A system for monitoring an athletic performance, comprising: an input system for: receiving, from one or more foot sensor systems, launch input data relating to a plurality of foot launch events during a first time period, wherein the launch input data includes a timestamp associated with each foot launch event; and receiving, from the one or more foot sensor systems, strike input data relating to a plurality of foot strike events during the first time period, wherein the strike input data includes a timestamp associated with each foot strike event; a processor system comprising: one or more processors; and memory storing computer executable instructions that, when executed by the one or more processors, cause the system to at least perform: determining, by the one or more processors, a first flight time associated with a first set of timestamps of a plurality of timestamps associated with temporally adjacent foot launch events and foot strike events; and determining, based on the first flight time, an energy expenditure of a user during the first time period, wherein the first flight time is a time when both a left foot and a right foot of the user are simultaneously not in contact with a ground surface. an input system for: receiving, from one or more foot sensor systems, launch input data relating to a plurality of foot launch events during a first time period, wherein the launch input data includes a timestamp associated with each foot launch event; and receiving, from the one or more foot sensor systems, strike input data relating to a plurality of foot strike events during the first time period, wherein the strike input data includes a timestamp associated with each foot strike event; receiving, from one or more foot sensor systems, launch input data relating to a plurality of foot launch events during a first time period, wherein the launch input data includes a timestamp associated with each foot launch event; and receiving, from the one or more foot sensor systems, strike input data relating to a plurality of foot strike events during the first time period, wherein the strike input data includes a timestamp associated with each foot strike event; a processor system comprising: one or more processors; and memory storing computer executable instructions that, when executed by the one or more processors, cause the system to at least perform: determining, by the one or more processors, a first flight time associated with a first set of timestamps of a plurality of timestamps associated with temporally adjacent foot launch events and foot strike events; and determining, based on the first flight time, an energy expenditure of a user during the first time period, wherein the first flight time is a time when both a left foot and a right foot of the user are simultaneously not in contact with a ground surface. one or more processors; and memory storing computer executable instructions that, when executed by the one or more processors, cause the system to at least perform: determining, by the one or more processors, a first flight time associated with a first set of timestamps of a plurality of timestamps associated with temporally adjacent foot launch events and foot strike events; and determining, based on the first flight time, an energy expenditure of a user during the first time period, wherein the first flight time is a time when both a left foot and a right foot of the user are simultaneously not in contact with a ground surface. determining, by the one or more processors, a first flight time associated with a first set of timestamps of a plurality of timestamps associated with temporally adjacent foot launch events and foot strike events; and determining, based on the first flight time, an energy expenditure of a user during the first time period, wherein the first flight time is a time when both a left foot and a right foot of the user are simultaneously not in contact with a ground surface. <EOS>