Guinea fowl are the darndest animals. Even though there's nothing wrong with their wings, they'll only take off if it's a last minute escape! But when it comes to running, the reluctant aviators are second to none; put a guinea fowl on a treadmill, and it just keeps going. So when Monica Daley needed to compare how running animals adjust when the going gets steep, she looked no further than the unconventional bird; they'd keep running uphill until they lost their grip. Focusing on a pair of muscles that span the ankle joint,Daley and Andrew Biewener wondered how the muscles functioned as the birds walked or trotted up an incline. Were the muscles matched, so that both worked harder as they ascended a treadmill-hill, or had the birds developed specialised muscles, which come into their own when trotting over different terrains (p. 2941)?

When an animal runs at a steady speed on the flat, it hardly takes any mechanical effort at all, storing energy in the leg's elastic tendons at the beginning of a step, and recovering it as the foot leaves the ground. An efficient runner can recover almost all of the stored energy, sprinting at virtually no mechanical cost. But going uphill is a different matter. The animal's leg muscles must work hard to raise it up a slope. Knowing that each muscle in the leg has a unique role in support and locomotion, Daley decided to focus on the gastrocnemius and a digital flexor at the guinea fowl's ankle,to compare how they worked on the flat with running up an incline.

Daley fitted the birds with a variety of sensors, to measure the force and work done by both ankle muscles as the birds walked and trotted on the flat,and up a relatively steep slope. Needing only a few steps of steady running at every speed, Daley soon had enough data to begin untangling how the muscles functioned as the birds ran.

Not surprisingly, the digital flexor with its long tendon was `an economic force producer' says Daley, doing little mechanical work, yet storing large amounts of energy in the tendon during each stride along the flat. However, as the birds began running uphill, the muscle failed to do more work. So a running guinea fowl couldn't modulate the digital flexor's function as it ran uphill.

Next Daley focused on the gastrocnemius muscle, but she was in for a surprise. The gastrocnemius muscle's tendon wasn't at all springy,contributing little as it ran on the flat. Yet it managed to increase the muscle's work output as the birds rushed up the simulated hill. Daley suspects that the gastrocnemius is better at modulating its work output than the digital flexor, and although it probably doesn't contribute much while the bird is running, it could help on the few occasions when the bird must leap to freedom.

But that wasn't the only surprise. When Daley took a closer look at the bird's digital flexor muscle's performance on the flat, she was amazed to see a startling degree of variation in the muscle's work output between steps. No two steps were the same, with some producing work while others didn't. But over a series of strides, the work averaged out; as expected, the bird produced no overall mechanical work as it ran on the flat. Daley explains that the work produced by this muscle is extremely sensitive to the bird's leg posture, so the muscle's work output is affected by subtle changes in posture between steps. Could this mean that the digital flexor is an entirely self-stabilising muscle, spontaneously reacting to each stride? Daley isn't sure, but if it is, it could keep the bird trotting over terrains where others would have tumbled.

Daley, M. A. and Biewener, A. A. (
). Muscle force-length dynamics during level versus incline locomotion: a comparison of in vivo performance of two guinea fowl ankle extensors.
J. Exp. Biol.