The behavior of aponeuroses during voluntary contractions is still poorly understood and results provided in the literature are controversial. Therefore, the aim of this study was to investigate the behavior of the tibialis anterior aponeurosis during passive movements and active isometric, concentric, and eccentric contractions in vivo. Based on previous findings, we hypothesized that the aponeurosis exhibits behavior that is not consistent with a serial alignment with the contractile element of the muscle. Nine subjects participated in the study and performed contractions on a Biodex-dynamometer. Two ultrasound probes were used to visualize the proximal and distal ends of the tibialis anterior aponeurosis from which length changes were calculated. The main findings were that: (1) During isometric contractions, aponeurosis lengths increased and decreased with increasing and decreasing forces by about 2.8 (±1.5) mm while tendon length changes were much greater and averaged 15 (±3.3) mm, (2) during passive movements, aponeurosis lengths did not change significantly, and (3) despite similar average torque changes during concentric and eccentric contractions, aponeurosis lengths changed by 2.2 mm in concentric but only by 1.2 mm in eccentric contractions. The results of this study did not provide clear evidence for or against a serial alignment of the aponeurosis with the contractile elements. However, at low activation levels during force ramp contractions, there was a small but consistent shortening of aponeuroses with increasing torque. Our findings suggest that aponeurosis length changes do not depend on force alone but depend critically on activation level and the type of contraction.