Friction sets the first ceiling on Spider-Man dreams. Real skin cannot generate the shear forces needed to hold a human body on smooth glass, so researchers turn to gecko-inspired dry adhesion, using van der Waals forces and microstructured pads. These systems can support a person on test rigs, but they demand large surface area and precise load angles that make free climbing on city walls impractical.
Tendon strength and muscle physiology impose the next limit. Human flexor tendons and forearm muscles fatigue quickly when asked to sustain body weight at extreme joint angles, and connective tissue tears long before comic-book levels of grip are reached. Exoskeletons and powered prosthetics can offload some tension, but they shift the problem to actuator torque and energy density in batteries, where current technology still falls short.
Web-swinging runs into harsher physics. A human acting as a pendulum experiences high centripetal acceleration and peak tension in the line; impact forces at each swing bottom risk joint dislocation and spinal injury. Synthetic fibers like aramids and ultra-high-molecular-weight polyethylene match or exceed steel in tensile strength-to-weight ratio, yet harness anchor points and building facades are not engineered for repeated dynamic loading at those magnitudes.
