Unilateral lower limb suspension does not mimic bed rest or spaceflight effects on human muscle fiber function

Abstract

. Unilateral lower limb suspension does not mimic bed rest or spaceflight effects on human muscle fiber function. J Appl Physiol 93: 354-360, 2002. First published April 5, 2002 10.1152/japplphysiol.01245. 2001.-We used Ca 2ϩ -activated skinned muscle fibers to test the hypothesis that unilateral lower leg suspension (ULLS) alters cross-bridge mechanisms of muscle contraction. Soleus and gastrocnemius biopsies were obtained from eight subjects before ULLS, immediately after 12 days of ULLS (post-0 h), and after 6 h of reambulation (post-6 h). Post-0 h soleus fibers expressing type I myosin heavy chain (MHC) showed significant reductions in diameter, absolute and specific peak Ca 2ϩ -activated force, unloaded shortening velocity, and absolute and normalized peak power. Fibers obtained from the gastrocnemius were less affected by ULLS, particularly fibers expressing fast MHC isoforms. Post-6 h soleus fibers produced less absolute and specific peak force than did post-0 h fibers, suggesting that reambulation after ULLS induced cell damage. Like bed rest and spaceflight, ULLS primarily affects soleus over gastrocnemius fibers. However, in contrast to these other models, slow soleus fibers obtained after ULLS showed a decrease in unloaded shortening velocity and a greater reduction in specific force. muscle atrophy; muscle disuse; muscle damage; weightlessness HUMANS EXPOSED TO SPACEFLIGHT experience skeletal muscle atrophy, particularly of the antigravity, postural muscles of the lower limbs and back (8). Efforts to understand the mechanisms underlying the adaptations of skeletal muscle to microgravity have been hindered by the limited number of human spaceflights and by the expense and logistical problems associated with studying humans in space. An alternative approach has been to use ground-based models of nonweight bearing that mimic the effects of spaceflight. The most commonly used ground-based model of spaceflight has been bed rest. This model appears to induce adaptations in muscle cells that are similar to those experienced during spaceflight (25-27). However, although ground-based bed rest eliminates or reduces many of the problems associated with human spaceflight, the procedure is still relatively expensive and logistically complex.An alternative ground-based model of non-weight bearing is unilateral lower limb suspension (ULLS). In this model, as originally described by Berg et al. (2), the treatment limb is flexed and suspended above the ground by the use of a shoulder harness. Subjects ambulate using crutches and are thus able to participate in most of their routine daily activities. The procedure reduces many of the expenses and logistical issues associated with bed rest studies, induces atrophy of the extensor muscles that comprise the treatment limb, and results in a decline in the voluntary neuromuscular strength of these muscle groups (1-3, 5, 11). Consequently, the procedure has been proposed as an alternative ground-based model of spaceflight (2, 5).In previous work, our laboratory reported t...

Disuse of the musculo-skeletal system in space and on earth

“…Also, after 21 days of immobilization VL fibre type I CSA declined by 13%, whereas both type IIa and IIx CSA was reduced by 10% (Hortobagyi et al 2000). This seems consistent with the findings of a reduction of 7% in soleus fibre diameter without any changes in the gastrocnemius muscle following 12 days of ULLS (Widrick et al 2002).…”

Effect of 23-day muscle disuse on sarcoplasmic reticulum Ca2+ properties and contractility in human type I and type II skeletal muscle fibers

“…A quite similar trend of specific force decrease was found in the type I fibers (Ϫ9%), although the difference did not reach the significance level. The specific force measurements were made in only a subset of the subjects, specifically in three male subjects, but importantly reflect quite similar trends as those observed in two previous studies examining specific force in muscle fibers of young male subjects after a period of ULLS ranging from 4 to 21 days (6, 21); those studies, using fibers that were chemically skinned and stored, found the specific force was reduced by ϳ13 to 20% in both type I and type II fibers (6,21,54). One possible explanation for the decrease of specific force following ULLS, which was not explicitly examined here, is a loss of total myosin content.…”

Whole muscle contractile parameters and thickness loss during 35-day bed rest

“…Similar results were found after 17-day spaceXight by Widrick et al (1999) in muscle Wbres the human soleus showing a 30 and 55% increase of the Vo for type I and type IIa Wbres, respectively. In contrast, a decrease in Vo of the human single Wbres was found after bed rest (Trappe et al 2004;Larsson et al 1996;Widrick et al 2002).…”