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Flex

Flex — Health & Safety

Last updated: 2026-06-10

1. Flex is not medical advice

Flex is a software tool that generates running training plans based on the information you provide and adapts them based on your activity. It is software, not a medical service.

We are not doctors, physiotherapists, sports physicians, or other licensed healthcare professionals. Our training plans are not a diagnosis, prescription, prognosis, or treatment for any medical condition.

Do not use Flex as a substitute for medical advice, examination, or treatment. If you have any questions about your health or whether a particular exercise is safe for you, talk to a qualified medical professional.

2. Consult a doctor before starting

You should consult with a qualified medical professional before starting any new training program, especially if any of the following apply to you:

  • You are over 45 years old (men) or 55 years old (women) and have not exercised regularly in the past year.
  • You have a heart condition, chest pain at rest or with exertion, or you have had a heart attack or heart surgery.
  • You have high blood pressure that is not well controlled.
  • You have diabetes (type 1 or type 2).
  • You have asthma, COPD, or another respiratory condition.
  • You have a musculoskeletal injury — currently or in the past 12 months — that affects how you walk or run.
  • You are recovering from surgery (any kind), or have an unhealed bone fracture.
  • You are pregnant, recently postpartum (less than 12 weeks), or trying to become pregnant.
  • You experience dizziness, fainting, irregular heartbeat, or unusual shortness of breath at rest.
  • You take prescription medication that affects your heart rate, blood pressure, or hydration.
  • Your doctor has previously told you to avoid running or vigorous exercise.
  • You have any other reason to doubt that vigorous exercise is safe for you.

If any of the above apply to you, do not use Flex until you have explicit clearance from your doctor.

3. While training: listen to your body

Pain is information. The following are signals to stop and seek help:

3.1 Stop immediately and seek emergency medical attention

Call 999 (UK) or 911 (US) — or your local emergency number — if you experience any of these during or after a run:

  • Chest pain, pressure, or tightness.
  • Pain or numbness radiating into your arm, jaw, or neck.
  • Sudden severe shortness of breath that does not resolve with rest.
  • Sudden severe headache, vision changes, or loss of consciousness.
  • Sudden weakness, numbness, or paralysis on one side of the body.
  • Heart palpitations that do not resolve within a few minutes of rest.
  • Loss of consciousness or near-fainting.

3.2 Stop training and consult your doctor before resuming

  • Sharp, localized pain in a joint or bone that worsens during the run.
  • Pain that persists for more than 48 hours after a workout.
  • Pain that changes your gait (you start limping or compensating).
  • Swelling, bruising, or warmth around a joint.
  • Symptoms of overtraining: persistent fatigue beyond what your plan justifies, resting heart rate elevated by 10+ beats per minute for several days, sleep disturbance, mood changes, frequent illness.
  • Any symptom your doctor has previously told you to monitor.

3.3 Reduce intensity or skip the workout

  • Mild muscle soreness lasting more than 24 hours.
  • Unusually elevated resting heart rate (in the app: the Daily Readiness chip flags this).
  • Poor sleep the night before (in the app: a red Readiness signal).
  • Active head cold, mild fever, or other illness.
  • Significant life stress or major schedule disruption.

The Flex adaptive engine will pick up some of these signals automatically and propose plan adjustments. Trust your body over the algorithm if there is a conflict. You can always skip a workout — the plan will adapt.

4. Environmental safety

Heat

Running in heat dramatically increases injury and cardiac risk. If the feels-like temperature exceeds:

  • 27 °C (80 °F): hydrate aggressively, slow down, consider shifting the workout earlier or later.
  • 32 °C (90 °F): replace runs with indoor / pool / cycle alternatives. The Flex weather signals will warn you.
  • 35 °C (95 °F): do not run outdoors.

Heatstroke is a medical emergency.

Cold

  • Below −5 °C (23 °F): cover skin, take care on ice, expect 20–30% slower paces.
  • Below −15 °C (5 °F): do not run outdoors without expert guidance and proper gear.

Altitude

Plans generated in our app assume sea-level training. If you live or travel above 1,500 m / 5,000 ft, expect reduced pace and increased heart rate at any given effort. Acclimate over 1–2 weeks before assessing whether the plan is achievable.

Air quality

Do not run outdoors when local AQI is above 150 (unhealthy). Move to indoor / treadmill alternatives.

Traffic and visibility

Wear reflective clothing in low light. Carry ID. Tell someone your route or use a live-location share for long runs in unfamiliar terrain.

5. Hydration and nutrition

Flex provides general guidance only. Your individual hydration and fueling needs depend on body composition, climate, sweat rate, prior nutrition status, and medical conditions. For specialized nutrition guidance — especially around long runs, marathons, and ultra distances — consult a registered sports dietitian.

Hyponatremia (over-hydration with insufficient sodium) is a real risk for long-event runners. Do not assume "more water is always better."

6. Pregnancy and postpartum

Running during pregnancy is generally considered safe for many people who were already runners pre-pregnancy, with explicit clearance from your obstetrician or midwife. Flex does not currently offer pregnancy-specific or postpartum-specific plan modifications. If you are pregnant or recently postpartum:

  • Get explicit medical clearance before continuing Flex.
  • Stop and consult your provider if you experience bleeding, fluid leakage, cramping beyond mild, decreased fetal movement (third trimester), or any other concerning symptom.
  • Do not return to running postpartum until your medical provider has cleared you — typically not before 6 weeks and often 12+ weeks, depending on delivery.

7. Minors

Flex requires users to be at least 13 years old (US) or 16 years old (UK / EU). If you are under 18, an adult guardian must agree to these Terms and supervise your training. Youth athletes have specific physiological considerations (growing bones, growth-plate injuries, RED-S risk) that are outside the scope of our software. Coaches and pediatricians should oversee youth running programs.

8. Eating disorders and RED-S

Running can interact with disordered eating and RED-S (Relative Energy Deficiency in Sport) in dangerous ways. If you have a current or past eating disorder, or if anyone (medical professional, family member, coach) has expressed concern about your eating or weight, please consult a specialist before using Flex. Do not use Flex as a weight-loss tool. Our plans optimize for performance, not weight reduction.

9. Cardiac screening

The American Heart Association and UK NHS both recommend cardiac screening for adults beginning a vigorous exercise program after extended sedentary periods. A simple resting ECG and a conversation with your GP is cheap insurance.

10. The acknowledgment block

When you accept this guidance in the app (a one-time acknowledgment shown before your first training dashboard session), you confirm:

I have read and understood Flex's Health & Safety guidance. I understand Flex is not a medical service and does not provide medical advice. I confirm that I do not have any of the medical conditions listed that would require doctor's clearance before starting, or — if I do — I have received that clearance. I will stop training and seek medical advice if I experience any of the warning signs described. I accept that the choice to run any given workout is mine, and I take responsibility for that choice.

The timestamp of your acceptance is recorded in your profile for audit purposes. You can review this Health & Safety guidance at any time at Settings → Health & Safety in the app, or at https://www.runwithflex.com/legal/health-safety.

11. Updates to this guidance

We will update this Health & Safety guidance as the science of training evolves and as our app gains new features. Material changes (new warning signs, new screening criteria, new safe-environment thresholds) will be announced 30 days in advance via email and in-app, and you will be asked to re-acknowledge. Non-material clarifications take effect when posted.

12. Contact

  • Safety concerns or to report a safety-related incident with Flex: privacy@runwithflex.com
  • Medical emergencies: call your local emergency number (999 UK, 911 US, 112 EU)

Be honest with yourself. Train hard, recover harder, and never sacrifice long-term health for short-term performance.

— The Flex team