Sagittal Plane Balance and Posture in Human Walking

David A. Winter 2. To absorb mechanical energy to protect vital organs and Department of Kinesiology joints (ankle during weight acceptance; knee at weight University of Waterloo acceptance, stance/swing transition, end of swing; hip during mid-stance) [8, 91. YAlALKING IS PROBABLY the single most important form Overlaid on the above energetic requirements are three of human movement that we all have in common, and it sub-goals related to a safe transit across the ground. is the one activity that we take most for granted until 1. Control of foot trajectory during swing to achieve safe something goes wrong with our neuromuscular or skeletal ground clearance and effect a gentle contact at the end of system. One of the major tasks in normal walking is that of swing. balance and posture; and because humans are unique as 2. Maintenance of support of the body against gravitational bipeds, with their center of mass well above ground level, the forces during stance [10, 11 1. demand of that task is extreme. 3. Maintenance of posture and balance of the head, arms, In the geriatric population [11 and in many gait pathologies, and trunk [12, 131. the fear of falling is severe. In response to this problem, there The motor patterns related to walking and running must have been scores of studies on posture during static balance, simultaneously accommodate all the above goals. The power with the focus of these studies to monitor the sway of the generation and absorption requirements can only be seen in body. In most cases, this has been done with a force platform the mechanical power curves that are derived from the and, by monitoring the center of pressure (C of P) of the body, product of each joint's moment of force and angular velocity. many researchers presume to have a handle on the center of The foot trajectory information is revealed in the dispacement gravity (C of G) of the body. Unfortunately, these measures kinematics of the foot, and the support and balance requireare not the same; the center of pressure merely reflects the ments are seen in the joint moments of force. The latter two response of the neuromuscular system (mainly ankle musrequirements can be considered as regulating functions that cles) to correct the C of G of the body [2-41. It has been serve to keep the upper body from collapsing vertically and shown that unstable amputees have significantly less sway from falling over in a horizontal direction. (as measured by the excursions of their C of P) than normals [5]. Thus, the use of C of P deviations as an "instability REGULATION OF VERTICAL AND HORIZONTAL index" is not justified. It appears that those with an intact POSITION OF HAT balance system feel free to allow their C of P to oscillate, but Vertical support is achieved by a general extensor thrust at with the knowledge that recovery is well within their control. all three joints during stance. A net support moment [101 has However, once the C of G has gone outside the area of the been defined as a quantitative measure of this total extensor foot, the body will start to fall and can only recover if one leg pattern. It is the algebraic sum of moments at the ankle, knee, moves to arrest the fall [2]. and hip, with extensor moments being given a positive In walking, the situation is drastically different. It is polarity. In all walking and running trials, the support moment recognized that these are balance problems in the lateral has been found to be positive during stance, negative during direction, but they are minor in comparison with the forward early swing, and positive during late swing [8, 91. The direction. With the large forward momentum (or kinetic variability of this net support pattern, as measured by a net energy) of the body, we are in an inherently unstable coefficient of variance (CV), is quite low compared with that situation. The head, arms, and trunk (HAT) constitute about measured at individual joints (especially the hip and knee 67 percent of body mass, with the C of G located about 67 moments). Such findings strongly suggest that the central percent of body height above ground level. The elderly nervous system (CNS) is programmed for a total limb recognize this fact as they alter their gait patterns by extensor pattern to control the amount of knee flexion during decreasing stride length and cadence [6]. Only momentarily stance. does the C of G ever come near the area of the foot. In fact, The regulation of posture and balance, on the other hand, Shimba [7] has shown in one normal walking trial that the C requires the control of position and acceleration of HAT in the of G did not pass within the area of either foot. Or, in running, horizontal direction as HAT travels along, alternately supwe have noted that the C of G passes from heel to toe in a ported on the two hip joints. Thus, the control of balance and matter of less than 70 ms when the center of pressure was posture is an anterior/posterior function that must be overlaid under the foot for 320ms. During static balance, there is only on top of the support function. Neurologically, one would one motor task; but during walking, the task of balance must hypothesize that the muscles should be programmed orthobe overlaid on top of the tasks of propelling the body forward, gonally, such that any alteration in the anterior/posterior supporting it during weight bearing, and controlling the foot moments of force should be independent of the support trajectory during swing. Thus, the dynamics of the control of pattern. Such a relationship has, in fact, been documented in posture and balance of the trunk is so drastically different in the trade-offs between the hip and knee moments and walking, that a comparison with static balance is almost between the knee and ankle moments. Between the hip and meaningless. knee, the trade-off is virtually one-for-one during the critical stance period. For example, on one trial, a subject might have NORMAL WALKING MOTOR PATTERNS an average 30 Nm extensor moment at the knee and a 15 Nm Five sub-goals of the motor system have been identified in flexor hip moment. On a successive stride, the knee extensor human gait. The first two relate to the energetics of propulmoment is seen to decrease to 10 Nm and the hip averaged 5 sion of the body: -Nm extensor (i.e., it became 20 Nm more extensor than at the 1. To generate mechanical energy at key points in time first stride). Thus, there was a net 20 Nm shift from anterior during the gait cycle (ankle at push-off, knee during midto posterior at both joints, but the net contribution to the stance, hip during late stance/early swing) [8, 91. support moment remained unchanged. This posterior shift