Voluntary activation level during lengthening, isometric and shortening contractions (angular velocity 60°/s) was investigated using electrical stimulation of the femoral nerve (triplet, 300 Hz) superimposed on maximal efforts. Recruitment of fiber populations was investigated using the PCr/Cr ratio of single characterized muscle fibers obtained from needle biopsies at rest and immediately after series of 10 lengthening, isometric and shortening contractions (1s on/1s off). Maximal voluntary torque was significantly higher during lengthening (270±55 Nm) compared with shortening contractions (199±47 Nm, P<0.05) but not different from isometric contractions (252±47 Nm). Isometric torque was higher than torque during shortening (P<0.05). Voluntary activation level during maximal attempted lengthening contractions (79±8%) was significantly lower compared to isometric (93±5%) and shortening contractions (92±3%, P<0.05). Mean PCr/Cr ratios of all fibers from all subjects at rest were 2.5±0.6, 2.0±0.7 and 2.0±0.7, respectively for type I, IIA and IIAX fibers. After 10 contractions the mean PCr/Cr ratios for grouped fiber populations (regardless of fiber type) were all significantly different from rest, 1.3±0.2, 0.7±0.3, 0.8±0.6 for lengthening, isometric and shortening contractions, respectively (P<0.05). The cumulative distributions of individual fiber populations after either contraction mode were significantly different from rest (P<0.05). Curves after lengthening contractions were less shifted compared to curves from isometric and shortening contractions (P<0.05), with a smaller shift for the type IIAX compared to type I fibers in the lengthening contractions. The results indicate a reduced voluntary drive during lengthening contractions. PCr/Cr ratios of single fibers indicated an hierarchical order of recruitment of all fiber populations during maximal attempted lengthening contractions.