Hip Moments During Level Walking, Stair Climbing, and Exercise in Individuals Aged 55 Years or Older

Subjects

Data were collected from 17 Caucasian men and 13 Caucasian women who were 55 years of age or older and had no identified musculoskeletal or neurological impairments. Subjects were recruited through newspaper advertising. The mean age of the subjects was 65.4 years (SD=6.02, range=55–75) (for men: X̄=66.6, SD=5.6, range=57–75; for women: X̄=63.7, SD=6.4, range=55–73). The mean weight of the subjects was 80.5 kg (SD=16.2, range=49.5–114) (for men: X̄=87.6, SD=13.5, range=71–114; for women: X̄=71.2, SD=15.0, range=49.5–95). The mean height of the subjects was 170.8 cm (SD=10.5, range=153–187) (for men: X̄=177.9, SD=6.1, range= 162–187; for women: X̄=161.3, SD=6.8, range=153–173).

Measurement System

Hip internal moments during level walking, stair climbing, and exercises were obtained with the QGAIT system.*^24,25 The QGAIT system is a software package that works in conjunction with the Optotrak motion system,^† a force plate, standardized radiographs (which provide information on the location of the joint centers), and anthropometric data.

Infrared-emitting diodes (IREDs) were placed over the greater trochanter, the lateral femoral epicondyle, the head of the fibula, and the lateral malleolus. Two probes, which projected anteriorly, were attached to each subject's thigh and shank, and an IRED was attached to these endpoints, providing 3 markers per limb segment. Three IREDs attached to a plastic anchor were placed at the level of fifth lumbar vertebra. A mechanical on-off footswitch placed under the right heel provided information regarding foot contact during gait and stair ascent and descent to define a gait cycle for normalization of the data.

Vertical and shear ground reaction forces were measured with an AMTI force platform.^‡ A 16-channel analog-to-digital board integrated with the Optotrak system allowed simultaneous collection of motion, force plate, and footswitch data. Force and motion data were collected at a frequency of 50 Hz.

Standardized radiographs^26,27 were used to calculate the discrepancy between the surface location of the IREDs and the positions of the hip and knee joint centers. Lead shots were placed over the bony IRED landmarks (greater trochanter, lateral femoral epicondyle, and head of the fibula). These lead shots were then located on radiographs; the discrepancy between the external bony landmark and the actual hip and knee joint centers was estimated, and a correction factor was calculated. The location of the ankle joint center was estimated as the midpoint of the distance between the lateral and medial malleoli, which was measured with a caliper.

The anthropometric data collected included each subject's height, weight, lengths of lower limb from greater trochanter and tibial plateau to the floor, thigh and calf circumferences, and shoe weight. These measurements are necessary for the calculation of thigh, shank, and foot center-of-mass locations; segment masses; and inertial properties.

Kinematic, kinetic, anthropometric, and joint center data were integrated in order to calculate net moments (in newton-meters) during level walking, stair climbing, and exercise. Hip moments were obtained by use of inverse dynamics with a link segment model. In the inverse-dynamics approach, the segments are assumed to be rigid bodies with hinge joints between them. In our model, the foot was considered part of the shank; therefore, our model was composed of shank and thigh segments only.²⁵ The moments were calculated first at the proximal end of the shank and then at the proximal end of the thigh. Therefore, moments of force were calculated first at the knee and then at the hip. The moments obtained during each activity were normalized by body weight, resulting in measurements expressed in newton-meters per kilogram. The rate of change in moments was obtained in all 3 planes by use of a program designed to locate the maximum slope during each exercise cycle and was reported in newton-meters per kilogram per second.

The QGAIT system was previously used to obtain measurements of knee moments and forces, and measurements of these gait variables were found to be accurate and reproducible.^24,25 The test-retest reliability of measurements of 3-dimensional moments of force at the hip during gait was determined in our laboratory for 10 subjects (5 men, 5 women) who had a mean age of 27 years. Testing involved 2 visits to the laboratory over a 2- week period. The hip moments demonstrated excellent repeatability between visits, with the 95% confidence intervals of the maximal point difference between curves for all 6 moments crossing zero in all planes.

The exercises we examined were those likely to generate high net moments and rate of change in moments at the hip, suitable for older individuals, and measurable by the system. The exercises included weight-bearing activities, such as left hip flexion, left hip flexion with a 1.13-kg weight around the left ankle, left hip extension, and left hip abduction while in single-limb stance on the right test leg. In addition, ascending stairs and descending stairs, basic or aerobic steps, lunge, and knee bend were investigated (Tab. 1). The non—weight-bearing activities included right test hip flexion, right test hip flexion with a 1.13-kg weight around the right ankle, right test hip extension, and right test hip abduction while subjects stood on the left leg (Tab. 1). Figures 1 through 3 demonstrate a subject performing flexion of the right test hip, basic step, and flexion of the left hip with weight, respectively. All subjects wore running shoes for all tests.

Figure 1.

Flexion of the right test hip showing the subject's right test leg in flexion.

Figure 2.

Basic step showing the subject's right foot on the foot plate and the left foot raised and going back to the stepper.

Figure 3.

Flexion of the left leg with a weight around the ankle showing the foot of the subject's right test leg on the foot plate while the left leg is in flexion.

Procedure

All subjects signed a consent form prior to participating in the study. Radiographs of the hip and knee were obtained first. The 3 anatomical landmarks (greater trochanter, lateral femoral epicondyle, and head of the fibula) were identified on the right leg and marked with an erasable marker, and a small lead shot was placed over each landmark for radiographic purposes. In the motion laboratory, IREDs were attached as previously described. A footswitch was placed under the right heel for gait and stair-climbing data collection.

Data from the right lower limb were collected during all activities. Level walking was performed by all 30 subjects to provide the baseline measure used for comparison with the other exercises. Data were collected for 24 subjects during ascending stairs. Descending stairs and the remaining exercises were randomized so that each subject performed 3 or 4 of the exercises under investigation. Subjects were provided with an opportunity to practice each activity prior to recording and were provided with rest periods as necessary between activities. Data for 5 trials were collected for each activity, and the average of the 5 trials was used in data analysis. One visit of approximately 2 hours was required for the data collection.

Data Analysis

A 2-way analysis of variance (ANOVA) (2 exercises × 3 planes) for repeated measures on all factors was carried out with the maximum peak moment and rate of change in moments obtained at the hip during the activities under investigation. Each exercise was compared with level walking. When the F ratio was significant, a paired-sample t test was conducted to determine where the differences between level walking and each of the exercises occurred. A t-test power analysis was performed when no significant difference was found. All statistical tests were considered significant at the .05 level.

Peak Internal Moments

The overall results obtained for the hip peak internal moments during level walking and other exercises are shown in Table 2. The ANOVA test comparing level walking with each of the exercises demonstrated a difference in moments between planes in each case. Differences for exercise were found when level walking was compared with ascending stairs, knee bend, lunge, basic step, and abduction of the left hip. The interactions between exercise and plane for comparisons between level walking and all the other exercises were significant, except for lunge. The paired-sample t test between level walking and each of the exercises was performed to compare means in each plane. All comparisons showed a t-test power above 75%.

Rate of Change in Moments

The mean rates of change in moments obtained during level walking and all the other exercises are shown in Table 3. The results of the ANOVA for the main effect of plane and for the interactions between exercise and plane were all significant.

In the frontal plane, descending stairs had a rate of change in moments comparable to the mean rate of change obtained during level walking (0.111 versus 0.067 N·m/kg/s). Similarly, basic step, knee bend, lunge, flexion of the right test hip with weight, flexion and extension of the right test hip, and abduction and extension of the left hip had rates of change in moments that did not differ from those obtained during level walking. All the other exercises had rates of change in moments lower than those obtained during level walking.

In the sagittal plane, none of the rates of change in moments during the exercises were higher than those obtained during level walking. The rate of change in moment with lunge did not differ from those obtained during level walking. All the other exercises generated rates of change in moments lower than those obtained during level walking.

In the transverse plane, the rates of change in moments during ascending stairs and descending stairs did not differ from those obtained during level walking. All the other comparisons resulted in rates of change in moments lower than those obtained during level walking.

Frontal Plane

The magnitudes of the internal abductor moments in several of the exercises (ascending and descending stairs, flexion of the left hip with and without a weight, abduction and extension of the left hip, and basic step) were comparable to those obtained during level walking. The rates of change in moments during descending stairs, abduction and extension of the left hip, and basic step also did not differ from those obtained during level walking. Therefore, in the frontal plane, all of these exercises could be recommended to promote diversity and high loads at the hip joint.

The internal abductor moment during level walking is attributable primarily to the medial shift of the body's center of mass as the weight is transferred to the stance limb. The abductors of the stance limb create a moment that limits the drop of the contralateral pelvis.²⁸ During descending stairs, when the foot strikes the step below, the abductors also have to generate moments with enough magnitude to sustain the body weight as it is transferred to the stance limb. The impact of the braking forces as the body moves down during descending stairs is probably comparable to or greater than the impact at heel contact during level walking, creating similar joint reaction forces and consequently moments.

Flexion of the left hip and flexion of the left hip with weight had almost no impact. The moments generated during these exercises, however, were similar to those generated during level walking. To sustain the body weight as the opposite leg goes into flexion apparently requires a level of activity comparable to that required of the same muscles during the stance phase of the gait cycle. Similarly, ascending stairs and abduction and extension of the left hip resulted in internal abductor moments and rates of change in moments that did not differ from those obtained during level walking. Therefore, these exercises may be an alternative to level walking in a program designed to promote high loads at the hip.

The magnitude of the internal adductor moment obtained in the frontal plane during level walking was, on average, very low and comparable to results reported in the literature.^29,30 The maximum peak internal adductor moment occurred during the swing phase of the gait cycle and was similar in magnitude to the peak adductor moment generated during knee bend. The functions of the adductors during the swing phase of gait are to maintain the body near the midline and to help the flexor muscles flex the hip.^31,32 Knee bend involves greater hip flexion range than does level walking. Thus, it is likely that the demand placed on the adductors during knee bend would be higher to assist the flexors during knee bend.

Similarly, lunge, flexion of the left hip with and without a weight, and abduction of the left hip also generated internal adductor moments that did not differ from those obtained during level walking. In addition, the rates of change in moments during knee bend, lunge, and abduction of the left hip were not different from those obtained during level walking. Thus, these exercises could be used in addition to level walking as components of a program designed to create high loads at the hip in the frontal plane.

Sagittal Plane

According to Andriacchi and Mikosz,³³ the highest external moments during most activities of daily living occur in the sagittal plane, in the direction tending to flex the joints. Thus, the demand is placed on the extensor antigravity muscles of the joint to counterbalance the external flexor moments.^33,34 The internal extensor moment obtained during level walking in the current study reached −0.89 N·m/kg. This magnitude is in agreement with those in other studies in which sagittal moments at the hip were measured.^29,30

The activities that generated the highest internal extensor moments in the sagittal plane were ascending stairs and lunge, although these values were not higher than those obtained during level walking. As the intensity and physical demands of a task increase, the external moments and the internal forces increase proportionally.

The fastest rise time in moments in the sagittal plane was observed during level walking. Lunge showed a rate of change in moment of a magnitude similar to that obtained during level walking. Therefore, in the sagittal plane, lunge and ascending stairs would be appropriate for generating high internal extensor moments at the hip.

Transverse Plane

The magnitude of the internal rotator moment during ascending stairs was higher than that obtained during level walking. Ascending stairs also generated a rate of change in moment comparable to that obtained during level walking. Therefore, ascending stairs would appear to be the best activity for promoting high loads with a fast rise time at the hip in the transverse plane.

Descending stairs and lunge resulted in hip internal rotation moments comparable to those obtained during level walking. Similarly, knee bend, lunge, and ascending and descending stairs generated hip external rotation moments comparable to those obtained during level walking. Consequently, these activities may be recommended as alternatives to level walking for enhancing mechanical loads at the hip in the transverse plane.

In general, the non—weight-bearing exercises (flexion of the right test hip, flexion of the right test hip with weight, abduction of the right test hip, and extension of the right test hip) generated lower moments of force at the hip than did the weight-bearing activities in almost all planes. Flexion of the right test hip with weight and extension of the right test hip generated moments that were not different from those obtained during level walking in the sagittal plane only. The remainder of these exercises (abduction of the right test hip and flexion of the right test hip) generated moments that were lower than those generated during level walking in every plane. In these types of exercises, the external moments are the product of the acceleration and the weight of the leg. External forces involved in the system are inertia, gravity, and forces attributable to the distal segment. Thus, it is not surprising that the moments generated during these exercises were lower than those generated during the weight-bearing test leg exercises. The weight placed around the ankle during flexion of the right test hip was an attempt to increase the moments generated at the hip. As the mass of the segment increases, the applied forces have to increase to change the state of motion of the segment. A weight of 1.13 kg, however, was not sufficient to promote high moments at the hip. How much more weight would need to be added to the lower extremity to generate peak moments higher than those obtained during level walking in each plane should be determined in future research.

Burr et al²² demonstrated that repetitive loading of up to 1.5 times body weight of the lower limb initiated bone remodeling in the vertebral columns of rabbits. On the basis of this result, Grove and Londeree¹⁰ hypothesized that activities that generate impact forces greater than 1.5 times body weight, such as jumping jacks, knee to elbow with jump, and running in place, would be required to maintain or increase the bone mass density of the proximal femur in women who are postmenopausal. Activities with an impact less than 1.5 times body weight, such as slow and fast walking, were thought to be insufficient for bone mass maintenance. On the basis of this information, none of the exercises investigated in this study would be recommended, because the moments generated in every plane were lower than 1.5 times body weight. Ground reaction forces, however, were the outcome measure reported in the study by Grove and Londeree.¹⁰ Ground reaction forces represent the net vertical and shear forces acting between the foot and the ground (force plate) and are equal in magnitude to the force that the body applies to the ground through the foot.^31,35 These forces are the sum of the mass × acceleration products of all body segments while the foot is in contact with the force plate.³⁵ These forces represent the total body reaction forces, rather than the forces at the hip joint specifically. In our opinion, these ground forces would not yield information regarding any treatment decision related to a specific joint.³⁵ The moments generated at the hip during the high-impact exercises described by Grove and Londeree¹⁰ could be lower than 1.5 times body weight. Therefore, the moments of force generated at the hip during each activity should be investigated prior to any exercise recommendation.

Woodward and Cunningham,²³ using an accelerometer attached to the ankle, showed that the rates of change in acceleration at the ankle during ascending and descending stairs were higher than those obtained during level walking. In our study, the rates of change in moments at the hip during descending stairs (frontal plane) and ascending stairs (frontal and transverse planes) were comparable to those obtained during level walking. A comparison between these 2 studies is difficult because both the measurement systems and the outcome measures used in the studies were different. In addition, the rates of change of acceleration reported by Woodward and Cunningham²³ do not reflect the rates of change in forces. The measurement of moment of force during various exercises provides an indication of the relative force acting at a joint.³⁴ Therefore, we suggest that the rate of change in moments is a reasonable measure of the rate of change of force and hence strains.