Fatigue and muscle activation.

"Increased muscle activation with decreased movement in a fatigued state may represent an effort to increase trunk stiffness to protect lumbo-pelvic-hip structures from overload"

No rocket science here . . . but good to remember that fatigue sets us all up for injury if one does not observe and listen to the signs of fatigue . . . . especially when athletic and loading demand is increasing rather than tapering at the same time as the fatigue is building. As we fatigue, compensation recruitment is supposed to generate more stiffness to protect the motor units. But, can this be at a cost ?

This study looked at whether fatigue may affect muscle recruitment, active muscle stiffness and trunk kinematics, compromising trunk stability. The purpose of this study was to compare trunk muscle activation patterns, and trunk and lower extremity kinematics during walking gait before and after exercise.

The study used surface EMG to look at the rectus abdominis, external oblique, erector spinae, gluteus medius, vastus lateralis, and vastus medialis in a group of otherwise healthy individuals.

Essentially the study concluded that:
"There was less trunk and hip rotation from initial contact to midstance after exercise. Neuromuscular fatigue significantly influenced the activation patterns of superficial musculature and kinematics of the lumbo-pelvic-hip complex during walking. 
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Gait Posture. 2016 Nov 9;52:15-21. doi: 10.1016/j.gaitpost.2016.11.016. [Epub ahead of print]

Muscle activation patterns of the lumbo-pelvic-hip complex during walking gait before and after exercise.

Chang M1, Slater LV2, Corbett RO1, Hart JM1, Hertel J1.

https://www.ncbi.nlm.nih.gov/pubmed/27846435