19.  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.

20. Example 10-7
Calculating 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).

21.  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).

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

23.  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.

24.  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.

25. 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.

26.  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.

27.  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.

29. Example 10-8
Classic 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.

30.  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

31.  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.

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

33.  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.

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

35.  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.

36.  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.

37. Hip Joint Compression with Contralateral
Cane 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.

38.  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.

39.  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.

40.  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.

41.  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.

42.  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.

43.  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).

44.  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.

45. Example 10-9
Hypothesized 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:

46.  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:

48.  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.

49.  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).

50.  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%.

51. 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.

52.  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.

53.  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.

54.  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.

55.  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.

56.  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.

57.  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.

58.  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.

59.  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.

60.  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.

61. End of Part - 4