“The body achieves what the mind believes” is a motivational saying that has been printed on many images. The saying is on many gym posters, and is of unknown authorship.
 
Kitty Consolo won the Revco-Cleveland Marathon in May 1985, and the Associated Press quoted her:
 
“But I have a little saying: ‘The body achieves what the mind believes.’”
 
“The old adage that ‘The body achieves what the mind believes’ is true” was printed in the book Practice Dynamics (1986) by Linda L. Miles.
 
         
20 May 1985, Huntsville (AL) Times, “Marathon Winner Overcomes Cyst,” pg, B-6, col. 3:
CLEVELAND (AP)—Marathon runner Kitty Consolo wasn’t sure how far she could push herself after undergoing an operation in March.
 
“I didn’t even expect to finish the race, let alone win it,” the 28-year-old said Sunday after winning the women’s Revco-Cleveland Marathon in two hours, 49 minutes and 13 seconds. “But I have a little saying: ‘The body achieves what the mind believes.’”
 
Google Books
Practice Dynamics
By Linda L. Miles
Tulsa, OK: PennWell Books
1986
Pg. 73:
The old adage that “The body achieves what the mind believes” is true.
 
Google Books
Keeping It in the Family:
Successful Succession of the Family Business
By James W. Lea
New York, NY: Wiley
1991
Pg. 188:
“The body achieves what the mind believes” is a YMCA motto for encouraging young athletes.
 
7 March 1994, Cincinnati (OH) Post, “Rijo’s snake oil makes scents” by Paul Daugherty, pg. 1C
All that matters is, Jose Rijo believes it works. “It’s the truth, buddy,” he explains. “I use it because it makes me feel good,” Rijo says. “Your body always does what it thinks your mind wants your body to do.” You mean that the body achieves what the mind believes? “Yes, indeed,” says Rijo.
   
16 November 1994, Richmond (VA) Times-Dispatch, “Swimmers Finding Hat Is Perfect Fit” by Harold Pearson, pg. N-8:
“As the saying goes, ‘The body achieves what the mind believes.’”
   
Google Books
No Chains Around My Feet:
Running as a Metaphor for Life
By Nomoya M Mahlangu
Bloomington, IN: AuthorHouse UK
2017
Pg. ?:
The body achieves what the mind believes. —Author Unknown
 
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The body achieves what the mind believes. 🙌 #MotivationalMonday #Motivation
8:00 AM - 4 Jun 2018
 
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Jennifer Pilates ✌🏻 💖 🌟
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“The Body Achieves What the Mind Believes” ✌🏻💖🌟 @JenniferPilates http://jenniferpilates.com  #Pilates #FridayMotivation #Fridayfeeling #MindBody #mindfulness #JoyTrain
12:20 PM - 8 Jun 2018