During running, the foot-ground impact results in a rapid deceleration of the lower extremity that is transmitted through the body from the foot to the head. Shock attenuation (S A) was defined as the reduction of shock wave energy between the foot and head. Shock energy is absorbed through bone, cartilage, and muscle, for example, with muscle having the greatest potential to absorb energy. The purpose of this study was to determine the effect of fatigue on SA. Two types of fatigue were investigated:​ (a) system, and (b) quadriceps. System fatigue was defined as a decreased force generation capability of the entire neuromusculoskeletal system brought on by a graded exercise test. Quadriceps fatigue was defined as a decreased force generation capability of the quadriceps muscles as a result of exhaustive knee extension exercises.

Two groups of ten subjects completed either a quadriceps or system fatigue protocol. A third group often subjects completed both fatigue protocols on separate days. All subjects completed at least one non-fatigued run followed by a fatigue protocol (either quadriceps or system) followed by at least one fatigued run. Acceleration data were collected (1000 Hz) using two surface mounted accelerometers, one attached on the forehead, the other on the distal end of the tibia.

Following both types of fatigue, SA was poorer than the non-fatigued condition (p<​0.05). During running following system fatigue compared to non-fatigued running, VO₂ was greater (p<​0.05), while there was no change in VO₂ during running following quadriceps fatigue (p>​0.05). Stride length was determined to be functionally unchanged during running following either fatigue condition. The components of SA, head and leg powers, were determined to be similar during running following both fatigue conditions. Mathematically, the changes in SA were explained by a small increase in head shock (HS) concurrent with a decrease in leg shock. It was concluded that running while fatigued resulted in a poorer S A compared to non-fatigued running. Furthermore, despite the small increase in HS following fatigue, it was concluded that the mechanism for managing fatigue during running resulted in maintaining HS within a limited range of shock.