ANALYSIS OF THE WEIGHT BEARING STRATEGY DURING GAIT USING THE SUPPORT MOMENT AND CONTRIBUTIONS TO THE SUPPORT MOMENT

Presented at NACOB 98:
North American Congress on Biomechanics
Canadian Society for Biomechanics - American Society of Biomechanics

University of Waterloo
Waterloo, Ontario, Canada
August 14-18, 1998

ANALYSIS OF THE WEIGHT BEARING STRATEGY DURING GAIT USING THE SUPPORT
MOMENT AND CONTRIBUTIONS TO THE SUPPORT MOMENT

S. Nadeau, S.J. Olney, L. Teixeira and I. McBride
School of Rehabilitation Therapy, Queen's University
Kingston, Ontario, Canada

INTRODUCTION

Supporting of the body weight (BW) during the stance phase is one important subtask of gait (Winter, 1991). Individuals spend more than half of the time (~60%) in stance while walking, and for the major part of this period, the support of the BW is only ensured by one limb. The stance phase of gait challenges several capabilities such as balance, muscular coordination, strength and mobility of the lower limbs, and therefore, differences are to be expected in the way individuals with different impairments support their BW. The purpose of the present paper is to determine the strategy used to support the BW during the stance phase of gait. The strategy was evaluated using the support moment and the contributions to the support moment applied to healthy and stroke subjects.

REVIEW AND THEORY

The support moment concept, described by Winter in 1980, provides a useful framework to study the strategy used to support BW during gait. Within this concept, the collapse of the lower limb during the stance phase is prevented by the extensor moments at the hip and knee and the plantarflexion moment at the ankle. Using the convention that these moments are positive, the support moment (MS) is defined as

MS=MH+MK+MA(Equation 1).

By looking at the different components of the MS, Winter concluded that individuals with similar kinematic patterns, could use their hip extensors and/or their knee extensors to prevent collapse of the lower limb during the first part of the stance phase. The plantarflexors were always the most important muscles used to produce the MS in the second part of the stance phase. Recently, Nadeau et al. (1997) developed an equation expressing, in percentage, the joint moments of the lower limb in relation to the MS. Using this equation, the contribution of each joint moment to the support of the BW for each instant in time is calculated and the strategy used to support the BW can be quantified. For example, the ankle joint's contribution (CMA), in percentage, is given by:

[MS - (MK + MH) / Ms] x 100 (Equation 2)

A positive percentage indicates that the joint moment works to prevent the collapse of the lower limb whereas a negative contribution means that the joint moment is tending to collapse the joint.

The results of equation 2 are easy to interpret and the method appears promising for quantifying the strategy used to support the BW during gait. However, before this method becomes widely used further analyses are needed. The purpose of this study was to apply the method to a large group of healthy subjects and to some stroke subjects having different gait patterns.

PROCEDURES

The natural gait pattern of twenty-two healthy adults [mean age (SD); 56.0 (± 20.3) yrs] and five subjects who had sustained a stroke (Table 1) was assessed in the sagittal plane of movement. Reflective markers were placed on the fifth metatarsophalangeal joint, lateral malleolus, heel, knee, hip and shoulder joints. The kinematic data were obtained with a videographic system (Peak Performance) while an AMTI force platform embedded in a 9 m walkway was used to obtain the ground reaction forces. The time-distance parameters were determined by foot-contacts or were defined during the digitizing process. A minimum of three walking cycles were collected for each subject. The joint moments at the hip, knee and ankle were computed using an inverse dynamic approach, then the MS and the contributions to the MS were calculated with equations 1 and 2, respectively.

Table 1. Characteristics of the 5 chronic stroke subjects

Subjects Age
(yrs) Speed
(m/s) Walking
Aid

S1 25 0.68 No

S2 42 0.74 No

S3 57 0.55 No

S4 51 0.81 No

S5 57 0.44 Cane

Subjects	Age (yrs)	Speed (m/s)	Walking Aid
S1	25	0.68	No
S2	42	0.74	No
S3	57	0.55	No
S4	51	0.81	No
S5	57	0.44	Cane

RESULTS

Results showed that the healthy subjects walked at a natural speed of 1.26 (± 0.17) m/s. The gait speed of each stroke subject is reported in Table 1. Figure 1 presents the MS and the contributions to the MS of each joint for the stance phase normalized to 100%. The strategy used by the healthy subjects to ensure the BW support during gait (Fig. 1A) was quite consistent between subjects (as indicated by the small SD). During early stance (before 10%), the hip extensors worked alone to support the BW. The high positive contribution of the hip extensors decreased rapidly and ended at 40% of the stance phase. From approximately 10% to 50%, the knee extensors and the ankle plantarflexors also contributed positively to support the BW. The contribution of the knee extensors increased and decreased progressively on each side of a peak value observed at about 10%. The contribution of the plantarflexors increased quite linearly from 20% to 80%. In the second part of the stance phase (50% to 90%), the plantarflexors worked alone to prevent the collapse of the lower limb. The contributions to the Ms of the hip, knee and ankle muscles ranged from -100 to 200%, -100 to 100% and, -50 to 200%, respectively. The highest contributions appeared, in general, at the beginning and at the end of the stance phase when the subject was in double limb support.

Figure 1 (A) Mean values of MS and Contributions to the MSfor the 22 healthy subjects walking at natural speed. (B) Individual data for the stroke subjects. Support Moment (A and B), Hip (A and B), Knee (A and B), Ankle (A and B) contributions.

Two stroke subjects (S1 and S4) had a very low MS (one includes negative values). For these two subjects, the contributions were very high and didn't have any meaningful significance. Thus, Figure 1B presents the contributions to the MS for only three stroke subjects (S2, S3, S5). Large variations were seen in the way the stroke subjects support their BW. Compared to the healthy, the ankle contribution began earlier whereas the hip contribution was increased in the first half of the stance phase. The knee showed three different patterns: Subject S5 had only a negative contribution, Subject S2 had almost entirely a positive contribution and subject S3 demonstrated a knee pattern close to that of the healthy group. Overall, these results suggest that stroke subjects used their hip extensors and their plantarflexors relatively more than their knee extensors to prevent the knee from collapsing during the stance phase of gait.

DISCUSSION

The results have shown that the MS and the contribution to MS could be used to assess the weight bearing strategy during gait of healthy subjects. The strategy was remarkably consistent from one healthy subject to another when the subjects walked at their natural speed. These findings agreed with those reported by Winter (1980). It was found that the stroke subjects used different strategies to support their BW. The difference was particularly marked at the knee where the contributions ranged from negative to positive values. Since stroke subjects had different impairments, variations in the way they supported they BW were expected. Two subjects, one supporting the BW using a hyperextended knee (S4) and the other using a hyperextended hip (S1) had very low values of MS. These kinematic deviations came with very large flexion moments at the knee and at the hip, respectively. Within the MS concept, flexion moments promote knee collapse and thus they are subtracted from the extensor moments. This explains the low MSobtained for subjects S1 and S4. However, this also reveals that the MS, as defined by Winter, might not give relevant information in some situations. In conclusion, these preliminary results indicate that, overall, the MS and the contributions to the MS is an interesting method to assess the weight bearing strategy during gait. However, it should be worth developing the MSconcept to include patients who have important kinematic pattern differences from normal subjects.

REFERENCES

Winter, D.A. J. Biomech, 13, 923-927, 1980.

Nadeau, S et al. Gait & Posture, 5, 21-27, 1997

Winter, D.A. The biomechanics and motor control of human gait. Waterlo, UW press, 1991

ACKNOWLEDGMENTS

S. Nadeau is supported by a post-doctoral fellowship from the Medical Research Council (MRC) of Canada and from Réseau Provincial de Recherche en Adaptation/ Réadaptation du Québec (REPAR-FRSQ).