Narici MV, Kayser B, Barattini P, Cerretelli P.

J Physiol. 1997 Feb; 499: 81P.

Istituto di Tecnologie Biomediche Avanzate, CNR, Milano, Italy.

The effect of simulated microgravity on the contractile properties of the human triceps surae (TS) was investigated on eight male volunteers (age 42.2 +/- 8.0 years, height 1.82 +/- 0.06 m, mass 82.2 +/- 12.1 kg) before, during and after 17-day head-down bedrest at -6 deg to the horizontal. Time-to peak torque (TPT), half-relaxation time (1/2 RT), rate of rise of torque (dT/dt), and twitch peak torque (PT(tw)) at 20, 15 and 5 deg of dorsiflexion (DF) and at 5, 15, 25 and 30 deg of plantarflexion (PF) were assessed during supramaximal percutaneous electrical stimulation of the TS. Maximum voluntary contraction (MVC) was measured using the twitch-occlusion technique. A current intensity of 60% of the supramaximal level was used to investigate the frequency-torque relation (FTR), during stimulation at 1, 10, 20, 30 and 50 Hz, and the fatiguability during intermittent TS stimulation at 20 Hz for 350 ms every second for 2 min. A fatigue index (FI) was calculated as the ratio of the final torque over the initial. This lower current intensity was used to reduce the subjects' discomfort at the high stimulation frequencies. All torque measurements were performed in isometric conditions. Ethical approval of the study was obtained by the NASA medical board. Values are presented as means +/- S.E.M. Significance of the data was tested with ANOVA followed by the Scheffe test of critical differences. After 8 and 13 days of bedrest, PT(tw) decreased on average by 23.6 +/- 5.3 and 16.0 +/- 3.9% (P < 0.05-0.01) across ankle angles from 20 deg DF to 5 deg PF; no significant changes were present at other joint angles. No significant changes were found in TPT and 1/2 RT during the bedrest and recovery, whereas a decrease in dT/dt ranging from 14.2 +/- 6.5 to 19.9 +/- 6.0% (P < 0.05-0.01) was observed during the bedrest. Changes in dT/dt were no longer significant once normalized for PT(tw). There was a tendency (P < 0.01) for an increase in MVC during recovery days 6 and 10. However, four out of eight subjects showed incomplete twitch occlusion during the MVC before bedrest. A decrease in the torque at 1, 10, 20, 30 and 50 Hz was found on days 8 and 13 of bedrest, and on days 2, 6 and 10 of recovery. The maximum decrease in tetanic torque at 50 Hz was on day 2 of recovery and was 18.1+/- 4.4% (P < 0.05-0.01). Changes in the FTR were no longer present once torque was normalized for the value at 50 Hz, thus showing no change in the shape of the FTR. The FI increased from the baseline value of 0.67 +/- 0.06 to 0.75 + 0.05 (P <0.05) on days 8 and 13 of bedrest. The decrease in fatiguability was due to a decrease in initial torque during the fatigue test since FI was inversely related to the initial torque at 20 Hz. It is concluded that 17 days of bedrest seem sufficient to produce a significant loss of torque of the TS. The greater loss of twitch torque mainly at dorsiflexion angles is indicative of changes in the angle-torque relation of the TS and may possibly be due to a decrease in muscle length during bedrest. Although no change in the shape of the normalized FTR was found, the decrease in fatiguability during bedrest and the negative correlation between FI and tetanic force at 20 Hz would favour the hypothesis of preferential fast-twitch fibre atrophy during bedrest.

Publication Types:

• Meeting Abstracts

Keywords:

• Abdominal Wall
• Animals
• Ankle
• Bed Rest
• Electric Stimulation
• Head
• Humans
• Male
• Muscle, Skeletal
• Muscles
• Time
• Torque
• Weightlessness
• NASA Discipline Musculoskeletal
• Non-NASA Center

Other ID:

• 98603921

UI: 102233454

From Meeting Abstracts