The knee and ankle extensors as human primary antigravity muscle groups are of utmost importance in a wide range of locomotor activities. Yet, we know surprisingly little about how these muscle groups work, and specifically, how close to their maximal capacities they function across different modes and intensity of locomotion. Therefore, to advance our understanding of locomotor constraints, we determined and compared relative operating efforts of the knee and ankle extensors during walking, running and sprinting.
Using an inverse dynamics biomechanical analysis, the muscle forces of the knee and ankle extensors during walking (1.6 m/s), running (4.1 m/s) and sprinting (9.3 m/s) were quantified and then related to maximum forces of the same muscle groups obtained from a reference hopping test that permitted natural elastic limb behavior.
During walking, the relative effort of the ankle extensors was almost two times greater compared with the knee extensors (35±6 vs 19±5%, P<0.001). Changing walking to running decreased the difference in the relative effort between the extensors muscle groups, but still the ankle extensors operated at a 25% greater level than the knee extensors (84±12 vs 63±17%, P<0.05). At top speed sprinting, the ankle extensors reached their maximum operating level, whereas the knee extensors still worked well below their limits, showing a 25% lower relative effort compared with the ankle extensors (96±11 vs 72±19%, P<0.01).
Regardless of mode of locomotion, humans operate at much greater relative effort at the ankle than knee extensor muscles. As a consequence, the great demand on ankle extensors may be a key biomechanical factor limiting our locomotor ability and influencing the way we locomote and adapt to accommodate compromised neuromuscular system function.