Dynamics of Changes in Muscle Architecture, Force, Strength-Velocity Properties of the Muscles of Lower Limbs in Humans under the Influence of a Three Week Unloading

Abstract

The present study examined the hypothesis that significant alterations in antigravity muscle architecture would occur with dry water immersion (DI) and that some structural changes may be seen in postural muscles because of the overall decrease in physical activity. Ten men (age (mean ± SE) – 24.5 ± 3.9 years, height – 176.1 ± 1.2 m, mass – 71.1 ± 3.4 kg) volunteered for the study. The healthy men underwent of DI for 21 days. All subjects did not use physical training during the exposure. The contractile properties of the muscle ankle extensors and flexors (maximal voluntary contraction (MVC) and strength-velocity relationship) were evaluated using an isokinetic dynamometer. The internal architecture of the triceps surae muscle of its two heads (medial (MG) and lateral (LG) gastrocnemius muscles) was determined by in vivo ultrasound (US) at ankle angles of –15° (dorsiflexion), 0° (neutral anatomical position), and +30° (plantarflexion) with an angle in the knee joint of 0°. In each position, longitudinal US of MG and LG were obtained at the proximal levels 30% (MG and LG) of the distance between the popliteal crease and the center of the lateral malleolus. US images were detected at rest for each ankle position, and the fiber length (Lf) and pennation angle (Θf) relative to the aponeurosis were determined. After DI the MVC muscle ankle extensors decreased from 122.6 ± 43.1 to 99.5 ± 22.7 N (19%). Although there was a significant no change in dorsiflexion. After DI with the ankle angle increasing from –15° to +30°, Lf changes from 43 ± 1 to 32 ± 2 mm (25.6%, p 0.01) for MG and from 45 ± 2 to 34 ± 1 mm (24.4%, p 0.01) for LG, and Θf increased from 21° ± 1° to 26° ± 2° (23.8%) for MG and from 14° ± 1° to 18° ± 2° (28.6%) for LG. Collectively, the present data suggest that the architecture and contractile capacity of human pennate muscle are interrelated, in vivo. The finding that amongst the antigravity muscles, the MG deteriorated to a greater extent than the LG is possibly related to the differences in relative load that this muscle possibly experiences during daily loading. Different Lf and Θf, and their changes by after unloading, might be related to differences in force-producing capabilities of the muscles and elastic characteristics of tendons and aponeuroses. Structural muscle changes can be considered as an adaptive process, in response to disuse.