Read the facility policy first
Gyms set their own rules about photography, video and tripods. Check them before recording, including any separate rules for classes or changing areas. Permission to use a phone for music or a training log does not automatically mean filming is permitted. If the policy is unclear, ask a member of staff.
Consider whether a recording is necessary for the question you are trying to answer. A trainer may be able to check something in person, or a written note may capture a machine setting. When video is useful and allowed, agree on what needs to appear in the frame so you do not record more of the room than necessary.
Do not film in changing rooms or toilets. On the exercise floor, avoid including other people and seek permission wherever appropriate rather than treating their presence as part of the background. Someone should not have to step out of a route or abandon equipment to avoid your camera.
Position the phone without creating a hazard
Use only an arrangement the gym permits. A tripod, bag or improvised stand can block a walkway or occupy space another user needs. Never balance a phone on moving equipment or place it where a weight could strike it. Ask staff about a suitable location if the required angle is difficult.
Do not adjust the camera while simultaneously handling equipment. Set up, check the view and put your attention back on the task. If the arrangement keeps falling or requires constant interruption, stop recording. Capturing an image is not a reason to use a setup that is awkward or distracting.
For remote feedback, ask the trainer how to send the file and how it will be handled. Avoid posting it publicly simply because that is the easiest upload option. Review the frame for other people and identifying details before sending anything.
Think about the start and end of the recording as well as its centre. A clip may capture people while you walk back from the phone or collect it afterwards. Review the whole file, not just the moment you wanted feedback on. If you cannot avoid recording someone, do not assume cropping later makes the original capture acceptable under the gym's rules.
Ordinary phone use also takes space
A phone can be useful for notes between exercises, but occupying a busy machine while scrolling unrelated content makes it harder for others to use the room. If you need to make a call or handle a longer message, move to an appropriate area after leaving the equipment ready for someone else.
Keep audio private and do not use a loudspeaker for calls on the gym floor. Put the device somewhere secure, not on a bench another person may need. The aim is not to ban a useful tool from your visit; it is to keep the surrounding people and equipment available for their intended use.
Turn off notifications on the visible screen if a recording could reveal private messages. Also check whether a reflective surface includes a wider part of the room than expected. Mirrors can make a carefully aimed camera show people behind it. A quick review before the actual recording helps identify those problems without making repeated takes around other users.