CNS fatigue is… not central. — Transcript

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1. 0:00You've probably heard claims like these:
2. 0:02“You should be very careful with deadlifts because they fry the CNS”, or
3. 0:06“Take a deload weak every couple of weeks because central fatigue accumulates”.
4. 0:10Most of the people making these claims have no idea
5. 0:13what CNS fatigue actually is.
6. 0:17Research finds that neuromuscular fatigue
7. 0:19after deadlifts is actually no greater than after squats or bench presses.
8. 0:23Even leg presses do not stimulate more neuromuscular fatigue than leg extensions.
9. 0:29Central nervous system fatigue is not what the name suggests at all.
10. 0:33Central nervous system fatigue is not actually central in nature.
11. 0:38Most of the fatigue from the CNS is actually local, peripheral.
12. 0:43The popular idea about CNS fatigue is that the central nervous system,
13. 0:47in particular - the brain,
14. 0:49has some dysfunction and therefore it affects your entire body.
15. 0:52In that case, if the fatigue is in the brain,
16. 0:55in the CNS, the effect is central, meaning it affects all of your body parts,
17. 0:59and therefore this type of central fatigue
18. 1:01would affect your biceps just as much as your quads.
19. 1:04However, most of CNS fatigue in exercise science
20. 1:07actually takes place in the motor neuron.
21. 1:09Now, this is technically called CNS fatigue
22. 1:12because the cell body of the motor neuron, which is what transmits
23. 1:16the electrical activity from the brain to the muscle to activate it,
24. 1:20lies in the CNS, specifically in the spinal cord.
25. 1:23It's not even in the brain, it's in the spinal cord.
26. 1:25But the axon, like a tentacle, extends outward into a muscle,
27. 1:29and it is that specific part of the chain where the fatigue takes place.
28. 1:34So it's the motor neuron that is fatigued,
29. 1:37and the effect of that is only the fibers that it innervates.
30. 1:40So it's a very specific part,
31. 1:42and it's just that muscle that is affected by the fatigue of the motor neuron.
32. 1:46But technically we call this CNS fatigue in exercise science
33. 1:50because the motor neuron is part of the CNS.
34. 1:53This confuses a lot of people because they naturally assume that
35. 1:57CNS fatigue, “Central Nervous System” fatigue is in the brain
36. 2:02and therefore must be central in effect.
37. 2:04It is not.
38. 2:05It is local.
39. 2:06The vast majority of CNS fatigue is related simply to muscle damage.
40. 2:10It is entirely local in effect, and it only affects the muscle group
41. 2:14that was trained.
42. 2:14In fact, the whole idea of central fatigue as a concept
43. 2:19is highly questioned in exercise science, especially for strength training.
44. 2:24For example, one study found that all of the CNS changes
45. 2:27that occurred after ten sets of squats were local in effect.
46. 2:32There was no evidence of central fatigue.
47. 2:34They were seen as changes,
48. 2:35but they were related to the motor neurons, not to the actual brain,
49. 2:40the overall capacity to direct the body as a whole.
50. 2:43The strongest refutation of the idea of central fatigue is probably
51. 2:47a 2021 meta-analysis of the entire literature that concluded that for strength training
52. 2:52central fatigue is generally trivial or absent, in contrast
53. 2:55to endurance training where we do actually have evidence of central fatigue.
54. 2:59If you think about it, that makes sense, because with endurance training,
55. 3:02you train the heart, the lungs and central systems a lot more.
56. 3:05Strength training is, by and large, local.
57. 3:08You train your biceps, the biceps gets fatigued, the biceps adapts.
58. 3:11There is no other process that occurs in the quads, for example.
59. 3:15When you train the quads, the quads adapt, there’s stress in the quads,
60. 3:18fatigue in the quad, growth in the quads,
61. 3:20nothing happens in the biceps.
62. 3:21Endurance training is a more central training in general, so it makes sense
63. 3:25that there could be central fatigue after endurance training,
64. 3:27but for strength training there isn't.
65. 3:30In the words of the researchers, they concluded: “Overall, the findings
66. 3:33do not support the existence of a general non-local muscle fatigue effect.”
67. 3:38What had happened on social media
68. 3:39is that they take a concept from exercise science, which is real CNS fatigue,
69. 3:43they don't understand what it means, and they conflate it with central fatigue,
70. 3:46and therefore they make it out to be some general boogeyman
71. 3:49with all sorts of implications that do not actually make any sense.
72. 3:52The absence of any central fatigue after resistance training
73. 3:56is actually very easy to verify yourself.
74. 3:58You can just do a lower body workout and then check your biceps curl performance.
75. 4:02Your biceps curls should not be any weaker because you did a bunch of leg extensions,
76. 4:06squats, or any other exercise for the lower body beforehand.
77. 4:10Similarly, after an upper body
78. 4:11workout, your leg extensions or your leg curls should not be any weaker
79. 4:14just because you trained other muscle groups.
80. 4:17When I teach this to my students
81. 4:18they often protest: “But I can clearly feel the fatigue.”
82. 4:22Well, what you feel is mental fatigue.
83. 4:24Mental fatigue is measured as a subjective sensation.
84. 4:28In exercise science fatigue refers to loss of force production.
85. 4:31So if your biceps curl 8 rep max is still the same, you can still do
86. 4:368 reps with your 8 rep max
87. 4:37then there is no neuromuscular fatigue, but there might still be mental fatigue.
88. 4:41We see big discrepancies between mental and physical fatigue in many scenarios.
89. 4:45For example, music can make you a lot more energized
90. 4:48and can get rid of mental fatigue,
91. 4:50but it doesn't do anything for neuromuscular fatigue, obviously.
92. 4:53Mental fatigue and loss of willpower sometimes called ego depletion
93. 4:57in psychological research, actually occurs even when you do
94. 5:00any other type of activity that's not physical at all, but it is
95. 5:03a “have to” activity, something you don't necessarily want to do.
96. 5:06Whereas if you're doing something that you really enjoy,
97. 5:08that you're passionate about, you don't suffer a lot of willpower depletion.
98. 5:11For example, you might experience a lot of fatigue
99. 5:13when visiting family members that you don't necessarily want to see,
100. 5:16or when you're in traffic and you're commuting.
101. 5:19Truck drivers experience a lot of mental fatigue, typically,
102. 5:21but they're not actually doing anything physically,
103. 5:23they're just sitting in their truck.
104. 5:24But they develop mental fatigue because it's boring.
105. 5:27In fact, boredom and mental fatigue go hand in hand very strongly.
106. 5:31Both occur during “have to” activities instead of “want to” activities.
107. 5:35Mental fatigue
108. 5:35can be thought of as a system that tells us that
109. 5:37there is a lack of instant gratification for what we are currently doing.
110. 5:41Going to the gym is the ultimate lack of instant gratification.
111. 5:44You have to push yourself past your comfort zone and suffer
112. 5:47immediate pain to get later future results.
113. 5:51As a result, in the gym we develop mental fatigue and even though mental fatigue
114. 5:55is just psychological, in large amounts it can actually impair performance.
115. 5:59So some people argue that this is all semantics.
116. 6:02It's all just giving the fatigue a different name,
117. 6:05but it is central fatigue.
118. 6:06While it is central fatigue,
119. 6:08it is physiologically a very different concept.
120. 6:10If there was indeed neuromuscular fatigue in the brain, that would warrant
121. 6:14a deload or some other reduction in training stress.
122. 6:17However, knowing that the fatigue is mental
123. 6:20has a completely different implication, because mental fatigue
124. 6:23you can get rid of with caffeine or music, or just pushing through
125. 6:27knowing that it is safe and that it is mental fatigue.
126. 6:30So I hope this video helps demystify a lot of the bro science about deadlifts,
127. 6:34frying your CNS, or needing deloads because of central fatigue accumulation.
128. 6:38In reality, central nervous system fatigue, despite what the name suggests,
129. 6:42is mostly local in effect, and it is nothing to worry about.
130. 6:46It is a normal part
131. 6:47of neuromuscular fatigue, and as long as your performance is okay, you are fine.
132. 6:51So in practice you just have to measure your performance.
133. 6:54As long as you are progressing in a gym, you are fine
134. 6:56and you don't have to worry about any CNS fatigue boogieman.
135. 6:59If you're tired of slop
136. 7:00and misinformation on social media, you will probably love my Online PT Course.
137. 7:04The link is in the description.
138. 7:06Or
139. 7:07you can keep watching my own YouTube slop
140. 7:09like this video about the theory that broke fat loss science.