Abstract
The purpose of this study was to formulate the dynamic properties of the human flexor pollicis longus muscle (FPL), including short-latency stretch reflex, for a prosthetic hand controller. First, we measured the tension responses of the FPL during isometric contraction. The subjects were four healthy men. While the subject maintained constant isometric force (Fc) in the FPL, we applied a small ramp stretch to the thumb. We measured stretch length X(t), force F(t) and IEMGs (rectified and smoothed EMGs) of the FPL and the extensor pollicis longus (EPL) muscles. Experiments were repeated with various stretch lengths (35 ms, 4 mm ; 50 ms, 4 mm ; 50 ms, 7 mm) and various levels of isometric force (2, 7.5, 15, 22.5, 30% of maximum voluntary contraction (MVC)). Next, we isolated the stretch length perturbation-evoked force attributed to the muscle and short latency stretch reflex Fv(t). We then identified the dynamics by using the Auto-Regressive eXogeneous (ARX) model, the input of which was X(t) and the output of which was Fv(t). We created a third-order ARX model by calculating AIC values. Parameters of the decided ARX model were estimated using a least squares approach. Finally, parameters (K, t1, t2) of the transfer function G(s), which were obtained from the ARX model, were calculated. An almost linear relationship was obtained between the gain K of G(s) and the constant isometric force (Fc). The range of t1 and t2 was determined by calculating the differences between the isolated forces Fv(t) and the calculated forces with the model Fv*(t).
