I have tried my best to monitor this for several weeks, but it is one of those things that changes when I am conscious of it My question is, suppose I am doing exercises that use a number of various muscle groups, this will be for a large number of reps, maybe 40. If one muscle groups starts to tire will a different muscle group increase, its share of the load?
Sort of.
Individual muscles consist of multiple groups/teams of muscle cells/fibers that all fire at the same time, controlled by the same motor neuron, called “motor units”. As some fatigue others are recruited to take on more of the job. That is spread across many muscles.
I would appreciate your explaining Charlie horse; when I feel one forming in a calf, my first instinct is to point my toes towards the floor — should make the calf muscle slack, no? No! Stretching the toes towards the knee (which should make the muscle tense, IMHO) makes the CH slack off. This is all counterintuitive! I’m at a loss! What gives?
To some extent.
If you are lifting a heavy weight with a squat or press or deadlift, a lot of muscles are going to come into play. The smaller muscles will tire before the big ones. The body will try to compensate, but this will limit the number of reps that a heavy weight that can done with decent form.
You have different muscle fibres with different twitch speeds, which are first used by different exercises depending on acceleration, degree of loading, time under tension and whether it involves anaerobic and/or aerobic metabolism. This is a pretty complicated topic, but is important for scheduling athletic training.
In practice, if seeking to make muscles bigger, you want to do more volume. For “big” exercises (squats etc.), this means doing them with free weights and moving to less challenging variations (hack squats, hack squats, etc.), then getting more help to compensate for the tired smaller “stabilizer” muscles by using the Smith machine, other gym machines, lowering the weight (step downs) and other strategies. These include using “heavy”, “medium” and “light” workouts for major muscles to aid in recovery that use progressively more machines, maybe more reps, certainly less weight (as a percentage of maximum).
My machine does not use weight; it only uses resistance. I am trying to decipher the experience. I will start off with say 5 reps at about 50% force applied to set my tempo for my reps. As I tire the applied force decreases while the effort continues to increase. after about 12 or 15 reps I can no longer maintain the tempo and am pushing at say 90% effort, I might go another 10 reps like that until I basically have nothing left. Yesterday I tried something different. I pushed at about 70% effort and did a lot more reps to see if more muscles would become engaged. I think my original way was better as I am not experiencing the tightness this morning that I have grown to expect following a workout.
A reasonable method would be to work up to as much resistance as you can do, and consider that amount “100% effort”.
You could then try “heavy workouts” where you do several small sets (1-5 reps) at 85-90% of the first amount. “Moderate workouts” where you do bigger sets (6-15 reps) at 50-70% of the first amount, and “light workouts” where you do a few sets at any weight. These will be approximate, but if working with intensity “heavy” workouts require more recovery time than light ones, assuming that those muscles feel tired the day after the workout. If the muscles do not feel tired the day after the workout, do more “heavy” workouts or increase the intensity by doing similar exercises.
You could also work up to 100%, then back off to 60% and every minute do five quick reps for as long as you can. This is one of my favourite strategies, because your 100% is different on good and mediocre days.
Doing moderate days at 70%, but more reps, is a good idea even if you do not feel the tightness. I would consider alternating between heavy workouts at 90-109% and easier ones with more reps at 70%.
I am not convinced that that is a reliable metric.
This.
It’s good to have some awareness of how you feel post workout, and not just stick on a routine that isn’t pushing you enough (as most people seem to).
For people just starting out, pretty much any workout will result in significant aches for the following days. As you start working out more frequently, that goes away, but it’s good to still feel something e.g. pushing a heavy door the next day and feeling a bit of warm feeling in the pecs that reminds you that you did chest exercises yesterday.
I sometimes also, at the end of the workout, do some of the motions of the exercises without any weights, and if I can’t feel significant fatigue then I’m not done.
ETA: this distinction between starting out and more frequent weightlifting might be responsible for you not feeling the same “tightness” @HoneyBadgerDC . I don’t think it’s typical to have aching muscles every time.
Some exercises are intended to isolate a single muscle or small group. Others have a broader target. For the exercise a single muscle or small group you should stop if other muscles get involved due to fatigue in the isolated muscles, but that depends on the particular exercise and it’s purpose.
I don’t experience aching muscles more of a pleasant tightness or slightly flexed feeling. I have been using that to gauge what areas need more work. This is coming up on my 3rd month of working out. It is starting to require more work to produce that same feeling.
Yeah I’d be careful about chasing that high, as there is risk of overtraining too.
FTR I am 46 and have been working out my whole adult life. I’m pretty shredded, but I’m not huge.
I can feel the muscles I worked out the next day, but only under load.
Feeling the muscle without doing anything is not typical and is only something I get occasionally after changing something significant in my routine.
But of course YMMV, I am just adding to the reference data 
I suspect because of my age (76) my recovery times are slower. It usually takes about 3 days for the tightness to subside. I haven’t worked out since I was a teenager. The past 10 years I haven’t had hardly any activity at all.
Soreness is a very poor metric for how much the muscles will benefit. Doing any new exercise with intensity can cause soreness. I rarely feel sore after a standard workout. Yet during two years of doing CrossFit, my shoulders ached most of the time. 
People often make the mistake of thinking they should always be using 100% of the weight they could lift. It is good to workout with focus and intensity, but you want the toughest workouts to be above 85%. Doing more volume at 60-70% has different benefits. You do not progress more by burning out your nervous system or not properly recovering between workouts using the same muscles.
A detailed discussion of type I, IIa and IIx muscle fibres and good advice about how athletes should structure a weightlifting program depending on their sport, goals and season is given in the kindle e-book “The Concentrated Loading System” available on Canadian kindle for under ten dollars. But unless you are a very serious lifter or athlete it is too detailed, and it assumes you already know a lot. Thibaudeau’s “BOSS system” is one I use myself and recommend, also great value, but again it is for people who have been lifting seriously for years. His “maximum muscle bible” and “black book” are also excellent but are harder to find and more expensive.
… probably overkill for a 76 year old successfully combating sarcopenia and for a large number of the rest of us just integrating strength training into our overall fitness plans targeting long healthspans. 
@HoneyBadgerDC the pump or tingle is not required to be continuing to progress. But if you like that feeling (and psychologically I do) the easiest way to keep getting it is to switch up your workouts. Move to an emphasis on the “eccentric” movement (slow controlled back down) for example, and other different, novel movements.
I know swimming will build muscle. 100’s of reps at low resistance and never going to complete failure. Thinking back I always felt very pumped after body surfing for an hour or so. I don’t remember that pump lasting very long but I did have a good muscular build.
Definitely so. But I mentioned these resources for the sake of completeness on the off chance someone wants to take a deep dive into the applied physiology. I opted not to summarize things here.
The emoji was there for a reason!
Your depth of knowledge is always appreciated. And the simple message that pretty much anything people do will give good results for healthspan, high volume low volume mixing it up whatever, so long as they keep trying to challenge themselves to do something more or better or different, is the lede.
I really need to take that deep dive. My new machine has nothing to really compare it to so I think that first source you posted might be very helpful.
Honestly, if you do not have access to free weights and a full gym it would help less. And it is targeted at fitness trainers who coach for a living. That said, it might be cheap in the US. it is definitely written by one of the most knowledgeable coaches, and I couldn’t fault anyone for buying it. A lot of stuff others post was taken from stuff he wrote - he is one of the best known and most successful coaches. He write the last two links posted; see if you enjoy his style (which can be slightly bro centric).
But you would want to know all the basics here before reading it. The very best books always start from first principles, briefly touching on the basics so in theory anyone could learn the more advanced stuff. This book is very good but doesn’t do that. It is very concise.
The basics:
https://www.verywellfit.com/weight uh-training-fundamentals-a-concise-guide-3498525