First off a review of what I consider to be exercise and what I do not.
There are many activities that people call exercise. Some are sports, some are recreation, some are sheer nonsense. All of them, may or may not provide a stimulus for the body to make an adaptation that will be beneficial to us in terms of health and/or functional ability, the latter, being a priority as we get older to maintaining quality of life.
Very few of them meet the criteria of safety, efficiency and effectiveness.
Sports and recreational activities do not, and the proper reason for pursuing these activities is if the individual seeks the enjoyment of the activity and the fulfilment that comes with accomplishments within reaching goals within that activity or sport. One should do so with the knowledge that these activities will present varying degrees of risk of acute or long term injury. Life, however is meant to be lived, and avoiding all risks is likely worse than being reckless. On the other hand, if someone is engaging in these activities and they don't actually enjoy them, but are doing so because they believe they are beneficial to them, then I have good news. Stop doing this... you are doing yourself more harm than good.
Proper exercise will go a long way towards minimizing the dangers of said sports/recreational activities by making muscles, joints, ligaments and bones as strong and as resistant to injuries as possible. "You use exercise to help you perform a sport/activity better or safer, NOT do the sport as exercise per se."
Proper exercise does not include high force activities of any type including steady state activities of any type or any explosive movements such as plyometrics or most crossfit movement, so pretty much most of what the personal trainer at the gym will have you do. (The crossfit movement has discovered the principle of intensity which is good, but the safety of most movements as any of their online forums will attest with their thousands of discussions on injuries.)
Proper exercise has been defined as
"Exercise is a process whereby the body performs work of a demanding nature in accordance with muscle and joint function, in a clinically controlled environment, within the constraints of safety, meaningfully loading the muscular structures to inroad their strength levels to stimulate a growth mechanism within minimum time." Ken Hurtchins
So with that preamble done, the topic of this blog is what are the different ways of inducing the aforementioned stimulus. Refer to previous blogs if you're not familiar with the fact that I believe such stimulus can and should be done with a minimum amount of volume and frequency; as little as 15 minutes a week, is not just something you can "get away with" but is necessary to allow the body the proper time to make the adaptation from the very intense stimulus.
One of the tenets of "deep inroad" is to exhaust the maximum number and types of muscle fibers, to provide the optimum stimulus, particularly the fast twitch fibers which have been shown to be most prone to adaptation.
One principle is that the set of an exercise should have a specific length of time, which may vary from individual to individual and even from body part to body part but usually falls between 30 seconds and as much as 3 minutes, in order to have an "orderly recruitment of fibers" starting with the slow twitch fibers and ending with the fastest twitch fibers but doing so before the initial slow twitch fibers have had a chance to recover and be used again. This makes sense. Finding the proper "time under load" requires some experimentation, and even though it has been suggested that each person has an ideal "tul", there is also evidence that there may be benefits with using different ones from time to time. I don't think we need to become overly obsessed with getting it perfectly.
Another principle is that the execution of the exercise should be such that through excellent form, and properly designed machines the targeted muscles receive a stimulus that is not only intense but that by not involving other muscles, small rest moments during the execution of a rep or set, the muscle will be stimulated maximally and most efficiently providing the highest quality stimulus, and theoretically the best results. Again, I believe this is valid, not only for effectiveness but also for safety purposes. This also in fact, may be very useful in research conditions that seek to eliminate variables completely in order to determine specifically how exercise affects and benefits us. However, in actual practice, a downside is that it requires the learning of such execution to become a fairly complex skill, special machines and usually a competent trainer to supervise the execution as even long time exercisers, will compromise form unsupervised as the exercise moves to the most difficult part of the set, as we get closer to muscular failure.
Finally, the principle that I like, is that by choosing certain exercises that are most demanding to the largest muscles of the body and challenge the body as a whole the most, will literally force the body to adapt because the signal that a "life threatening stress" (how your body perceives it) will be so strong as to kick all adaptive resources maximally into gear. I'm talking about exercises like the barbell squat, and deadlifts, particularly the trap bar deadlift. The most sophisticated machine with the most competent trainer and "perfect" form, may maximally stimulate fibers for the targeted muscle group, but I don't believe will ever provide the overall stimulus imparted for example by heavy deadlifts or squats. (Arthur Jones, inventor of Nautilus, referred to this an an indirect effect.) Again, however as with the two previous principles, there is a downside... one must approach these exercises with extreme caution and learn them properly to avoid injury. The trap bar deadlift is a huge improvement over the traditional barbell deadlift in this respect, however, it must be treated with the utmost respect.
One has to choose between their priorities of safety, effectiveness and accessibility to proper equipment and trainers.
I will conclude also by stating that the foregoing has dealt with the concept of the best stimulus to adaptation, but all this becomes "straightening deck chairs on the Titanic", if the organism cannot adapt, because the individual is sleep deprived, (very common in today's society) or their nutritional practices are terrible. (both being important, but the former taking precedence over the latter.) (See previous posts on sleep, and also highly recommend book: "Lights Out: Sleep, Sugar and Survival".
All for now, just some quick thoughts.