Key points Fast sarcomere‐level mechanics in contracting intact fibres from frog skeletal muscle reveal an I‐band spring with an undamped stiffness 100 times larger than the known static stiffness. This undamped stiffness remains constant in the range of sarcomere length 2.7–3.1 µm, showing the ability of the I‐band spring to adapt its length to the width of the I‐band. The stiffness and tunability of the I‐band spring implicate titin as a force contributor that, during contraction, allows weaker half‐sarcomeres to equilibrate with in‐series stronger half‐sarcomeres, preventing the development of sarcomere length inhomogeneity. This work opens new possibilities for the detailed in situ description of the structural–functional basis of muscle dysfunctions related to mutations or site‐directed mutagenesis in titin that alter the I‐band stiffness. Force and shortening in the muscle sarcomere are due to myosin motors from thick filaments pulling nearby actin filaments toward the sarcomere centre. Thousands of serially linked sarcomeres in muscle make the shortening (and the shortening speed) macroscopic, while the intrinsic instability of in‐series force generators is likely prevented by the cytoskeletal protein titin that connects the thick filament with the sarcomere end, working as an I‐band spring that accounts for the rise of passive force with sarcomere length (SL). However, current estimates of titin stiffness, deduced from the passive force–SL relation and single molecule mechanics, are much smaller than what is required to avoid the development of large inhomogeneities among sarcomeres. In this work, using 4 kHz stiffness measurements on a population of sarcomeres selected along an intact fibre isolated from frog skeletal muscle contracting at different SLs (temperature 4°C), we measure the undamped stiffness of an I‐band spring that at SL > 2.7 µm attains a maximum constant value of ∼6 pN nm−1 per half‐thick filament, two orders of magnitude larger than expected from titin‐related passive force. We conclude that a titin‐like dynamic spring in the I‐band, made by an undamped elastic element in‐series with damped elastic elements, adapts its length to the SL with kinetics that provide force balancing among serially linked sarcomeres during contraction. In this way, the I‐band spring plays a fundamental role in preventing the development of SL inhomogeneity. Key points Fast sarcomere‐level mechanics in contracting intact fibres from frog skeletal muscle reveal an I‐band spring with an undamped stiffness 100 times larger than the known static stiffness. This undamped stiffness remains constant in the range of sarcomere length 2.7–3.1 µm, showing the ability of the I‐band spring to adapt its length to the width of the I‐band. The stiffness and tunability of the I‐band spring implicate titin as a force contributor that, during contraction, allows weaker half‐sarcomeres to equilibrate with in‐series stronger half‐sarcomeres, preventing the development of sarcomere length inhomogeneity. This work opens new possibilities for the detailed in situ description of the structural–functional basis of muscle dysfunctions related to mutations or site‐directed mutagenesis in titin that alter the I‐band stiffness.