Muscle Pain as a Regulator of Cycling Intensity

Effect of Caffeine Ingestion

Alexander R. Gonglach; Carl J. Ade; Michael G. Bemben; Rebecca D. Larson; Christopher D. Black

Disclosures

Med Sci Sports Exerc. 2016;48(2):287-296. 

In This Article

Abstract and Introduction

Abstract

Caffeine ingestion improves endurance time trial performance. However, the ergogenic mechanism of action remains unresolved. One potential explanation for caffeine's performance-enhancing effect is an improvement in work for a given amount of muscle pain.

Purpose: To test this hypothesis, participants performed two studies in which they regulated exercise intensity based on feelings of muscle pain.

Methods: Thirteen young men were asked to regulate exercise intensity based on feelings of "moderate" muscle pain (a "3" on a 0–10 pain scale). After three familiarization trials, either caffeine (~5 mg·kg−1 body weight) or placebo were administered before a moderate pain trial. Nine caffeine "responders" were retested and ask to regulate their exercise intensity at a "strong" pain level (a "5" on a 0–10 pain scale). A caffeine (~5 mg·kg−1 body weight) or placebo was again ingested before exercise.

Results: Participants performed more work (P = 0.008) and covered more distance (P = 0.008) at a higher average power output (P = 0.009) and V̇O2 (P = 0.019), for an identical amount of "moderate" muscle pain in the caffeine condition. When exercising at a rating of a "5," caffeine did not increase total work, distance covered, or V̇O2 for an identical amount of "strong" pain in the nine caffeine "responders."

Conclusions: Our findings indicate caffeine increases work performed during exercise, eliciting a moderate amount of a pain. However, a threshold level of muscle pain may exist above which antagonism of adenosine receptors alone does not induce a hypoalgesic effect.

Introduction

Caffeine improves performance in a wide variety of sports (for review, see[[9,16]]), but the mechanism(s) behind this ergogenic effect remains unresolved. Early research[11] postulated altered metabolism and subsequent sparing of muscle glycogen could explain caffeine's ergogenic effect,[11] but recent research has refuted this initial hypothesis.[19] Caffeine's ability to improve maximal voluntary strength (MVC) and local muscular endurance (e.g., repetitions to failure and/or time-to-exhaustion) (for review, see[32] have also been hypothesized to account for improvements in performance. Improved strength, through resistance training, has been shown to improve endurance performance,[33] and consistent with this idea, recent findings from our lab demonstrated concomitant increases in strength and time trial performance following caffeine ingestion.[7]

A reduction in pain sensitivity via antagonism of central and peripheral A1, A2a, and A2B adenosine receptors,[31] termed hypoalgesia, has also been hypothesized as a potential mechanism of caffeine's ergogenic effect. Multiple studie[15,24,25,27] have demonstrated reductions in ratings of muscle pain during cycling at fixed percentages of peak oxygen uptake (V̇O2peak) when exercise intensity and pain ratings are "moderate," but the effect seems to be lost during heavy fixed-intensity exercise when pain ratings are "strong" or greater.[7,22] During time trials, where participants are asked to cover as much distance as possible in a fixed amount of time[22] or cover a given distance as quickly as possible,[2,3] caffeine consistently improves performance but does not alter pain ratings. We have recently demonstrated hypoalgesia during 30 min of fixed, moderate intensity exercise after caffeine ingestion that did not persist when a 10-min time trial was performed immediately after the 30-min bout.[7] These seemingly contradictory findings can possibly be explained in two ways: 1) a threshold level of pain exists above which caffeine is not hypoalgesic likely because of the collective nociceptive inputs being too great for antagonism of adenosine receptors alone to reduce pain, or 2) participants may regulate their work output during exercise, in some part, based on their perceptions of muscle pain—masking any hypoalgesic effect of caffeine, but allowing for greater work to be performed for a given level of pain.

The use of muscle pain to regulate exercise intensity during endurance exercise is an important but understudied topic[23]—especially in relation to caffeine ingestion. O'Connor et al.[26] demonstrated a moderate (a rating of "3" on a 0–10 scale) level of pain can be produced and sustained for 20 min during cycling when participants were allowed to adjust their work rate. "Fixing" muscle pain in this manner would allow researchers to determine if caffeine ingestion altered work performed for a given level of pain. As such, the purpose of this study was to determine if caffeine ingestion altered the amount of distance covered and work performed during a fixed pain time trial. Pain was fixed at a moderate level ("3" on a 0–10 scale) during an initial cycling protocol and at a strong level ("5" on a 0–10 scale) during a subsequent protocol. It was hypothesized that caffeine would improve distance covered and work performed during cycling at a moderate but not a strong level of muscle pain.

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