If a person stands in a doorway and presses the backs of his hands outwards against the doorframe as hard as he can for a minute, and then steps out, he will experience an involuntary levitation of the arms (Salmon, 1914; Kohnstamm, 1915; Sapirstein et al. 1937; Hagbarth & Nordin, 1998). We have measured the effects of the force and duration of the previous voluntary contraction on the magnitude of this after-contraction (AC).

With local ethics committee approval, six healthy male subjects who had given their informed consent (means ± S.D.: age 25 ± 1.2 years; height 169.7 ± 2.8 cm; weight 69.3 ± 10.1 kg) performed isometric voluntary contractions (VC) of the abductors of the right arm with the glenohumeral joint positioned at 30 deg of abduction. Force was registered by a transducer placed against the dorsal surface of the hand. At the end of the VC the transducer was withdrawn by a pneumatic ram to allow the AC abduction to occur. Joint angle was calculated from wrist position measured with a draw-wire transducer. EMG was recorded from m. deltoid.

Three experiments were performed on separate days:

(1) Subjects exerted VC forces of 20, 40, 60, 80 , 90 and 100 % of maximal voluntary contraction (MVC) for 15 s. Only one subject exhibited an AC.

(2) Subjects exerted VC forces of 30, 40, 50, 60 and 70 % of MVC for 60 s. All six subjects produced an AC for all conditions. The final AC abduction angles for each of these conditions were (means ± S.D., n = 6) 50 ± 13, 50 ± 7, 62 ± 13, 73 ± 12 and 92 ± 9 deg, respectively.

(3) Subjects maintained 60 % of MVC force for durations of 15, 45, 60 and 75 s. All six subjects experienced an AC following the 60 and 75 s trials. Four experienced an AC after the 45 s trial and two of these experienced it after the 15 s trial. For the four subjects who experienced an AC after 45, 60 and 75 s, the AC peak arm abductions were 60 ± 34, 74 ± 39 and 87 ± 26 deg, respectively.

The results indicate that a threshold VC duration is required in order to produce an AC. Beyond this threshold the magnitude of the AC was proportional to the force generated during the VC.

Hagbarth, K. & Nordin, M. (1998). J. Physiol. 506, 875-883 . abstract

Kohnstamm, O. (1915). Neur. Centbl. 34, 290-291.

Salmon, A. (1914). Atti della Accad. Medico-Fisica Fiorent. 78-91.

Sapirstein, M.R., Herman, R.C. & Wallace, G.B. (1937). Am. J. Physiol. 119, 549-556.