The increase in passive torque during muscle stretching may constrain the range of motion of a joint. As passive torque can vary substantially among individuals, the present study examined whether the relative lengthening of the myotendinous structures of the medial gastrocnemius (MG) during passive stretching differs among individuals. Sixteen subjects performed passive stretching of the plantar flexor muscles from ankle angles ranging from 10 degrees plantar flexion (-10 degrees ) to 30 degrees dorsiflexion (+30 degrees ). Changes in passive torque, muscle architecture (fascicle length and pennation angle) of the MG and electromyographic activity of MG and soleus were recorded. The results showed that passive torque produced by the plantar flexors increased exponentially (r(2) = 0.99; P < 0.001) with ankle dorsiflexion, whereas MG fascicle length increased linearly from 57.6 +/- 9.1 to 80.5 +/- 10.3 mm (P < 0.001), and pennation angle decreased linearly from 21.2 +/- 4.2 to 14.4 +/- 3.1 degrees (P < 0.001) when the ankle joint angle was moved from -10 degrees to +30 degrees . The relative contribution of muscle (fascicles and aponeuroses) and tendon elongation to the change in length of the muscle-tendon unit (MTU) at 30 degrees dorsiflexion was 71.8 and 28.2%, respectively. However, the adjustment differed across individuals during MTU lengthening; in subjects (62.5%) with small, passive stiffness, the elongation of the free tendon was less and that of the fascicles larger than for subjects (37.5%) with greater stiffness. In conclusion, the results indicate that the strain of muscle and tendon varies among individuals, and difference in the relative compliance of these structures influences MTU lengthening differently during passive stretching.