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 (Ͼ3 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 Ϯ 7 vs. 34 Ϯ 5%) and IIa fibers (11 Ϯ 6 vs. 31 Ϯ 5%), whereas type IIx fibers were more frequent (40 Ϯ 13 vs. 7 Ϯ 3%) compared with CTRL subjects (P Ͻ 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 Ͻ 0.01). Consequently, absolute peak power was greater in type IIa (46%; P Ͻ 0.05) and IIa/IIx fibers (118%; P Ͻ 0.01) of the SCI group, whereas normalized peak power was higher in type IIa/IIx fibers (71%; P Ͻ 0.001). Ca 2ϩ 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. chemically skinned fibers; unloaded shortening velocity; fiber power; passive tension; spinal cord injury MUSCLE UNLOADING OCCURS in a variety of conditions, such as immobilization, disease, paralysis, or exposure to microgravity. The absence of normal weight-bearing activity induces a rapid decrease in muscle mass and strength, especially of antigravity muscles (1, 25). Muscle atrophy induced by unloading is associated with several structural changes, such as modifications of the myosin heavy chain (MHC) isoform expression, inducing fiber-type transitions toward a higher proportion of fast type II fibers. Experiments based on single fiber models have demonstrated great sensitivity in detecting also a certain degree of functional variability of fibers expressing the same MHC isoforms, especially when the pattern of muscle activity changes. Human studies involving 17 days of bed rest (27, 28) or spaceflight (26) revealed that maximal single fiber force (P 0 ) from the soleus muscle was decreased, mainly as a result of a decline in fiber cross-sectional area (CSA), and that maximal unloaded shortening velocity (V 0 ) was increased. Consequently, single fiber power was either maintained or depressed, depending on the study participant or fiber type (29), with fibers expressing type I MHC being generally more affected. Muscle unloading up to 4 mo in a long-term bed-rest study induced a similar ...