2017年12月24日 星期日

【觀念】關於乳酸的三個迷思



乳酸一直被認為是運動員的天敵,它讓我們深蹲時大腿如燒灼般疼痛,讓我們跑完全馬的隔天雙腳痠軟。許多人用熱敷、緩和運動、按摩、加壓腿套等方式來加速乳酸的排除,這些做法也確實有「部分」的效果能減緩痠痛,促進恢復。


不過,乳酸對肌肉及運動表現的影響,其實沒有那麼簡單直觀,史考特整理了關於乳酸的三大迷思,包括:

  1. 乳酸會累積在體內多久?
  2. 乳酸會造成疲勞與痠痛嗎?
  3. 乳酸是無用的代謝廢物嗎?


希望讓各位對乳酸有更進一步的認識。











甚麼是乳酸?


名不正則言不順,首先我們要來說清楚乳酸到底是甚麼。


在氧氣充足的情況下(註1),大部分的葡萄糖會完全氧化,產生能量及二氧化碳。但在氧氣供應量相對不足時,葡萄糖只能部分燃燒並產生乳酸(Lactate)。




在過去乳酸被認為是一種代謝廢物,它會使肌肉細胞的pH值下降(就是變酸的意思),並造成肌肉痠痛、無力。


但2004年美國學者Robergs提出不同的見解:他認為乳酸上升與pH下降同時發生,並不代表兩者間有因果關係。部分證據顯示,乳酸不但不會使肌肉變酸,反而能中和酸性,使pH下降的兇手應該另有其人。


儘管細節部分尚未釐清,但傳統認為乳酸會使肌肉酸化的想法,可能是錯誤或不完整的。


(註1:過去認為乳酸是身體氧氣不足的情況下,被迫使用無氧呼吸所產生的代謝廢物,但近年來的研究開始質疑這個說法。目前認為除了氧氣不足之外,糖解作用加速、乳酸清除速率降低、快縮肌群被徵召都是乳酸濃度升高的原因。)




乳酸清除的速度比你想得快


按摩熱敷、緩和運動皆被認為能加速乳酸清除,消除肌肉痠痛,但真有必要這麼麻煩嗎?


2010年針對8位耐力型運動員所做的研究顯示,運動後60分鐘不管有穿還是沒穿壓縮腿套,血液中的乳酸濃度都會回復到運動前的水準。



有趣的是,運動剛結束時穿壓縮腿套的運動員乳酸濃度反而稍高一些,想利用壓縮腿套排酸的朋友可能需要重新考慮一下。


血液中的乳酸大概需要一個小時來恢復,但肌肉內的乳酸呢?速度很可能更快!


1994年的研究採用6位年輕男性的左右腳做為實驗及對照組,在劇烈運動至疲勞後,兩腳分別進行「緩和運動」及「完全休息」兩種復原模式,並重複採樣肌肉組織(痛痛!)來看哪一種對乳酸排除較有幫助。





學者發現,輕微活動對肌肉乳酸排除有幫助,不過即使是完全休息的那腳,運動後10分鐘其實也恢復得差不多了。針對越野滑雪所做的研究也發現,10分鐘的收操就能讓肌肉乳酸排得清潔溜溜。


根據以上研究,我們知道肌肉與血液中的乳酸在運動後很快就消失了(註2),超過一小時後再來按摩、拉筋、泡熱水澡、壓縮腿套對排乳酸是沒有幫助的。


(註2:上述研究都是針對有氧運動所做,目前我還沒有看到重量訓練後的乳酸清除的數據。不過,我猜測重量訓練與有氧運動的的乳酸清除率應不致相差太多。)




乳酸造成痠痛與疲勞?


遲發性肌肉痠痛(Delayed-onset muscle soreness, DOMS)是每個運動員都有的經驗,做完劇烈、或是不熟悉的運動後,隔天肌肉才會開始酸痛,所以才有「遲發性」之說。史考特個人的經驗是在運動後12小時會開始有感,24-48小時達到酸痛的頂點,之後才慢慢開始改善。


從時序上判斷,乳酸在運動後一小時內就已經清除完畢,但痠痛至少要12個小時才會產生,要說乳酸造成遲發性肌肉痠痛,真的是有點怪怪的。


早在1983年就有學者質疑過乳酸與肌肉痠痛的關係,他們發現上坡跑雖然產生較多乳酸,但選手的痠痛感不顯著。相反地,輕鬆地下坡跑只產生一點點乳酸,但隔天選手們痠痛到哀哀叫。





照這樣看來,乳酸並不會造成運動後的遲發性肌肉痠痛,但乳酸應該會造成運動「當下」的痠痛與疲勞吧(註3)?


不一定喲!


2004年一篇研究顯示,在運動前服用乳酸竟然能幫助跑者在高速下跑更久,這明顯與「乳酸造成疲勞」的假設不符。2011年的研究也重現了這個結果:比起安慰劑,乳酸能讓運動員在高強度的心肺能力測試中持續更久。







甚至在著名的「科學」期刊上,有一篇文章以「乳酸:最新的體能增強藥物」為名,來說明乳酸增強運動表現的能力。


如果上述資訊使您感到困惑,請記住這點就好:乳酸不會造成酸痛,也不是運動疲勞的主因。


(註3:肌肉細胞內pH值下降目前被認為是運動時肌肉痠痛的原因,回到我們文章一開始所提到的,乳酸本身不會造成pH下降,所以運動的酸痛與疲勞都不該被歸咎到乳酸身上。)




乳酸是無用的代謝廢物?


過去的科學觀念一直認為乳酸是人體在缺乏氧氣下,被迫「飲鴆止渴」的產物,但隨著時代演進,這個觀念受到嚴厲的挑戰。


現在科學家明確知道,乳酸不是一無是處的垃圾,心臟、肌肉、肝臟、大腦都能利用乳酸作為能量來源。







在氧氣不足的環境下,無氧代謝產生的乳酸可以在身體其他地方作為有氧代謝的原料,有點「你的垃圾是我的黃金」的意味。這樣「一條龍」形式的產業鍊,讓全身器官在劇烈運動時一起參與代謝,乳酸是重要的中間產物,絕非亟需清除的污染源。






(譯1:我吃了我的大便,這樣你就不必清理了。)
(譯2:我的黃金就是我的黃金)





結語


本文起先的題目是「關於乳酸的三個迷思」,沒想到文章寫完後竟然整理出五個重點,看來運動生理學還是個不斷在翻新的科學領域啊!


本文的重點回顧:

  • 乳酸不一定是運動時肌肉與血液變酸的原因。
  • 乳酸在一個小時內就會回復到運動前的濃度,一般人實在不需擔心乳酸排除的問題。
  • 乳酸不會造成遲發性肌肉痠痛。
  • 乳酸未必會造成運動時的疲勞,部分研究甚至認為乳酸能促進運動表現。
  • 乳酸不是代謝廢物,人體許多重要器官都能以乳酸作為燃料。


各位運動員,別再錯怪乳酸囉!


46 則留言:

  1. 多謝分享新知!原來我對乳酸的認識全都有很大的問題啊...

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  2. 那麼酸痛的原因到底是⋯

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  3. 一堆跑者在參加馬拉松賽後,都會說我要小跑一下,排一下昨天比賽的乳酸....

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  4. 對啊,那麼運動後12至48小時內發生的酸痛感到底成因為何?困惑啊> <"

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  5. 西方的醫學知識早在很久以前就知道乳酸對人體有益的。在1930年代,由乳酸鹽組成的林格乳酸鹽溶液 (Ringer Lactate Solution)就用於醫院靜脈滴注(俗稱吊鹽水, 打點滴...)。史考特醫師, 對嗎? 西方科學家只使用乳酸來測定運動強度,現在常見的運動飲料甚至含有乳酸鹽。但是,東方醫學一直經率地簡單將乳酸與運動酸痛(疲勞)聯繫起來。可能是兩者都含有酸字吧.

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  7. 那麼酸痛的原因到底是⋯

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  8. 那麼酸痛的原因到底是⋯

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