Why sportswear is sewn with flat seams
Short answer: During exercise the fabric moves a lot, the skin is damp, and an ordinary seam can cause chafing within half an hour. Flat seams remove the ridge entirely, so nothing is left to press or rub against the skin.
Movement and moisture together
In sport the same movement repeats thousands of times, and sweat softens the skin. Those are the two worst conditions for chafing at once. Even a small unevenness in the fabric is therefore clearly felt when it rubs against damp skin through a whole workout.
What an ordinary seam does
An ordinary seam lays two fabric edges on top of each other and stitches them together, leaving a small ridge. The ridge is thicker than the fabric around it and lifts slightly from the skin. With every movement it shifts back and forth and wears the same narrow strip of skin.
The flat seam does not build up
Edge to edge gives the same thickness as the fabric around it. There is no thickening that can press and no ridge that can rub. The seam lies flat against the skin and follows the fabric as you move, instead of forming a fixed line of resistance. See bamboo tennis socks for men.
Placement still matters
Even a flat seam should sit away from the places that move most. That is why seams are laid diagonally or completely outside the movement zone, for example behind the shoulder instead of on top. A good seam is both flat and placed where the skin has the least to do.
Seams that follow the body
Some garments are cut so the seams fall at the side of the body rather than under the arm or along the inner thigh. That makes a difference, because the biggest movements go forward and back. If you check where the seams lie, you can quickly judge what a garment was made for. See bamboo undershirts for men.
Are flat seams just as strong
Slightly weaker, but strong enough when placed correctly. A flat seam has fewer layers of fabric to hold on to, and therefore demands a lot of the thread and technique. If it is well made, it withstands normal use and washing. Check that the stitches sit evenly and the seam is closed all the way round.
Can ordinary underwear be used for sport
For light activity, yes. On long outings the seams matter a lot, because chafing can appear even if it did not on a short walk. If you know sore spots from sportswear, the seams are a good place to start, and you can test a new model on a short outing first.
What about seamless models
They solve the problem completely, but they cost more and come in fewer cuts. Seamless clothing is knitted in one piece, so there are no seams to think about. The range is smaller, though, and the fit is not always as tailored as it can be with seams. See bamboo compression stockings.
How to check a seam
Turn the garment inside out and feel the seam with your fingertip while pulling the fabric lightly. If it feels hard, thick or sharp, it can chafe. If it is flat and soft and lies evenly, that is often a good sign. Also move around in the clothes before you decide.
Frequently asked questions
Are flat seams just as strong?
Slightly weaker, but strong enough when placed correctly. Look for even stitches and a closed seam, and wash by the care label so the seam stays flat and neat for longer.
Can ordinary underwear be used for sport?
For light activity, yes. On long outings the seams matter a lot, because chafing can appear after hours of movement, so test a new model on a short outing before taking it on a long one.
What about seamless models?
They solve the problem completely, but they cost more and come in fewer cuts. Choose them if seams are your biggest problem, and check that the fit still sits right when you move.
You can find the full range at Bamboo-Clothing.com.
