Running economy, defined by de Oliveira Assumpcao, et al 2013, as the energy demand for a given velocity of submaximal training, will determine how two athletes of equivalent VO2max will race. Now, few people only train running but incorporate sports activity, weight training, plyometrics and other forms of training. This creates muscle damage which can impact your running economy.
The authors reviewed different studies. The literature currently demonstrates that while a single bout of lower extremity strength training can decrease running economy severely (20-30%), repeated bout effects and adaptation mediate this decrease. Adapted strength training athletes lose 0-5% of their running economy, depending on the type of training they are performing.
Similarly, downhill running for overspeed work has been incorporated in many running programs but is also known to create soreness from the eccentric activation. Studies again showed that running for 30 min at a -15% grade generated a 20-30% drop in running economy in athletes unaccustomed to downhill running. Once adaptation had taken place, the drop in performance was 0-7%.
Given that high intensity weight training can produce >90% VO2max while downhill running was measured to average at 65% of VO2max, it has been hypothesized that the 2 mechanisms involved are separate and require different adaptations.
This study provides a basis for what athletes and coaches have known for ages. When integrating a new component into your training, everything suffers for a bit. As you get better and more accustomed to the new stimulus, your whole game comes back and has the potential to improve.