Which muscle passively flexes the hock when the stifle is flexed?

Which muscle passively flexes the hock when the stifle is flexed?

• A. Flexor Carpi Ulnaris
• B. Peronius Tertius
• C. Gastrocnemius
• D. Tibialis Anterior

Answer: (B)

The important idea here is the reciprocal linkage between the stifle and the hock. In many quadruped hind limbs, the fibularis tertius (also called peronius tertius) forms a bridge between these two joints so that motion at the stifle tends to drive motion at the hock in the same direction. When the stifle flexes, the fibularis tertius tendon is stretched across both joints and pulls on the tarsal bones, causing the hock to flex as a passive result of that stifle movement. This is the mechanism behind the hock following the stifle during flexion. Other muscles listed don’t create this reciprocal effect. For example, the gastrocnemius crosses both joints but its action tends to extend the hock, not flex it in response to stifle flexion. Forelimb muscles or other hind limb muscles don’t form this specific stifle–hock linkage, so they don’t produce the same passive hock flexion when the stifle is flexed. So the muscle responsible for passively flexing the hock when the stifle is flexed is the fibularis tertius (peronius tertius).

The important idea here is the reciprocal linkage between the stifle and the hock. In many quadruped hind limbs, the fibularis tertius (also called peronius tertius) forms a bridge between these two joints so that motion at the stifle tends to drive motion at the hock in the same direction.

When the stifle flexes, the fibularis tertius tendon is stretched across both joints and pulls on the tarsal bones, causing the hock to flex as a passive result of that stifle movement. This is the mechanism behind the hock following the stifle during flexion.

Other muscles listed don’t create this reciprocal effect. For example, the gastrocnemius crosses both joints but its action tends to extend the hock, not flex it in response to stifle flexion. Forelimb muscles or other hind limb muscles don’t form this specific stifle–hock linkage, so they don’t produce the same passive hock flexion when the stifle is flexed.

So the muscle responsible for passively flexing the hock when the stifle is flexed is the fibularis tertius (peronius tertius).