The Importance of a Flat Surface

Playing on bumpy ground sucks; it makes the ball way more unpredictable.

Safety issues

• https://www.active.com/soccer/articles/how-to-avoid-indoor-soccer-injuries-879361

General

Rolling your ankle

• https://www.gftskills.com/7-tips-on-ankle-injury-prevention-rehabilitation-for-soccer-players/
  • Never play in running shoes, you'll be at a much higher chance of rolling your ankle.
• https://www.ussoccer.com/stories/2014/03/17/13/38/prevent-common-injuries-in-soccer-ankle-sprains
• I strongly suspect that being overweight greatly increase

Tearing your ACL

• 2014.07.09 - CBC - Wrong soccer cleat could increase ACL knee injury risk
  
  • He was wearing outdoor cleats with very tall studs while playing on turf, and when he tried to turn in-place the shoes wouldn't release from the ground.

Playing surfaces

Grass

Turf

Cancer

• Wikipedia - Artificial turf–cancer hypothesis
  • In 2014, Amy Griffin, soccer coach at the University of Washington, surveyed American players of the sport who had developed cancer. Of 38 players, 34 were goalkeepers, a position in which diving to the surface makes accidental ingestion or blood contact with crumb rubber more likely, Griffin has asserted. Lymphoma and leukemia, cancers of the blood, predominated.
  • In a statistical study of the list of soccer players with cancer provided by UW coach Amy Griffin, public health researchers for the State of Washington found that the rates of cancer were actually lower than was estimated for the general population. While they did not state any conclusions on the safety of this form of artificial turf, they did recommend that players not restrict their play due to the presumed health benefits of being active.

Wood

Tarmac

Concrete

Soccer shoes

For turf

• Lists
  • https://www.atopdaily.com/best-soccer-turf-shoes/
  • https://athletegearzone.com/best-turf-soccer-shoes/
• Specific shoes
  • Adidas Mundial Team turf shoes
    • Amazon - adidas Performance Mundial Team Turf Soccer Cleat
• You don't want to use flat-soled shoes if you can avoid it because you won't be able to grip the surface as well as someone who's using turf shoes (i.e. shoes with mini-stubs / mini-cleats on the bottom).
  • Source
• However as far as I can tell using flat-soled shoes doesn't appear to be unsafe.
• Never use cleats meant for real grass on turf fields, because the cleats on shoes meant for grass are longer (to provide better traction on grass) and will grip the turf too much, which can lead to injuries like a torn ACL if you try to turn and the shoe doesn't release from the ground.

Drills

• Practice your balance: (one of my favorites) Stand on one leg, and use the other leg (by itself) to guide the ball around the leg you're standing on, as fast as you can without losing the ball or your balance. Do it 20 times for each foot. It'll usually be broken into four movements, corresponding to the sides of an invisible square around the leg you're standing on. This is a great drill for indoor soccer.
  
• Improve your sprinting ability: Hold onto a wall and stand on the foot further from the wall. Then, with your other leg, pull your knee up to stomach-level (so your thigh is oriented perpendicular to your torso) and then force it back down to a straight position, as fast as you possibly can, fifty times. Then do it for the other leg. I noticed a huge difference in my indoor-soccer sprinting ability after doing this drill for a few weeks.
  
• Practice side-hits (for passing / shooting): Set up two posts for goals, maybe ~2-5 feet apart (pick whatever distance apart provides a challenge). Have them right up against the wall. Stand a few feet away (~5?) and, using one foot, pass the ball against the wall 20 times. Then switch to the other foot. Then step back a few feet and repeat. Practice keeping the ball low on the ground; to have the ball come straight back (instead of popping into the air) you'll need to hit it very precisely, which is good practice.
  
• (less important) Roll the ball up on your foot, let it sit there for a second, then let it roll off (practice getting the ball on your foot)
  
• Practice your knee-juggling (less important) - Drop the ball from your hands at neck-level, then bring up your knee to bop the ball straight up
  
• drop the ball from your hands at stomach-level and kick it straight up (practice for juggling)
  
• Practice around-the-world (less important): Use your standing leg to get some up-motion to take the weight of the ball off your foot when you begin to swing it around