Purpose <p>In isolated animal muscles, contraction intensity and tissue stiffness have been shown to be related to gearing. However, these relationships remain poorly understood in humans. This study aimed to clarify how contraction intensity and velocity impact gearing and their relationship with tissue properties in humans.</p> Methods <p>Fifty-two healthy young adults participated in this study. We measured the shear elastic modulus of the medial gastrocnemius muscle, its aponeurosis, and the Achilles tendon, as well as muscle echo intensity. The shear elastic modulus of the aponeurosis was used for analysis as the ratio of transverse to longitudinal directions. Belly segment gear was quantified using ultrasound images of the medial gastrocnemius during isokinetic and isometric plantarflexion.</p> Results <p>During isometric contraction, belly segment gear under 60% MVC (1.054 ± 0.029) was significantly greater than that under 20% MVC (1.029 ± 0.044). In contrast, during isokinetic contraction, no significant difference between belly segment gear under 100°/s (1.053 ± 0.024) and that under 20°/s (1.056 ± 0.021) was observed. During lower-intensity, higher-velocity isokinetic contractions, greater belly segment gear was associated with a higher transverse to longitudinal stiffness ratio of the superficial aponeurosis (β = 0.274, <i>p</i> = 0.038), lower tendon stiffness (β=–0.312, <i>p</i> = 0.021), and lower echo intensity (β=–0.308, <i>p</i> = 0.021). Furthermore, mediation analysis revealed that the effect of aponeurosis stiffness on torque through belly segment gear was significant (indirect effect = 4.35; 95% confidence interval, 0.11–10.09).</p> Conclusion <p>Belly segment gear was associated with contraction intensity and connective tissue properties.</p>