Fatigue-Independent Alterations in Muscle Activation and Effort Perception During Forearm Exercise: The Role of Local Oxygen Delivery
Oxygen Conforming Response , Local Oxygen Delivery , Muscle Activation , Perception of Effort , Voluntary Exercise
In exercise, the sensations of effort to make the muscles do exercise can contribute to an individual’s decision to stop exercise or decrease exercise intensity. Skeletal muscle fatigue is one mechanism by which the effort required to continue exercise can be increased. In order to continue exercise with fatigued skeletal muscle we must increase the amount of electrical signals sent to the muscle. This increase in electrical signals sent to the muscle is perceived by the exercising individual and is interpreted as increased perception of effort to maintain the same exercise intensity. Another mechanism which may also require increased electrical activity sent to the muscle and therefore may increase perception of effort for the same exercise intensity is exercise where oxygen delivery is suboptimal, resulting in reduced oxygen levels (oxygenation) in the muscle. When human and animal muscle is electrically stimulated, force produced for a given electrical stimulation decreases when muscle oxygenation is reduced. When muscle oxygenation is restored force produced is also rapidly restored for the same electrical stimulation. This rapid adjustment of force for the same electrical stimulation with changes in muscle oxygenation is referred to as the oxygen conforming response (OCR). However, the existence of an OCR in voluntary human exercise has not been established and it remains to be determined if our perceptions of effort will follow changes in electrical signal sent to the muscle as a result of the OCR. We tested these unanswered questions by intermittently reducing forearm blood flow (FBF) during human (n = 16) handgrip exercise. When FBF was reduced, electrical activity and perception of effort increased. Interestingly, upon the first restoration of FBF neither electrical activity nor perception of effort were restored. We interpreted these findings to be consistent with skeletal muscle fatigue development during the first compromise to muscle oxygenation. Upon the second restoration of FBF both electrical activity and perception of effort were restored, which is consistent with an OCR and its ability to rapidly modify perception of effort in voluntary human exercise. Through its effects on effort perception, the OCR may therefore have important implications for exercise behaviour.