A new theory on stress and human performance is proposed in which physical and cognitive stressors enhance the level of neuromotor noise in the information-processing system. The neuromotor noise propagates in time and space. A 2nd assumption states that such noise facilitates easy tasks but disrupts complex tasks. In 4 experiments, 2 graphic tasks (number writing and graphic aiming) were crossed with 2 stressors (cognitive stress from a dual-task situation and physical stress in the form of loud auditory noise). Reaction time (RT), movement time (MT), and axial pen pressure were measured. In the RT phase, stress was predicted to lead to decreased RT with easy tasks and to increased RT with difficult tasks. In the execution phase, biomechanical adaptation to enhanced levels of noise was expected to manifest in higher levels of limb stiffness. In all 4 experiments, an increase of axial pen pressure with higher levels of stress evidenced the generality of biomechanical adaptation as a response to stress. RT and MT showed differential effects among the 4 experiments.