Determinants of Human Gait: A Review

1. Determinants of Human Gait: A Review Role of Knee/Ankle Coupling in Stability, Control & Propulsion Gordon J. Alderink, PT, PhD Grand Valley State University Cook-DeVos Center for Health Sciences Grand Rapids, Michigan USA Background Saunders and co-workers originally described six determinants (D1, D2, D3, etc) of gait as precise movements by the pelvis, hip, knee and ankles that theoretically minimized vertical and horizontal excursion of the body’s center of mass (COM), thus, reducing the energy cost of walking. However, it has recently been suggested that although these movements certainly occur, some of them may play little or no part in optimizing energy cost. Furthermore, there is evidence that a flattened COM trajectory increases muscle work and force requirements. Proponents of the dynamic gait perspective suggest that an inverted pendulum model of gait better explains the mechanical work and, therefore, metabolic costs of walking. Because of the complexity of human gait, mathematical models to describe or simulate normal walking have been justifiably simplified. For example, Saunders’s determinants of gait provide only a one-dimensional explanation of how humans may control energy expenditure while walking. Since recent evidence suggests that these gait determinants may not play a major role in controlling energy cost, one might examine gait determinants from a different perspective. Relevant to humanoid research energy cost may not be as important to consider as propulsion and control (stability).Furthermore, we might need to consider the interdependence of kinematic, kinetic and dynamic factors with regard to energy cost and control. Purpose The purposes of this presentation are: 1) review the determinants of gait; 2) review the dynamic walking perspective (inverted pendulum model); 3) review static and dynamic postural/gait controls; and 4) consider knee/ankle coupling (D3, D4 & D5) as crucial determinants for a stable, smooth dynamic human gait. Determinants of Gait D1. Pelvic Rotation. Rotation of the pelvis about a vertical axis reduces the angle of hip flexion and extension, minimizing the rise and fall of the hip joint, and, thus, elevation of COM during a stride. D2. Pelvic Obliquity. If the pelvis were to remain level during a stride, the rise and fall of the hip joint associated with flexion and extension would force the trunk to rise and fall as a function of the average elevation of both hips (stance and swing). Pelvic tipping about an antero-posterior axis resulting in a downward slope of the pelvis toward the swing leg reduces the cranial excursion of the trunk. D3. Knee Flexion in Stance. Early stance knee flexion effectively keeps hip height constant, thus reducing the height of the apex of the COM. D4. Ankle Mechanism. The apex of the COM is lengthened at initial contact by a dorsiflexed ankle (1st rocker). D5. Foot Mechanism. The leg is lengthened at the end of stance as the ankle moves from dorsiflexion to plantarflexion (heel rise or 3rd rocker), thus reducing COM vertical displacement. D6. Lateral Displacement of Body. Slight physiologic knee valgus reduces the walking base of support (BOS), thus minimizing side to side displacement of the COM. Kuo suggests that a flattened COM, as dictated by the six determinants, increases the muscle work, force requirements, and, therefore, energy costs of walking. Although the determinants do reduce COM excursion in a compass gait, Della Croce et al., Kerrigan et al., and Gard & Childress concluded that most determinants play little or no role in reducing COM and energy cost. Kerrigan et al. demonstrated that only D5 optimized the height of COM. Baker et al. suggested the optimization of energy expenditure during gait was not related to lowering the COM, but related to maintaining phase relationship and relative amplitude of the gravitational and kinetic energy of the body • Dynamic Gait Perspective • Dynamic walking simplifies the study of gait and offers a constructive perspective, i.e., yields predictions independent of experimental data. Since the determinant model fares poorly, Kuo suggests examining an inverted pendulum model: • The single support leg behaves like an inverted pendulum to transport the COM with relatively little muscle force and work (much less than the gait favored by the determinants theory) • Walking like an inverted pendulum requires a step-to-step transition, which require work to redirect COM velocity • Forced leg motion produces a trade-off in step-to-step transition costs vs energy cost related to force production • Kuo proposed a refined interpretation of the inverted pendulum gait using muscular-driven models that can be described using four intervals of stance phase (Figure 1). • Role of D3, D4 & D5 in Stability, Control & Propulsion • The knee and ankle/foot are comprised of ~30 synovial joints with 6 DOF movement. Each joint plays a unique interdependent role in the initiation and maintenance of a stable, controlled, smooth efficient gait. • Movement is produced/controlled actively (muscle) and passively (joint morphology & periarticular soft tissues). Muscle stiffness is controlled by its material and active intrinsic properties, and reflexes (joint mechanoreceptors, GTO’s and muscle spindles). Muscle actions account for 50% to 95% of the vertical ground reaction force (GRF) generated in stance phase; GRF’s translate into relatively high joint reactions forces, e.g., ~2.5 x BW at hip in single limb support. Physical Stress Theory suggests that the human body will attempt to attenuate high joint stresses. • Static (posture) and dynamic (gait) balance is provided by ankle/hip and hip/knee/ankle strategies, as well as visual and vestibular input. During gait “reflex” activity (at a metabolic cost) at the hip, knee and ankle control antero-posterior, and at the hip control medio-lateral, acceleration of the head, arms and trunk, at the same time other essential kinematic events are taking place, e.g., joint motion, step length, toe clearance, etc. • Let’s also examine D3, D4 & D5 and muscle requirements, using the refined inverted pendulum model proposed by Kuo (Figure 1): • From collision to rebound (~initial contact through loading response), the knee is flexing as the ankle is plantarflexing. During this subphase hip and knee extensors are main contributors early in stance, as are the ankle dorsiflexors. • From rebound through preload (~midstance to terminal stance) the knee remains extended as the tibia moves over the “fixed” foot (ankle dorsiflexion). The gluteus maximus, vasti, soleus and posterior gluteus medius make substantial contributions to knee extension, while the ankle plantarflexors provide primary support in late stance and is a major factor in producing forward body progression. • From pre-load through push-off (~terminal stance to preswing) the knee rapidly flexes as the ankle begins to plantarflex. During this time period, the iliopsoas and gastrocnemius are the largest contributors to peak knee flexion velocity during double support. Apparently, the sartorius and gracilis can assist in producing optimal knee angular velocity. • In conclusion, it appears likely that D3, D4 & D5 are important determinants to control COM excursion, metabolic costs, joint stresses, and provide stability. Robotic (humanoid) research might be furthered as a profound understanding of the interdependent nature of human gait mechanics is realized. • References • Baker R et al., 8th International Symposium on the 3-D Analysis of Human Movement, 2004. • Della Croce U et al., Gait & Posture, 14: 79-84, 2001. • Ferber R et al., Gait & Posture, 16: 238-248, 2002. • Gard S and Childress D, Gait & Posture, 5: 233-238, 1997. • Gard S and Childress D, Arch Phys Med Rehabil, 80: 26-32, 1999. • Kerrigan C et al., Arch Phys Med Rehabil, 82: 217-220, 2001. • Kuo A et al., Exerc Sport Sci Rev, 33: 88-97, 2005. • Kuo A, Human Movement Science, 26: 617-656, 2007. • Magee D et al., Scientific Foundations and Principles of Practice in Musculoskeletal Rehabilitation, Saunders Elsevier, 2007. • Mueller M and Maluf K, Phys Ther, 82: 383-403, 2002. • Piazza S, J NeuroEng & Rehab, 3:5, 2006. • Rose J and Gamble J, Human Walking (3rd ed.), Lippincott Williams & Wilkins, 2006. • Whittle M, Gait Analysis, An Introduction (4th ed.), Butterworth Heinemann Elsevier, 2007. • Winter D, Gait & Posture, 3: 193-214, 1995. Figure 1. Four subphases of stance illustrating instances of joint work and trajectory of COM (Kuo A et al., Exerc. Sport Sci. Rev. 33 (2), 88-97, 2005). • Work is required to redirect the COM between pendular arcs so that positive work is performed by the trailing leg before or simultaneous with negative work by the leading leg. Metabolic cost depends not on COM displacement per se, but on COM redirection between steps and the rate of work and metabolic energy expenditure are related to step length and width. • With the inverted pendulum model sagittal plane passive dynamic properties may provide stability. However, when more degrees of freedom are added to the model significant active control may be needed to stabilize lateral motion. • It can be argued that those utilizing a dynamic walking model (a compass gait in itself) misinterpreted Saunders et al. explanation for a “relatively flat COM trajectory.” Dynamic walking replaces one simple model with another one, which certainly can produce complete gaits, but cannot model human gait complexity. Muscle-driven forward simulations of normal and pathological gait call into question the ability of simple dynamic models to characterize gait. For example, muscle models incorporating force-length and force- velocity properties of muscle can best explain static and dynamic biped perturbations. Furthermore, dynamic simulations to perform muscle- induced segmental acceleration and power analyses have shown: • 1) muscles do substantial work in raising the COM in early stance, and 2) the interdependency of joint power transfers. Finally, one-, two- and three- dimensional dynamic models, because of their simplicity, do not account for the interdependent role of joint receptors, soft tissue controls (ligament and muscle), and 6 DOF joint movements. • While I concur that simple models can be constructive, they do not take into account the multitasking nature of the integrated neuro-sensory- musculo-skeletal human that locomotes smoothly, while minimizing physical stress, i.e., Physical Stress Theory, and metabolic costs. Therefore, let’s examine, in a different way, three of the gait “determinants.”