Experimental case study with a modified ski boot: Does the spoiler design influence muscle activity and kinematics during landing?

For the investigation of the boot-induced anterior cruciate ligament injury an experimental case study with a prototype ski boot was performed. The back spoiler of this boot was designed to yield by only a few degrees when a certain level of backward moment was reached. A mechanic switch was used to activate or to block this mechanism. Over a prepared hill, a single subject performed ten jumps with an activated mechanism and ten jumps with a blocked spoiler mechanism in randomized order. During the jump and landing, an electromyograph (EMG) of eight lower extremity muscles was recorded and synchronized to two S-VHS cameras that were located for two-dimensional kinematic analysis. The goal of these experiments was to quantify the muscular and kinematic effects caused by the new spoiler characteristics and also to establish a method for their judgment. For the quantification of the muscular effects, an activity quotient was defined and, for the analysis of the kinematics, the time history of three body angles and their variation between the trials were found to be appropriate. The results of this case study revealed clear differences between the two treatments. In particular, the activity quotient of the hamstring muscles showed a remarkable increase with the spoiler mechanism activated. The findings encourage further investigations in this direction, for example, a randomized field study using the derived method.