The aim of resistance training is to provoke neuromuscular adaptations. These adaptations usually result in gains in strength and muscle mass and are accompanied by fatigue. Fatigue can be defined as an inability to generate maximal voluntary force. In practice, it is reflected in the number of fewer reps we can do in the umpteenth set, or the lesser load we can move the day after a heavy workout, for example. Neuromuscular fatigue generated by training is caused by peripheral fatigue and central fatigue.
Peripheral fatigue is localised to the muscle(s) directly involved in the effort being made. The mechanical stress on the fibres and the accumulation of various metabolites during exercise decrease the capacity of a muscle to produce force. Central fatigue is fatigue of the central nervous system (so-called "nervous fatigue"), i.e. the motor cortex and spinal cord. Central fatigue is not the feeling of physical and mental soreness experienced after a workout, but the inability to recruit a muscle to its maximum capacity. It is caused by a decrease in motor neuron activity, an increase in inhibitory afferent feedback and a reduction in individual motor neuron response. In contrast to peripheral fatigue, central fatigue affects the whole body. It can be induced by heavy, light and even cardiovascular endurance training. It is often said and written that it is more difficult and longer to recover nervously than muscularly (i.e. from central fatigue than from peripheral fatigue). But is this really the case?
To try to answer this question, a team of British researchers studied the impact of strength and power training on fatigue and recovery in high-level athletes. To do this, the researchers recruited 10 athletes (4 women and 6 men) who were track and field specialists, international sprinters or long jumpers (100m: 10.44 ± 0.37s and 11.73 ± 0.34, for men and women, respectively), and used to strength training (1RM Squat: 190.0 ± 38.0kg and 107.5 ± 12.0kg, for men and women, respectively).
Each athlete performed two sessions: one focused on maximal strength training and one focused on power training. Each session consisted of 3 exercises, 4 sets of 5 repetitions with 3 min rest: squat, split squat and push press. The load used in the strength session corresponded to an RPE (rate of perceived exertion) of 16-17, i.e. very hard. For the power training, the athletes used 30% of the load used in the strength training.
Before, 10 minutes immediately after and 24 hours after the session, all athletes were subjected to various tests to assess the level of fatigue and recovery. So, a vertical jump test with countermovement (without arms), a maximum isometric voluntary contraction test during knee extension and a central-activation-ratio assessment (CAR) were performed. Blood lactate was also measured before and 4 minutes after the session. Note that the vertical jump test and the CAR test are more or less direct indicators of central nervous system activity and fatigue. The CAR test is performed by creating a superimposed stimulation during the maximum isometric contraction test. CAR is the ratio of the maximum isometric contraction force to the sum of this force and the superimposed stimulation force. A result of 1 indicates full central activation.
The main results of this study show that maximal isometric strength was decreased after the strength session up to 24 hours after the workout, but this was not observed after the power session. In contrast, no change was observed for measures related to central fatigue (the vertical jump test and the CAR test), regardless of the session. These results are mainly explained by the greater mechanical work performed during the strength session, as well as the higher blood lactate concentration achieved indicating a more metabolically demanding workload.
The reduction in maximal isometric strength with no change in central activation or vertical jump performance suggests that the mechanisms responsible for the post-training decline in strength are primarily due to peripheral fatigue rather than central fatigue. Other studies have shown that nerve fatigue is usually measurable just after the end of exercise but disappears quickly. Thus, since here the researchers waited 10 minutes to perform the post-workout tests (so as not to compromise the measurements, as muscle pH and ischaemia could influence action-potential propagation and contractile function), it is possible that nervous fatigue was present after the end of the session but that the body had recovered before the tests.
This study shows that even in high level athletes, a strength training does cause muscle fatigue (and this is greater than for a power training), caused by the high mechanical tension on the muscle fibres and the high mechanical workload, but it does not necessarily cause nervous fatigue. And if there is nervous fatigue, it disappears very quickly after the end of the exercise (other studies corroborate these results).
Thus, the fatigue that is felt after a heavy session is mainly linked to muscular fatigue which can impact our muscular performance for up to 72 hours. However, this does not mean that nervous fatigue does not exist. It has been observed in numerous studies, particularly during long cardiovascular efforts. Furthermore, some studies indicate that it could be caused by the production of certain metabolites, such as muscular ammonia, which in excess in the blood could be neurotoxic and cause central fatigue.
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