EMG and force analysis of the Kohnstamm phenomenon

The Kohnstamm (or “floating limbs”) phenomenon refers to the sensation of lightness and involuntary movement of the arms that follows the sustained, static muscle contraction pushing against an immovable resistance, classically demonstrated by pushing the arms outward against a door frame. This study employs surface EMG and force plate analysis to assess patterns of muscle activation and examine the effect of shoulder and hand position on the phenomenon. EMG and force were recorded while subjects pressed for 30 seconds against frames with force plates attached. Subjects were instructed to give maximal effort for the full period. Subjects repeated the experiment at two different frame widths (1.00m and 1.25m), which were presented in random order. Our preliminary results found that ~85% (33/39) of subjects experienced the phenomenon, while ~15% did not. Of those who experienced the phenomenon, ~80% self-reported that the effect was greater for the narrower frame width. Statistical analysis (2-way ANOVA) of force plate data did not show significant fatigue (decline in force) during the 30-second hold, nor were any significant differences between in force between the left and right limbs detected for the initial groups studied. Likewise, no significant difference in force or fatigue was detected between the individuals who experience the phenomenon and those who do not, although there is a large amount of variability in the data. EMG indicates that the phenomenon occurs from re-engagement of shoulder muscles after a brief period of quiescence following the end of the sustained contraction. Although the deltoid is commonly regarded as the primary muscle to contribute to the Kohnstamm effect, the greater perceived effect at the narrower frame, combined with EMG analysis suggests that the supraspinatus has an equal or greater contribution to the phenomenon. UNCA Office of Undergraduate Research This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.