For more than ten years, the Kinematic Theory has been exploited successfully in applications
dealing with handwriting processing. The theory uses Delta-Lognormal profiles to
model the behaviors of the motor control and the neuromuscular system involved in the production
of rapid movements. Its main underlying assumption is that the cumulative time delays of
the propagated impulse responses, recorded along different points on a neuromuscular system are
linked by proportional relationships. The study reported in this paper aims at using the physiological
knowledge of the upper arm to deeply investigate this assumption. First, the concept of a
subsystem is clarified and justified. Then, to observe the proportionality hypothesis, surface electromyographic
signals (SEMG) recorded from the upper limb, during the execution of rapid
handwriting movements, are analyzed. The results reveal proportional regressions, with strong
correlation, between the cumulative time delays of the muscle activities, calculated from SEMG
envelopes. This finding highlights the relationships that exist between the macro-delays observed
on the SEMG bursts and constitutes an observation proof of the proportionality hypothesis. These
developments add further support to the usefulness of the Kinematic Theory as a tool for the
study and understanding of human movement
