Often synonymous with torque, a moment is the turning effect produced by a force and is calculated by multiplying the perpendicular force by the distance from the pivot (or axis of rotation). Resistance in strength training produces an external moment, whereas muscles produce an internal moment to counteract the external moment. Moments are usually measured in Newton-meters. [1]
A dorsiflexion moment (DM) means that a force (internal or external) is trying to rotate the prosthesis in the direction of dorsiflexion (same for a planterflexion moment – PM). In a normal ankle joint, the muscles recruited for either dorsiflexion or planterflexion would be responsible for these moments.
However, in prosthetic gait, having a high heeled shoe increases the dorsiflexion moment because just prior to heel strike, the foot approaches the ground in a dorsiflexed position and proceeds to foot flat (planterflexion) immediately after the heel strike. If the shoe heel is low, the angle of dorsiflexion (DM) is also low. Conversely, if the heel is high, the (DM) is equally high. With excessive dorsiflexion, additional knee flexion is required to allow the foot to reach the floor after heel strike.
Planterflexion (PF) in the form of ‘foot flat’ immediately follows heel strike. If the prosthetic foot has a low heel or is in an attitude of plantar flexion already (PM), foot flat will occur prematurely. PF of the foot is restricted by an overly stiff heel cushion (or a plantar-flexion bumper). The stiff heel or bumper does not absorb the impact of the heel striking the floor, thus tending to produce abrupt and excessive knee flexion.
In the case of a solid-ankle cushion-heel (SACH) foot with an excessively soft heel cushion, there will be a momentary delay between heel strike and the initiation of knee flexion. The knee will begin to flex only after the heel cushion has been fully compressed. With a single-axis ankle, an excessively soft heel bumper will allow the prosthetic foot to plantar-flex too rapidly and thus slap the floor. This abrupt contact of the foot with the floor will tend to decrease the range of knee flexion. [2]
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