An upper arm model for simulated weightlessness

This investigation examined the effects of 4 weeks of non-dominant arm unlo ading on the functional and structural characteristics of the triceps brach ii muscle of six normo-active college-age males (age: 23 +/- 1 years, heigh t: 176 +/- 4 cm, weight: 76 +/- 6 kg). The primary intention of this study was to determine if arm unloading is an effective analogue for simulating t he effects of weightlessness on human skeletal muscle. Subjects were tested 2-3 days preceding unloading in a standard arm sling and following removal of the sling. The sling was worn during waking hours to unload the arm. Su bjects were allowed to remove the sling during sleep and bathing. Torque pr oduction (Nm) during maximal isometric extension at 90 degrees significantl y declined (P < 0.05) in response to unloading (53.93 +/- 5.07 to 47.90 +/- 5.92; 12%). There was no significant change (P > 0.05) in the force-veloci ty attributes of the triceps over the other measured velocities (1.05, 1.57 , 2.09, 3.14, 4.19, 5.24 rad.s(-1)). Cross-sectional muscle area (CSA) of t he upper arm was smaller (44.3 +/- 2.7 to 42.4 +/- 2.5 cm(2); 4%) following 4 weeks of unloading (P < 0.05). Histochemical analysis of individual musc le fibres demonstrated reductions in fibre CSA of 27 and 18% for type I and type II fibres, respectively. However, these changes were not statisticall y significant. Electrophoretic analysis of muscle samples revealed a signif icant increase (40 +/- 7 to 58 +/- 4%, pre- and post-, respectively) in myo sin heavy chain (MHC) type II isoforms following unloading. Reductions in t ype I MHC isoform composition failed to reach statistical significance (P < 0.08). Amplitude of the integrated electromyographic (IEMG) signal during maximal isometric contraction of the long head of the triceps decreased by 21% in response to the 4-week unloading period (P < 0.05). The changes in t riceps, muscle structure and function found with arm unloading are similar in magnitude and direction to data obtained from humans following exposure to real and simulated weightlessness. These findings demonstrate that arm u nloading produces some of the effects seen in response to weightlessness in muscles of the upper arm and provides potential for an additional model to simulate the effects of microgravity on human skeletal muscle.