Figure 2.

Muscular activity and illusion of external resistance when voluntarily bringing the arm down against the Kohnstamm lift. (a) The muscular contraction in the deltoid muscle is reduced when bringing the arm down voluntarily against the Kohnstamm. Notice the absence of detectable change in the antagonistic Pectoralis muscle when the arm is brought down versus when the arm rises up. When pushing down against a resistance introduced by the experimenter, the Pectoralis muscle was clearly activated. (b) The integrated EMG of the deltoid muscle during a Kohnstamm lift (Kohn) and when instructed to bring the arm down after the induction of a Kohnstamm (Kohn + V down). (c) A representative deltoid EMG trace showing a second Kohnstamm lift re-occurring after the arm has been brought down voluntarily against the Kohnstamm. After a push (partial view), the instruction to relax (black arrow) induces the first Kohnstamm (1) and subsequently the muscular activity is reduced to bring the arm down voluntarily (grey arrow). Relaxing again induces a second (2) or even third (3) involuntary arm lift. (d) Mean EMG allowed us to compare the deltoid muscle activity in the first and the second Kohnstamm to the baseline (grey line, thickness represents s.e.m.). (e) Perceptual ratings for the resistance felt during the downward movement of the arm (n = 21). Scale: 1, no resistance; 50, strong resistance. (f) Volunteers also rated a list of options on a 1–5 scale. A rating of 1 indicates strong disagreement with the descriptor, and 5 indicates strong agreement. (g) A conceptual sketch based on the key findings presented here. The inhibitory voluntary commands (−) win over the weaker involuntary excitatory outputs (+) presumably (dashed line) in the motor cortex. The efference copy of the voluntary generator is available for perception. The involuntary movement generator does not produce efference copy available for perception, and its contribution is misattributed to external sources (*p < 0.05).