Changes in myoelectric activity after prolonged muscle disuse

Kurzfassung

Introduction Strongly restricted muscle activity or complete immobilization result in decrease in muscle mass and force, i.e., muscle atrophy. In this study, we tested whether muscles excitability in the lower extremities is also affected through 21 day of inactivity during bed rest (BR). As a control was used the activity in m. biceps brachii (BB). Methods Voluntary (EMG) and evoked (m-wave) muscle activity were recorded in 8 subjects before BR, at the day after the BR as well as 5 days after the BR (REG) in m. gastrocnemius (GA), m. vastus lateralis (VL) and BB. Torque and EMG were measured during sets of 3 maximum isometric contraction at different angles. During breaks m-waves were excited. The rate of the muscle fatigability was tested in VL during sustained knee extension for 30 s at 50 % MVC. Results In VL the torque decreased after the BR by about 22 % from 261±7 to 205±7 Nm, (P<0.05) and partly recovered during REG to 234±7 Nm (P<0.05). The median power frequency of the EMG (MF) was lower after the BR (84±2 vs. 94±2 Hz). The duration of the m-wave increased from 20.1±0.2 to 20.8±0.2 ms (P<0.05) after the BR. Both parameters did not recover during REG. Signs of muscle fatigability in VL were not different before and after the BR neither for the torque nor for myoelectrical parameters. In GA decrease in torque was about 14% (P<0.05), the MF was not different, the m-wave duration rose from 13.5±0.3 to 14.6±0.3 ms (P<0.05). There were no changes in torque and properties of electrical activity in BB. Discussion One of the reasons for the loss of muscle force due to disuse should be a decrease in the protein synthesis (1). Additionally, our data show that the decrease in force in postural muscles due to BR is accompanied by signs of decrease in the muscle excitability. Thus, muscle atrophy due to disuse apparently involves not only impairment of the contractile apparatus but also the loss of sarcolemmal excitability. References Caiozzo VJ, Haddad F, Lee S, Baker M, Paloski W and Baldwin KM. Artificial gravity as a countermeasure to microgravity: a pilot study examining the effects on knee extensor and plantar flexor muscle groups. J Appl Physiol 107: 39-46, 2009.