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A Comparison of Current Biomechanical Terms

Susan Sienko Thomas, B.Sc.
Terry J. Supan, C.P.O.

Prosthetics and orthotics has evolved over the years from a craft into a profession. Prosthetics is now defined as the science of external limb replacement and orthotics the science of external support of the limbs and torso. Technological advances, in conjunction with an increase in education and experience, have provided the knowledge necessary for the production of improved designs. Concurrent with the developments in the field of prosthetics and orthotics is advancement in objective evaluation techniques.

Computer and video technology, in the form of motion analysis systems, are being used to assess complex gait patterns. Information relative to joint motion, muscle activity and force production is readily available from these complex systems. The objective quantification of the gait pattern enables the prosthetist/orthotist to assess improvements in device design and, ultimately, the benefits of the device to the patient.

Although gait, both normal and pathological, is one of the most studied of all human movements, the terms used to describe it vary from researcher to researcher and profession to profession. Despite the effort to devise a set of biomechanical terms which would be unified throughout the world, no unique definitions have been agreed upon. This is because professionals use the biomechanical terms which are unique to their own circumstances. This paper compares and contrasts the biomechanical terms which are presently used by prosthetists and orthotists and those used in the field of motion analysis.

Overall, the field of prosthetics and orthotics uses a standard set of terms which are unified within the profession to describe gait. This same situation does not exist within the area of gait analysis. The biomechanical terms used in the area of gait analysis are influenced by the ongoing research of three individuals: Drs. Perry,⁴ Sutherland,⁵ and Winter⁷ who have had a significant role in the development of gait analysis as a field and a profession. Due to the fact that these individuals influence the ongoing research in the area of biomechanics today, the terms in which they differ will be contrasted. This will provide a better understanding of the current biomechanics literature.

There are several terms which are commonly used in describing gait that are unified from profession to profession. A gait cycle is defined as "the movements and events that occur between successive heel contacts of the same foot." It contains the two phases of stance and swing. The periods and events which subsequently occur within a gait cycle vary between individuals. A period is defined as "a time frame which is initiated by or contains a significant event."

Prosthetists and orthotists describe three periods for the stance phase (Figure 1) which are: heel-strike, mid-stance, and push-off. These are used because of their relevance to the prosthetist and orthotist when assessing a gait pattern. Heel-strike is the time frame which occurs from heel contact to foot-flat. During this time, the force vector is posterior to the ankle and knee and anterior of the hip. During this time, the limb is starting to develop stability.

Midstance is the time frame between footflat to heel-off. During this time, the force vector moves anterior to the ankle and posterior to the knee and hip, and should provide optimal stability in the device. The final period is push-off. It represents the time frame from heel-off to toe-off. During this time, the force vector is in front of the ankle and changing from anterior to posterior at the knee. During this period, the device gradually reduces stability, allowing for toeoff. The swing phase contains the two periods of acceleration and deceleration. The acceleration period is the time in which the limb is moving forward rapidly to increase the stride length. The deceleration period is the time in which the limb is slowed down in preparation for heel contact.

Dr. Jacqueline Perry describes five stance phase periods and three swing phase periods (Figure 2) . The stance phase periods are as follows: initial contact, loading response, mid-stance, terminal-stance, and pre-swing. The swing phase periods are initial-swing, mid-swing, and terminal-swing.

Initial contact is defined as the moment when the foot touches the floor. The loading response is the reaction of the limb as it absorbs the impact. The period of single limb support during which the body progresses over a stationary foot is mid-stance. Terminal-stance is the period in the gait cycle in which the body moves ahead of the supporting foot and weight begins to fall on the contralateral limb. The final stance phase period of pre-swing is the transitional period of double support, during which the limb is rapidly unloaded in preparation for swing.

Initial swing is the point in which the limb is lifted from the floor and initial advancement of the thigh to achieve toe clearance and forward propulsion is assumed. During mid-swing, the limb is advanced further in order to achieve a vertical tibial position. Continued tibial advancement toward full knee extension, deceleration of the thigh, and maintenance of the foot position are included in terminal swing. This completes the full cycle from initial contact to terminal swing.

These period definitions are used by Dr. Perry as representative terms to describe what is happening at each specific point in the gait cycle.

Sutherland describes three periods of stance which are very useful in the clinical evaluation of gait (Figure 3) . Initial double support, single limb stance, and second double support combine to complete the stance phase. Double support is the period of time during walking when both feet are in contact with the ground. This occurs twice during a normal cycle. Single support is the time when only one limb is in contact with the ground. The remainder of the periods which complete the swing phase of the gait cycle are those described previously by Perry.

The final set of period definitions proposed by Winter, include the three stance phase periods of weight acceptance, midstance, and push-off, and the two swing phase periods of lift-off and reach (Figure 4) . The period of weight acceptance is the time between initial contact and maximum knee flexion of the support limb during stance. The period between weight acceptance and push-off is defined as mid-stance. Push-off is the period late in stance when the lower limb is pushing away from the ground and ankle plantar flexion occurs. The push-off period occurs shortly after the event of heel-off and ends with toe-off.

After toe-off, the two swing phase periods occur. Lift-off is the period during early swing, occurring between the events of toeoff and mid-swing. The reach period then follows and completes the gait cycle.

Important features in the gait cycle may be described as either a period or an event. The definitions associated with important events in a gait cycle are more consistent between professions. Two alternatives for relating the gait events are commonly used.

Prosthetists and orthotists define the gait events which are important to them in the determination of appropriate alignment. These same events are also used by Winter to describe gait (Figure 5) . Heel contact or strike is the first event in the gait cycle. Footflat, heel-off, and toe-off follow the initial event. Heel contact is the instant in which the heel of the foot makes initial contact with the ground. This event is followed by footflat which is the first instant when the foot is flat on the ground. During stance, there is an instant in which the heel leaves the ground. This instant is defined as "heel-off." The final event in the stance phase is "toe-off." It is the instant when the toe of the foot leaves the ground in preparation for swing.

The swing phase has one critical event which occurs during its time, which is midswing. The event of mid-swing is the midpoint in time between toe-off and initial contact.

The alternative set of gait events are commonly used in the clinical evaluation of gait (Figure 6) . The stance events include foot strike, opposite toe-off, reversal of fore-aft shear, opposite foot strike and toe-off. A foot strike event is the instant in which the foot strikes the ground. This occurs on both feet during the gait cycle. Toe-off is the instant in which the toe leaves the ground. Again, this occurs bilaterally during a single cycle. Reversal of fore-aft shear is the point during single limb stance in which the shear force, as measured by the force plates, reverses from aft to fore in preparation for opposite heel contact. The swing phase, as described by Sutherland, has no significant events defined.

This summarizes all the important periods and events which are commonly used to describe gait, both normal and pathological. Although investigators may define an event somewhat differently, it is usually stated in a term which is unique to their situation.

When comparing normal and pathological gait, the kinematic variables of velocity, stride length, step length, and cadence are used to determine variations from the norm. The definitions for these terms appear to be similar in all professions. An individual's velocity is the average distance (in meters) traveled per second. The horizontal distance covered by the same point on the foot from initial contact to ipsilateral initial contact is considered the stride length. A stride length contains two steps which can be defined as the distance measured from one point on the foot to the same point on the contralateral foot. The number of steps taken per minute is a measurement of cadence. Each of these kinematic variables are useful in determining gait abnormalities and maintain the same definition throughout all professions.

With the field of prosthetics and orthotics changing rapidly in both the materials utilized in manufacturing and device design, there is an increased need to assess the kinetics of the device. Measurements of moments, power, and energy provide an accurate quantitative measurement of the internal actions of the device.

A biomechanical moment of force (torque) is described as the net result of all muscular, ligament, and function forces acting to alter the angular rotation of the body.⁷ During the assessment of normal gait, the angles assumed by the joints do not reach extreme limits, therefore, minimizing the frictional forces. Thus, the net amount can be interpreted as the muscle force acting on the body. However, there is a large degree of discrepancy in the convention of moments used by prosthetists and orthotists and those used by biomechanists.

The present teaching in prosthetics and orthotics defines a moment as the movement which occurs as the result of force vector position. For example, during midstance, the force vector passes anterior to the ankle and posterior to the knee and hip. The prosthetist/orthotist, based on his teachings, would state that a dorsiflexion moment is occurring at the ankle, a flexion moment at the knee and an extension moment at the hip (Figure 7) . This convention in moment definition is in contradiction with the definition of a biomechanical moment which reflects the net muscle activity occurring at that point in time.

Biomechanically, when the force vector passes anterior to the ankle, posterior to the knee and hip, it facilitates ankle dorsiflexion, knee flexion and hip extension. In order to prevent an excess in any of the joint movements, the antagonist muscle fires, thus creating a moment in the opposite direction. In other words, when the force vector passes posterior to the knee, causing the knee to flex, the quadriceps or knee extensors begin to fire, creating a knee extension moment (Figure 8) . Therefore, the moment reflects the muscle activity which is occurring at the specific point in time.

The convention of moments is not the sole contributor to the confusion in this area. Another area of controversy with respect to moments is that of calculation. Several investigators, including some in the field of prosthetics and orthotics, have stated that a moment is the result of force times perpendicular distance. This allows extremely high moments to be calculated and subsequently, unrealistic values in the assessment of muscle action. The net muscle moment can be more accurately calculated by using an inverse dynamics solution of the link segment model described by Bresler & Frankel¹ and Winter.⁸ A comparison between the force times perpendicular distance equation and the inverse dynamics method has been investigated by Wells.⁶ The results indicate that moments about the ankle are quite similar despite the method used for calculation; however, as one moves from the ankle to the knee, the magnitude of the moment is greater when using the force times perpendicular distance method, rather than inverse dynamics.

By utilizing the inverse dynamics method of calculating moments, a more accurate relationship between muscle activity, energy and moments can be determined. In order to determine which muscle group is dominant during the stance phase of gait, it is necessary to understand the biomechanical conventions used to determine moment of force as they seem to contradict the convention of moments presently used by the prosthetist and orthotist (Figure 9) . Figure 9 illustrates the standard convention of the moment of force. Counter clockwise moments, as calculated at the proximal end of each segment, are positive, and the clockwise moments are negative.⁷ Therefore, a knee extension moment is positive while an ankle plantar flexor and hip extensor moment are negative. The determination of this moment of force about each joint provides an indication as to the net effect of all internal forces which include muscle, ligament and friction. In comparison to knee and hip moments, the moments about the ankle have been found to be very consistent from individual to individual. Variations in knee and hip moments can be explained by the increase in the number of link segments from the point of force. As the number of links increase, a greater number of adjustments can be made by the individual at both the hip and knee to react to the flexion or extension movement.

To illustrate the usefulness of calculating moments, it is noted that there is a gradual increase in the plantar flexor moment throughout the stance phase, indicating the significant role the plantar flexors play in gait. For the prosthetist, this would allow one to determine whether the prosthetic limb and foot simulate gastrocnemius activity by the moments they create. Evaluation of the moments about each of the joints will allow the prosthetist or orthotist to determine whether the device is simulating the muscle action which it is replacing.

A further biomechanical calculation which may aid the prosthetist and orthotist is power. Recent claims have been made by the developers of prosthetic feet that their foot "absorbs and generates energy." Winter states that the only way to determine which muscle groups are generating energy and which ones are absorbing energy is through a mechanical power analysis. The determination of power is through a mechanical power analysis. The determination of power is the result of the joint moment of force, times the angular velocity (Pj= Mj*Wj). If P is positive, this indicates a concentric contraction, and a negative power indicates eccentric contraction. When power is negative, the muscles are absorbing energy, and if it is positive, the muscles are generating energy. Again for the prosthetist, this biomechanical tool is useful in determining which prosthetic foot provides the optimum combination of energy storage and energy release. This measurement will also determine whether the feet are actually performing in the manner which they claim.

Therefore, the advances made in biomechanical technology, specifically in the area of moments of force and power determination, can further aid the technological advances being made in the field of prosthetics and orthotics. Biomechanical evaluation of the new devices on the market can also further disclose the accuracy of the claims made by the manufacturers of the devices and the benefits for patients.

By understanding the biomechanics from basic to complex, the prosthetist and orthotist can provide the individual with the optimum treatment device. With the knowledge of the differences in biomechanical terminology between professions, communication will be enhanced. Based on the advances in both the fields of biomechanics and prosthetics and orthotics a strong working relationship can be foreseen between the two.