Variation of recruitment nonlinearity and dynamic response of ankle plantarflexors

The ankle plantarflexor muscles of paraplegics may be trained to provide balance without support from the hands (in the laboratory environment) with the controller based on a two-block Hammerstein muscle model. This paper presents data on the variations of the recruitment curve block and linear dynamics block with electrode position, among various individuals and with fatigue. The tests were conducted in six groups: ‘a’ tests of a neurologically-intact subject were repeated on one day several times to record the effect of muscle fatigue; ‘b’ the same individual kept electrodes attached for a week and the muscle was identified every day; ‘c’ the same subject attached electrodes at marked positions every day for a week prior to identification; ‘d’ another normal attached electrodes at notionally the same positions over a period of one week; ‘e’ three normals and ‘f’ two paraplegics. Measurements were made with the Wobbler apparatus, in which the subject is supported upright in a standing posture. When comparing tests of fresh muscle every day, little difference was found between the nonlinear recruitment curves and linear dynamics of groups ‘b’ and ‘c’. In fatigued muscle the dynamics were slower. When the electrode position was not carefully reproduced, and over a longer period, significant differences in nonlinearity appear in the curve shapes (group ‘d’) and a similar amount of variation occurs between normals, between paraplegics, and from normals to paraplegics. The paraplegic curves show wider deadbands. The effect of prolonged stimulation on normals is slight but on paraplegics it is significant.