SUMMARY

Load carriage during walking (e.g., a backpack), is prevalent in everyday tasks as well as industrial and military settings. During loaded walking, the need to support additional weight and maintain dynamic balance induces immediate changes in joint mechanics and lower body muscle activations patterns. How the acute changes in workload of lower-limb muscles manifests during longer duration, fatiguing walking bouts remains unclear. Based on evidence that load carriage re-distributes the mechanical demands toward distal muscles (e.g., ankle plantarflexors) and away from proximal muscles (e.g. hip extensors), we hypothesized that distal muscles would fatigue at a faster rate than proximal muscles. To test this hypothesis we recruited 8 young heathy adults and compared the rate of change in the mean power frequency (rMPF), a common biomarker of muscle fatigue, for key ankle and hip muscles over a prolonged 30 min walk, during loaded and unloaded conditions. As expected, load carriage caused an immediate increase in recruitment of active muscle volume that was larger for ankle versus hip muscles. However, contrary to our hypothesis, we found a larger negative rMPF (i.e. more fatigue) for hip extensors (e.g., biceps femoris = -.697 +/- .458 Hz/min ) than for the ankle extensors (e.g., soleus = .077 +/- .150 Hz/min) during loaded walking (p = 0.0022). This finding suggests the possibility for a motor control strategy that acts to prioritize reliance on proximal muscles during long, highly demanding walking bouts in order to limit fatigue in key distal muscles that are important for efficient propulsive power output.