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1. Introduction
The blood lactate responses during incremental test preceded by a high-intensity exercise exhibit an U-shape pattern. The exercise intensity associated to the minimum blood lactate concentration ([bLac]) during test was first suggested to be the equilibrium point between blood lactate production and removal [1] and has been called lactate minimum (LM) intensity [2]. The LM test has been applied in several exercise modes and conditions [2–7] and shown to be associated to the maximal lactate steady state (MLSS) [3, 4, 7, 8] which is the gold standard among protocols of aerobic fitness evaluation derived from the [bLac] responses to exercise [9, 10].
Despite the variations on procedures for the [bLac] elevation [7], the length of the recovery period preceding the incremental test [11], and the number of incremental stages, the validity of LM as an index of MLSS was well demonstrated in running, cycling, and swimming [2, 4, 8], both in laboratory and field conditions [2, 5, 6]. However, to our knowledge the LM protocol has not been applied on walking as an exercise mode yet.
Walking is an exercise mode practiced by most people of any age or aerobic fitness level [12]. It is clear that walking and running are markedly different in terms of ground reaction force, ground contact time, duty factor, and patterns of mechanical energy fluctuations [13, 14]. Also, human walking is always performed with at least one foot in contact with the ground, which leads to a lower bouncing impact compared with running or jogging. During running exercise the eccentric exercise-induced muscle damage can also be larger than during walking, and this damage can lead to greater inflammatory process and muscle injury [14, 15]. Hence, walking produces less risk for musculoskeletal lower extremity injury than running because it is associated with lower reaction forces in low extremities tendons and joints [13, 15–19].
Walking tests have been used and suggested for physical fitness assessment, training prescription, and rehabilitation in different populations, using maximal and submaximal tests and/or exercises [20–22]. Thereby, the ability of indentifying an exercise intensity that evaluates the aerobic fitness using the LM protocol in walking test can be one more important option and also be applied to people that cannot perform maximum running test because of...