Nike has
Nike Men's Flex Run 2021 Road Running Shoes (Black/Dark Smoke Grey/White, CW3408-002) on sale for $51.97 - $12.99 (25%) off when you apply promo code
BFRIDAY on the checkout page =
$38.98.
Shipping is free on $50+ orders for Nike Members (
free to join).
- Note: Sizing and availability may vary.
Thanks to Community Member
SickerDealer for sharing this deal.
About this shoe:
- Stretchy mesh upper adapts for flexible, natural movement
- Sole grooves promote smooth, natural foot motion
- Lightweight foam cushioning feels soft under each step
- Wider midfoot improves shaping and support
- Heel cushioning and pull tab add daily comfort
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Edit: I asked AI
The Nike Flex Run 2021 (CW3408-002) has a soft, cushioned feel, but it is not as pillowy or "marshmallow‑soft" as an Adidas Ultraboost, and it is generally considered a light, flexible daily trainer/lifestyle shoe rather than a max‑cushion model like Ultraboost. It can be knee‑friendly for some people and uses dual‑density foam designed to feel plush, but it is not specifically engineered as a medical or high‑protection knee shoe, so fit, your running form, and your existing knee condition matter more than this specific model.
## How soft is the Flex Run 2021?
- Nike describes the shoe as using "soft yet durable foam" with an "extra‑cushy" core section visible through the outsole grooves, giving it a plush feel underfoot.
- Lab and wear‑test reviews describe the midsole as dual‑density: a softer foam core surrounded by firmer foam, producing a very soft step‑in feel but with more structure than a true max‑cushion shoe; testers note it feels soft and comfortable but not as bouncy or responsive, and not as deeply cushioned as high‑end max‑cushion shoes.
- Overall, it sits more in the "soft, flexible everyday trainer / lifestyle" category, whereas Ultraboost is known for a thicker, springy Boost midsole and is widely perceived as softer and more cushioned under load.
## Knee protection: what evidence says
- Clinical research on people with knee osteoarthritis shows that cushioned sport shoes can reduce reported knee pain and improve function over several months compared with less cushioned "sham" shoes, so some cushioning can be beneficial for knee comfort.
- Other biomechanical studies on highly cushioned "maximalist" shoes show that very thick cushioning can sometimes increase leg stiffness and impact loading at faster speeds, suggesting that more cushioning is not always better for joint forces.
- Reviews of running footwear and injury risk emphasize that "appropriate" shoes depend on individual factors (body weight, gait, speed, foot structure, and existing pathology); cushioning can help comfort, but it is not a guaranteed way to prevent knee injury by itself.
## Is Nike Flex Run 2021 a good choice for knee protection?
- Pros for knees:
- Soft dual‑density foam and flexible sole can make walking and easy jogging feel smooth and comfortable, which may help reduce perceived knee discomfort if you are coming from very firm shoes.
- The shoe is light and flexible, making it more suitable for shorter, easy runs or walking where you want comfort without a heavy, rigid platform.
- Limitations / cautions:
- It is not a stability shoe and does not have notable motion‑control features, so if you overpronate significantly or need more support for alignment, this model may not keep your knee tracking as well as a structured or stability shoe.
- The foam, while soft, is not as substantial as high‑end max‑cushion shoes, so heavier runners or those doing high mileage may want a more protective midsole with better long‑term support and shock attenuation.
- Knee protection depends heavily on fit, your gait, strength, and training load; shoes alone cannot guarantee protection and, if the shoe does not match your mechanics, it could even aggravate symptoms.
## Practical recommendation
- If you mainly walk or do short, easy runs and prefer a soft, flexible, low‑price shoe, the Flex Run 2021 can be a comfortable option, but expect it to feel less soft and less "cloud‑like" than Ultraboost.
- For someone actively trying to protect vulnerable knees (past injury, osteoarthritis, or frequent pain), it is usually better to:
- Choose a shoe with adequate cushioning plus the right level of stability for your gait (often from "daily trainer" or "max cushion but not overly soft" lines, sometimes in a stability version).
- Get fit at a running shop or by a physical therapist who can watch your gait and suggest models that keep your knee tracking comfortably.
If you tell roughly how much you weigh, whether you mostly walk vs run, and whether your knees hurt on the inside, outside, or front, a more targeted category (neutral vs stability, softer vs firmer) can be suggested.
Sources
Nike Flex Run 2021 Men's Road Running Shoes https://www.nike.com/t/flex-run-2...oes-M8SZXP
Nike Flex Run 2021 | Features, Performance Test & More! - YouTube https://www.youtube.com/watch?v=AwEiD3M-qfI
The most flexible running shoes - Solereview https://www.solereview.
Cushioned Footwear Effect on Pain and Gait Characteristics of ... - NIH https://pmc.ncbi.nlm.ni
Can the "Appropriate" Footwear Prevent Injury in Leisure-Time ... - NIH https://pmc.ncbi.nlm.ni
Nike Mens Flex Run 2021 Road Running Shoes – ShopCGX https://shopcgx.com/products/cw34...smoke-grey
Cut in half: Nike Flex Run 2021 Review | RunRepeat https://runrepeat.com/nike-flex-run-2021
Running in highly cushioned shoes increases leg stiffness and ... https://www.nature.com/articles/s...18-35980-6
The art of choosing the right running shoe: a review article https://jfootankle.com/JournalFoo...19
Debunking Running Shoe Myths - Runners' Edge Alaska https://runnersedge.phy
Nike Flex Run 2021 Men's Road Running Shoes https://www.nike.com/ph/t/flex-ru...oes-9F8kgv
Nike Flex Run 2021 Men's Road Running Shoes. Nike SG https://www.nike.com/sg/t/flex-ru...oes-9F8kgv
Nike Flex Run 2021 Black White Running Shoes CW3408-002 ... https://www.ebay.com/itm/326727605351
Nike Men's Flex 2021 RN Running Shoes - Exchange https://www.shopmyexcha
Nike Flex Run 2021 - YouTube https://www.youtube.com/watch?v=hNo_z6K
Nike Flex Run 2021 Black/White CW3408-002 Men's ... - eBay https://www.ebay.com/itm/177408240624
NIKE FLEX RUN 2021 | UNBOXING AND PERFORMANCE REVIEW https://www.youtube.com/watch?v=NjFfGEd
Max cushioned shoes bad for runners knee? : r/RunningShoeGeeks https://www.reddit.com/r/RunningS...ners
Nike Flex Run 2021 Men's Road Running Shoes https://www.nike.com/jp/en/t/flex...oes-dC2SF4
The Effect of Running Shoes on Lower Extremity Joint Torques https://www.sciencedire
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Edit: I asked AI
The Nike Flex Run 2021 (CW3408-002) has a soft, cushioned feel, but it is not as pillowy or "marshmallow‑soft" as an Adidas Ultraboost, and it is generally considered a light, flexible daily trainer/lifestyle shoe rather than a max‑cushion model like Ultraboost. It can be knee‑friendly for some people and uses dual‑density foam designed to feel plush, but it is not specifically engineered as a medical or high‑protection knee shoe, so fit, your running form, and your existing knee condition matter more than this specific model.[1][2][3][4][5]
## How soft is the Flex Run 2021?
- Nike describes the shoe as using "soft yet durable foam" with an "extra‑cushy" core section visible through the outsole grooves, giving it a plush feel underfoot.[6][1]
- Lab and wear‑test reviews describe the midsole as dual‑density: a softer foam core surrounded by firmer foam, producing a very soft step‑in feel but with more structure than a true max‑cushion shoe; testers note it feels soft and comfortable but not as bouncy or responsive, and not as deeply cushioned as high‑end max‑cushion shoes.[7][2][3]
- Overall, it sits more in the "soft, flexible everyday trainer / lifestyle" category, whereas Ultraboost is known for a thicker, springy Boost midsole and is widely perceived as softer and more cushioned under load.
## Knee protection: what evidence says
- Clinical research on people with knee osteoarthritis shows that cushioned sport shoes can reduce reported knee pain and improve function over several months compared with less cushioned "sham" shoes, so some cushioning can be beneficial for knee comfort.[4]
- Other biomechanical studies on highly cushioned "maximalist" shoes show that very thick cushioning can sometimes increase leg stiffness and impact loading at faster speeds, suggesting that more cushioning is not always better for joint forces.[8]
- Reviews of running footwear and injury risk emphasize that "appropriate" shoes depend on individual factors (body weight, gait, speed, foot structure, and existing pathology); cushioning can help comfort, but it is not a guaranteed way to prevent knee injury by itself.[5][9][10]
## Is Nike Flex Run 2021 a good choice for knee protection?
- Pros for knees:
- Soft dual‑density foam and flexible sole can make walking and easy jogging feel smooth and comfortable, which may help reduce perceived knee discomfort if you are coming from very firm shoes.[2][3][1]
- The shoe is light and flexible, making it more suitable for shorter, easy runs or walking where you want comfort without a heavy, rigid platform.[3][7]
- Limitations / cautions:
- It is not a stability shoe and does not have notable motion‑control features, so if you overpronate significantly or need more support for alignment, this model may not keep your knee tracking as well as a structured or stability shoe.[9][7]
- The foam, while soft, is not as substantial as high‑end max‑cushion shoes, so heavier runners or those doing high mileage may want a more protective midsole with better long‑term support and shock attenuation.[7][3][9]
- Knee protection depends heavily on fit, your gait, strength, and training load; shoes alone cannot guarantee protection and, if the shoe does not match your mechanics, it could even aggravate symptoms.
## Practical recommendation
- If you mainly walk or do short, easy runs and prefer a soft, flexible, low‑price shoe, the Flex Run 2021 can be a comfortable option, but expect it to feel less soft and less "cloud‑like" than Ultraboost.[1][2][3][7]
- For someone actively trying to protect vulnerable knees (past injury, osteoarthritis, or frequent pain), it is usually better to:
- Choose a shoe with adequate cushioning plus the right level of stability for your gait (often from "daily trainer" or "max cushion but not overly soft" lines, sometimes in a stability version).[4][5][9]
- Get fit at a running shop or by a physical therapist who can watch your gait and suggest models that keep your knee tracking comfortably.
If you tell roughly how much you weigh, whether you mostly walk vs run, and whether your knees hurt on the inside, outside, or front, a more targeted category (neutral vs stability, softer vs firmer) can be suggested.
Sources
[1] Nike Flex Run 2021 Men's Road Running Shoes https://www.nike.com/t/flex-run-2...oes-M8SZXP
[2] Nike Flex Run 2021 | Features, Performance Test & More! - YouTube https://www.youtube.com/watch?v=AwEiD3M-qfI
[3] The most flexible running shoes - Solereview https://www.solereview.
[4] Cushioned Footwear Effect on Pain and Gait Characteristics of ... - NIH https://pmc.ncbi.nlm.ni
[5] Can the "Appropriate" Footwear Prevent Injury in Leisure-Time ... - NIH https://pmc.ncbi.nlm.ni
[6] Nike Mens Flex Run 2021 Road Running Shoes – ShopCGX https://shopcgx.com/products/cw34...smoke-grey
[7] Cut in half: Nike Flex Run 2021 Review | RunRepeat https://runrepeat.com/nike-flex-run-2021
[8] Running in highly cushioned shoes increases leg stiffness and ... https://www.nature.com/articles/s...18-35980-6
[9] [PDF] The art of choosing the right running shoe: a review article https://jfootankle.com/JournalFoo...19
[10] Debunking Running Shoe Myths - Runners' Edge Alaska https://runnersedge.phy
[11] Nike Flex Run 2021 Men's Road Running Shoes https://www.nike.com/ph/t/flex-ru...oes-9F8kgv
[12] Nike Flex Run 2021 Men's Road Running Shoes. Nike SG https://www.nike.com/sg/t/flex-ru...oes-9F8kgv
[13] Nike Flex Run 2021 Black White Running Shoes CW3408-002 ... https://www.ebay.com/itm/326727605351
[14] Nike Men's Flex 2021 RN Running Shoes - Exchange https://www.shopmyexcha
[15] Nike Flex Run 2021 - YouTube https://www.youtube.com/watch?v=hNo_z6K
[16] Nike Flex Run 2021 Black/White CW3408-002 Men's ... - eBay https://www.ebay.com/itm/177408240624
[17] NIKE FLEX RUN 2021 | UNBOXING AND PERFORMANCE REVIEW https://www.youtube.com/watch?v=NjFfGEd
[18] Max cushioned shoes bad for runners knee? : r/RunningShoeGeeks https://www.reddit.com/r/RunningS...ners
[19] Nike Flex Run 2021 Men's Road Running Shoes https://www.nike.com/jp/en/t/flex...oes-dC2SF4
[20] The Effect of Running Shoes on Lower Extremity Joint Torques https://www.sciencedire
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