Motivation or Demotivation
Benjamin Franklin is responsible for a variation of the expression " there are no gains without pains." Before his time, Franklin believed that 45 minutes of exercise a day was necessary to maintain good health. http://thebenjaminfranklindiet.com/blog, Poor Richard (1734).
Jane Fonda popularized the expression along with "Feel the Burn" in her exercise videos. Her intent was to motivate exercisers to push beyond the point of comfort.
Pushing your body past your comfort zone is necessary to see strength and endurance gains. However it is important to know when and how much to push yourself on any given day. We feel differently each day so although most regular exercisers have a routine, anything can be modified.
Good Pain
There are different types of workout pain. There is the good kind and the bad kind. Good pain is that which results from using muscles that you normally don't use. I often think I exercise every muscle in my body but then will do a new routine and will feel sore the next day. I have had days where the muscle soreness is extreme and I wince whenever I move but that is rare. Is is also a strangely good feeling knowing that my muscles are getting stronger.
It is also a misconception that you should stop working out when you experience post-exercise soreness, which, by the way, can occur 2 days after the activity. You can exercise using different muscles. Never continue to exercise areas of your body that are clearly injured.
Bad Pain
Before we get to periodization, it is important to mention the 'bad' pain. If you are exercise-obsessed like I am, you often ignore pain signals. I don't want to ever interrupt my workouts. This has definitely come back to bite me. Of course, deciding when to stop is a judgement call. So, I often wait too long. I jumped off of a curb awkwardly during a run several years ago but told myself to keep going. I ran another 3 miles back to my car. My whole ankle turned purple and became hot and swollen. These are typical symptoms of injury. An x-ray revealed a fracture. It took me 6 months to recover and begin to walk normally. If I had taken action sooner I could have dramatically cut the recovery time. I have behaved similarly on multiple occaisons. This was during the days when running was my only form of exercise. After the fracture, tearing my quadriceps muscle, and a lot of lecturing, I started to cross-train so opposing muscles would strengthen and reduce the imbalance. I was very resistant. Looking back however I realize it was one of the most important behavior modifications I internalized.
If you have pain from an injury, "no pain, no gain" does not apply. Injuries can be accute or caused by overuse of a particular muscle or muscle group. An acute injury is usually easy to recognize - it is a distressing feeling often caused by intense or damaging stimuli (Wikipedia, https://en.wikipedia.org/wiki/Pain), like falling on ice and breaking your wrist. Overuse syndrome can be harder to diagnose because it happens over time but can be equally painful and sometimes trickier to treat. It occurs over time for a variety of reasons. A medical professional should be consulted if you experience either type of pain that does not improve with RICE (rest, ice, compression, and elevation) or other home treatments. https://en.wikipedia.org/wiki/RICE_(medicine). Be leary of internet suggestions unless you know they are from reputable medical sources.
Most aerobic work involves the large muscles of the lower body. However you can still work out if you are injured by doing upper body cardio and strength as well as chair cardio and strength. If you experience a chronic injury it helps to take an anti-inflammatory ( if you can tolerate this), like Ibuprofen, within the first 24 hrs. Icing on and off for 20 minute periods, rest, and elevation of the affected area above the heart, will all help to reduce inflammation and speed recovery. After 24 hours, I often find heat works better for me but the bottom line is to use what works for you.
Cross-training
Cross-training is very important for overall fitness. When you run, you are using mostly quadricep muscles (agonist muscle) but that leaves your hamstrings vulnerable (antagonist muscle). This is an example of muscle imbalance. Your quads are getting stronger but not your hamstrings, which are the opposing muscles. Biking is a great cross-training activity for runners. No matter what you do, variablility will help stave off injuries. Keep in mind that everyone gets older and you want to develop habits and activities that you can maintain for your entire life. Your physical fitness routine should include the following:
Cardio-vascular activities such as walking, running, jumping rope, aerobics classes, biking etc.
Strength exercises with body weight, free weights, or machine weight systems
Flexibility - stretching exercises - dynamic for warm up and static for cool down (https://www.hss.edu/conditions_dynamic-static-stretching.asp#:~:text=Dynamic%20stretches%20are%20controlled%20movements,up%20to%20about%2045%20seconds.)
Balance- exercises to help maintain your balance as you age
Form First
Another major factor to consider is using good form. The quality of the exercise you are performing is more important than the quantity of the exercise(s). This includes all forms of exercise. If fatigue causes you to compromise form, you have reached your limit and should back off until you can slowly build up to the new level.
I remember thinking I didn't want to waste my time on flexibility (stretching) or balance. It didn't feel like 'working out'. Now I realize how they contribute to a well-rounded fitness plan and are important for preventing injury and maintaining fitness. As you age; cardio-vascular, strength, flexibility, and balance all deteriorate and effort is required to maintain the benefits.
Periodization
Moving on to Periodization. An additional risk of only doing one particular exercise activity is adaptation. Our bodies get used to an activity that is repeated over and over without alteration and that activity becomes less effective in increasing fitness gains. Our bodies like to be confused - go figure. I don't like to be confused but my body says "surprise me".
From : Fitness.org https://www.acefitness.org/education-and-resources/lifestyle/blog/6660/periodized-training-and-why-it-is-important/
“Periodizing” your training is the key. Instead of doing the same routine month after month, change your training program at regular intervals or “periods” to keep your body working harder, while still giving it adequate rest. For example, you can implement periodization training for your strength-training program by adjusting the following variables:
The number of repetitions per set, or the number of sets of each exercise
The amount of resistance used
The rest period between sets, exercises or training sessions
The order of the exercises, or the types of exercises
The speed at which you complete each exercise
Different exercises
Research shows the results.
A frequently cited study conducted at the Human Performance Laboratory at Ball State University has shown that a periodized strength-training program can produce better results than a non-periodized program.
CONCLUSION
I am not sure pain is the word we should be using for the exercise objective this refers to. You shouldn't feel debilitaing pain or push yourself to the point where you are using poor form in whatever exercise you are performing. Manage your exercise intensity to be maintainable but challenging. Try to slowly push yourself to the next level, vary your exercise activities and HAVE FUN (do something you enjoy - you are more likely to maintain it.)