. muscular fatigue,

. mental fatigue. 

Sensorial fatigue and muscular fatigue are relatively well defined. These can be objectified and measured. Mental fatigue however is more difficult to characterize and to assess. It must be attributed to the highly subjective character of fatigue: one feels "tired", but it is sometimes difficult to translate this sensation into objective manifestations. Indicators therefore remain essentially subjective. Performance tests can however show degradation in those mental functions (memory, attention, etc.) that are related to fatigue.
One of the main characteristics of fatigue lies in the fact that it is a phenomenon that progressively builds up with time (fatigue accumulation). Fatigue is not necessarily related to an excessive workload, to rest, which is too short, or even to disturbed sleep.
A given task may generate feelings of fatigue for certain subjects and lead to no particular fatigue whatsoever in others. This refers us back to the psychological aspects of motivation and satisfaction in the task to be accomplished. For these aspects, great inter-individual differences are observed.
 
Recovery from fatigue, i.e. return to a rested condition, is also poorly known and is associated with inter- and intra-individual differences.
Thus, in certain cases and for certain subjects, recovery seems to be very long in spite of sufficient sleep of satisfactory quality.
These aspects must therefore be subject to in-depth research in order to determine all factors influencing level of fatigue and its manifestations and consequences.
In particular, the accumulation of fatigue remains unexplored even if it is of capital importance in aviation. Moreover, there is a lack of documented studies on the consequences of successive jet lags when operating in opposite directions over long periods of time.
 
 

 
 
 
 
In spite of the fatigue aspects which are still poorly known, certain manifestations  (i.e. ↓ lowering, decrement or decrease, ↑ rising, increment or increase) have been identified in the physiological and psychomotor fields.
 
In the physiological field
 In the psychomotor field
 
↓ in body temperature
 ↓ in memory
 
↓ in muscular force or strength
 ↓ in communications skill
 
↓ in binocular vision
 ↓ in ocular tracking           (following objects with eyes)
 
↓ in volume of circulating blood
 ↓ in attention span
 
↓ in muscular glycogen (energy stores)
 ↓ in personal care
　
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