Submaximal running energetics are maintained despite local muscle fatigue

When running, metabolic cost increases as muscles are simultaneously fatigued. However, the contribution of an individual muscle group to fatigue-related increase in metabolic costs remains unclear. We investigated the metabolic consequence of running with local plantar flexor or knee extensor fatigue and associated neuromuscular control strategies. Recreational and experienced male runners (N=20) completed two sessions (one per muscle group), with each including two 10 min running bouts: without and with local fatigue ([~]20% reduction in peak joint torque). Net metabolic power and muscle activity (initial and final minutes) were determined. Metabolic power was unaffected by plantar flexor (p=0.367) or knee extensor (p=0.607) fatigue in both cohorts. Plantar flexor fatigue recovered during the fatigued run (p=0.033), while knee extensor fatigue only recovered for the recreational cohort (p=0.009; experienced: p=0.826). With plantar flexor fatigue, plantar flexor muscle activity was unchanged between runs (p[≥]0.312), however initial soleus activity was greater in the unfatigued than fatigued run for experienced runners (p=0.022), and initial medial gastrocnemius activity was greater in the unfatigued than fatigued run for the combined cohort (p=0.009). With knee extensor fatigue, knee extensor muscle activity was mostly lower in the unfatigued than fatigued run (p[≤]0.009), except for final vastus lateralis activity, which was unchanged between runs (p=0.061). Therefore, muscle groups respond with different activation strategies when fatigued. Running with plantar flexor or knee extensor fatigue, at levels like those induced by prolonged running (10-42 km), does not increase metabolic power and thus, submaximal running energetics may be maintained despite local muscle fatigue. NEW & NOTEWORTHYWhile muscle fatigue is suggested to increase the metabolic cost of running, the individual contributions of key lower limb muscle groups have not been explored. We examined responses after fatigue of only the plantar flexors or the knee extensors. Results indicate that local fatigue did not affect the metabolic power of male runners for either fatigued muscle group. These findings enhance our understanding of running performance and the interaction between fundamental criteria dictating human locomotion.