Effect of long-term muscle paralysis on human single fiber mechanics

This study compared human muscles following long-term reduced neuromuscular activity to those with normal functioning regarding single fiber properties. Biopsies were obtained from the vastus lateralis of 5 individuals with chronic ( > 3 yr) spinal cord injury ( SCI) and 10 able-bodied controls ( CTRL). Chemically skinned fibers were tested for active and passive mechanical characteristics and subsequently classified according to myosin heavy chain ( MHC) content. SCI individuals had smaller proportions of type I ( 11 +/- 7 vs. 34 +/- 5%) and IIa fibers ( 11 +/- 6 vs. 31 +/- 5%), whereas type IIx fibers were more frequent ( 40 +/- 13 vs. 7 +/- 3%) compared with CTRL subjects ( P < 0.05). Cross-sectional area and peak force were similar in both groups for all fiber types. Unloaded shortening velocity of fibers from paralyzed muscles was higher in type IIa, IIa/IIx, and IIx fibers ( 26, 65, and 47%, respectively; P < 0.01). Consequently, absolute peak power was greater in type IIa ( 46%; P < 0.05) and IIa/IIx fibers ( 118%; P < 0.01) of the SCI group, whereas normalized peak power was higher in type IIa/IIx fibers ( 71%; P < 0.001). Ca2+ sensitivity and passive fiber characteristics were not different between the two groups in any fiber type. Composite values ( average value across all fibers analyzed within each study participant) showed similar results for cross-sectional area and peak force, whereas maximal contraction velocity and fiber power were more than 100% greater in SCI individuals. These data illustrate that contractile performance is preserved or even higher in the remaining fibers of human muscles following reduced neuromuscular activity.