An apparatus, comprising: a processor; an interface; an oxygenation sensor, configured to be positioned proximate an area of skin of a user, the oxygenation sensor further configured to output data indicative of a muscle oxygenation of a body tissue of the user; a non-transitory computer-readable medium comprising computer-executable instructions that when executed by the processor are configured to perform at least: receive muscle oxygenation data from the oxygenation sensor during an exercise session comprising at least a first portion comprising an intermittent activity; calculate a change in muscle oxygenation as a difference between a current muscle oxygenation value and a previous muscle oxygenation value obtained from the same tissue and during the same exercise session; calculate a rate of change for the change in muscle oxygenation; based only on the rate of change in muscle oxygenation and using a graph of average rate of change in muscle oxygenation against measured time to exhaustion obtained from exercise trial data, calculate an estimated time to exhaustion for the user, wherein exhaustion is defined by when the user can no longer continue exercising as confirmed by measurements of one or more parameters selected from lactate, maximal heart rate and respiratory exchange rate; and based upon the estimated time to exhaustion for the user, cause an output configured to notify the user to alter behavior to result in a different time to actual exhaustion by portraying to the user the estimated time to exhaustion for behavior modification. a processor; an interface; an oxygenation sensor, configured to be positioned proximate an area of skin of a user, the oxygenation sensor further configured to output data indicative of a muscle oxygenation of a body tissue of the user; a non-transitory computer-readable medium comprising computer-executable instructions that when executed by the processor are configured to perform at least: receive muscle oxygenation data from the oxygenation sensor during an exercise session comprising at least a first portion comprising an intermittent activity; calculate a change in muscle oxygenation as a difference between a current muscle oxygenation value and a previous muscle oxygenation value obtained from the same tissue and during the same exercise session; calculate a rate of change for the change in muscle oxygenation; based only on the rate of change in muscle oxygenation and using a graph of average rate of change in muscle oxygenation against measured time to exhaustion obtained from exercise trial data, calculate an estimated time to exhaustion for the user, wherein exhaustion is defined by when the user can no longer continue exercising as confirmed by measurements of one or more parameters selected from lactate, maximal heart rate and respiratory exchange rate; and based upon the estimated time to exhaustion for the user, cause an output configured to notify the user to alter behavior to result in a different time to actual exhaustion by portraying to the user the estimated time to exhaustion for behavior modification. receive muscle oxygenation data from the oxygenation sensor during an exercise session comprising at least a first portion comprising an intermittent activity; calculate a change in muscle oxygenation as a difference between a current muscle oxygenation value and a previous muscle oxygenation value obtained from the same tissue and during the same exercise session; calculate a rate of change for the change in muscle oxygenation; based only on the rate of change in muscle oxygenation and using a graph of average rate of change in muscle oxygenation against measured time to exhaustion obtained from exercise trial data, calculate an estimated time to exhaustion for the user, wherein exhaustion is defined by when the user can no longer continue exercising as confirmed by measurements of one or more parameters selected from lactate, maximal heart rate and respiratory exchange rate; and based upon the estimated time to exhaustion for the user, cause an output configured to notify the user to alter behavior to result in a different time to actual exhaustion by portraying to the user the estimated time to exhaustion for behavior modification.