# Cycling Biomechanics and Its Relationship to Performance
## 6 Measures of Muscle Fatigue  
Fatigue can be defined as a reduction in muscle force or power that occurs with exercise [72]. It corresponds (1) to a reduced ability to maximally activate the muscles and as a feeling of tiredness and exhaustion (central aspect, factors above the neuromuscular junction), and (2) to the deterioration of the muscle functions (peripheral aspect, factors below the neuromuscular junction). A decrease in maximal power output during short sprints is therefore a simple test to assess both the central and peripheral aspects of fatigue during cycling [25]. Conversely, the subjective perception of fatigue, as evaluated by analog scales (e.g., Borg scale of perceived exertion), is a simple means of assessing the central aspect of fatigue.  
Central fatigue can also be evaluated using the twitch-interpolation technique, which consists of stimulating the nerve supplying the muscles during a voluntary contraction and comparing the gain in force associated with the stimulation [73]. Peripheral muscle fatigue is in general evaluated by the change in muscle force elicited by electrical stimulation of the muscle itself, using surface electrodes, or by the stimulation of its motor nerve [72]. However, these methods require specific equipment in order to be performed immediately after dynamic cycling tests [73]. It must be stressed also that during complex movements, coordination is a non-negligible factor in performance. Central fatigue might alter the ability to properly execute the technique, which would decrease the performance, although the muscle force _per se_ might not be affected.  
EMG has been used to assess neuromuscular fatigue by analyzing the change in the mean or median frequency of the spectrum of the EMG signal. The spectrum of the EMG signal refers to its frequency content obtained after a Fourier decomposition of the signal. Mean or median frequencies generally decrease with fatiguing contractions because of the slowing of the action potentials at the surface of the muscle membrane (sarcolemma). However, such an analysis of the surface EMG during fatiguing dynamic contractions has strong limitations (e.g., movement of the electrodes, non-stationarity of the signal) [62] and should be interpreted with caution.  
An increase in EMG signal amplitude has also been used as an indicator of fatigue. Amplitude of the EMG signal increases with fatigue at a constant and non-maximal power output [74]. This increase is attributed to the progressive recruitment of additional motor units and/or to changes in the properties of the sarcolemma. In both cases, this indicates the presence of muscular fatigue. However, in order to confirm that the increase in EMG is linked solely to muscle fatigue and not to changes in technique, it is important to ascertain that movement kinematics were not altered and that no form of compensation occurred, which is difficult to achieve in most cases.  
In conclusion, fatigue can be assessed easily by measuring a reduction in performance during maximal efforts or using analog scales. However, using such assessments the origin of the decrease in performance remains unknown, as it may originate from changes in muscle coordination, an inability to fully activate the muscles or altered muscle functions. Finally, the use of the EMG signal is not recommended for the analysis of muscle fatigue.