Anterior cruciate ligament (ACL) injuries are still seen frequently on the slope. The backward fall may cause an ACL rupture. To reduce the load on the knee, the design of a ski boot with a “giving-way” spoiler is discussed. Past studies have excluded the importance of muscle activity during the backward fall. Therefore, this research tends to determine muscle activity depending on the spoiler flexibility. Electromyographic measurements of six test subjects were combined with a two-dimensional motion analysis under controlled laboratory conditions. One ski boot with a stiff spoiler was compared with three flexible spoiler prototypes. The signal of four muscles (m. vastus med., m. vastus lat., m. biceps fem., m. tibialis ant.) was recorded bilaterally. Analysis of the experiments with the stiff spoiler shows a muscular imbalance between quadriceps and hamstring muscles. Unlike this, the flexible spoiler leads to a more balanced muscle activity pattern. Especially the m. biceps femoris shows more activity.
These results could possibly explain the mismatch of calculated mathematical models with the actual injury mechanism. Additionally, they allow the conclusion that appropriate ski boot design stimulates muscle activity and may protect the knee—in particular the ACL.