In summary, if you're only interested in a basic understanding of HIT methodology and where much of it originated I would suggest starting with a far less technical book. I suggest starting with the last published edition of Ellington Darden's "The Nautilus Book" and perhaps "Total Fitness: The Nautilus Way". If you like what you read and want to dig a little deeper into the evolution of HIT read Darden's more recent book, "The New High Intensity Training: The Best Muscle-Building System You've Never Tried". If the gears in your head are in high gear after that and you really want to get DEEP into what evolved from the original Nautilus protocol _then_ you go for "Superslow" or preferably "The Renaissance of Exercise: A Vitruvian Adventure Volume 1". When your grasp of all the aforementioned material is truly solid then move on to Doug McGuff's writing. McGuff's ideas do not surpass or supplant Hutchins' but rather sharpen the points with brilliant thoughts and clinical observations from a medical physician's perspective. Doug McGuff, MD published his "Ultimate Exercise: Bulletin #1" in the late 90's and later updated that with "Body by Science: A Research Based Program to Get the Results You Want in 12 Minutes a Week", both of which are hugely valuable contributions to the literature on HIT methodology and philosophy. His article about "Stoicism in Training" is critical reading.

CrossFit Games: The sport of fitness has arrived (or so claims Reebok, the official sponsor of the CrossFit Games). Each summer the CrossFit Games test participants with a barrage of physical challenges and workouts, ranging from swimming and running to pull-ups and handstand walks (sorry, Kobayashi, hot-dog eating has yet to make an appearance). Participants accrue points over the events, and the male and female winners are crowned World’s Fittest Man & Woman. Sectional and Regional qualifiers narrow the field before the annual Games Weekend.
If you haven’t strength trained regularly, muscle loss may now reach critical levels, interfering with balance, gait, and other daily activities. But if you take up strength training, those changes are reversible: A number of studies including adults in their 70s have found that progressive strength training two or three times per week can lead to such improvements as increased muscle mass, more ease with everyday activities like climbing stairs and carrying groceries, and reduced joint pain.
3.  Move v-e-r-y s-l-o-w-l-y but smoothly. "Explosive" movement is not only nonproductive, but also dangerous. Plus, moving slowly eliminates momentum, which ensures constant muscle loading. Make a movement last about ten seconds. (A chin-up should take about ten seconds from the lowest to the highest point, and then another ten seconds from the highest to the lowest point. Same goes for a push-up.) There is nothing to be gained from fast movements. Moving slowly prevents injury. (There are over 30 million exercise related injuries annually in this country; most of these can no doubt be attributed to high-force movement.) Keep your movements low-force and high-intensity. An analogy: If you attempt to lift your car quickly, you will likely injure yourself even if using proper form. If you try lifting it slowly and intensely, your chances of injury are nil. Think of how you drive your car over speed bumps... fast will cause damage to the car's suspension, slow will not.

Perception of effort, defined as “the conscious sensation of how hard, heavy, and strenuous exercise is” [23, 24], was measured during the incremental test (at the end of each minute) and during the time to exhaustion tests (at the end of the warm-up and every 30 s) using the 15 points RPE scale (Borg 1998). Standardized instructions for the scale were given to each subject before the warm-up. Briefly, subjects were asked to rate how hard they were driving their leg during the exercise (leg RPE [8, 24, 25]). Subjects were also asked to not use this rating as an expression of leg muscle pain (i.e., the intensity of hurt that a subject feels in his quadriceps muscles only).
^ Jump up to: a b Kyu, Hmwe H; Bachman, Victoria F; Alexander, Lily T; Mumford, John Everett; Afshin, Ashkan; Estep, Kara; Veerman, J Lennert; Delwiche, Kristen; Iannarone, Marissa L; Moyer, Madeline L; Cercy, Kelly; Vos, Theo; Murray, Christopher J L; Forouzanfar, Mohammad H (9 August 2016). "Physical activity and risk of breast cancer, colon cancer, diabetes, ischemic heart disease, and ischemic stroke events: systematic review and dose-response meta-analysis for the Global Burden of Disease Study 2013". BMJ. 354: i3857. doi:10.1136/bmj.i3857. PMC 4979358. PMID 27510511.
One way repeated ANOVA was used to compare time to exhaustion between sessions (S1, S2 and S3). Relative reliability was calculated with the intraclass correlation (ICC) model (3, 1) [27]. Absolute reliability was calculated with the typical error of measurement (the standard deviation of the change scores divided by [28, 29]). Bland and Altman’s 95% limits of agreement were also used (calculated for S1 vs S2, S1 vs S3 and S2 vs S3) as an additional representation of measurement error and to identify the presence of heteroscedasticity [19]. As data were heteroscedastic, both raw data and log transformed Bland and Altman’s plots are presented. Limit of agreement ratio (LOA) was also calculated from the log transformed data as follow: LOA = (1.96 × SDdiff / grand mean) × 100; where “SDdiff” represents the SD of the differences between tests (S1 vs S2, S1 vs S3, S2 vs S3) and “grand mean” represents (mean S1 + mean S2 + mean S3)/3. As time to exhaustion data were heteroscedastic, we also calculated the coefficient of variation (CV) for each subject as follow: CV = 100×(SD of the three measurements)/(mean of the three measurements). Mean CV for all subjects were also calculated. We also calculated the smallest worthwhile change (0.2 × between subjects SD) [21].

Chronic stress and the subsequent chronic peripheral glucocorticoid secretion plays an important role in the desensitisation of higher brain centre response during acute stressors because it has been shown that in acute (and also chronic) immobilisation, the responsiveness of hypothalamic corticotrophin-releasing hormone (CRH) neurons rapidly falls.26 These adaptation mechanisms could be the consequence of changes in neurotransmitter release, depletion of CRH and/or desensitisation of hypothalamic hormonal release to afferent neurotransmitter input.26 Indeed, concentrations of CRH are elevated, the number of CRH-secreting neurons in the limbic brain regions is increased and the number of CRH binding sites in the frontal cortex is reduced secondary to increased CRH concentrations following chronic stress.25

Now that you understand the vast amount of categories available to you, you can start narrowing the field of choices down by looking at the specifics of exercise videos and deciding which ones will be something you will not only enjoy but continue to use long term. If you are starting out as a beginner to exercise, then you should expect to upgrade your exercise videos as your fitness levels increase. You don’t have to stay with a beginner video when your fitness level advances to intermediate or advanced. Below are some factors to look at when looking at individual videos.
Thus, little is known about the effects of monitored vigorous exercise in elderly people. While significant benefits for basic motor tasks (such as balance and gait) can be achieved through different kinds of physical activity (i.e., stretching exercises, treadmill, Pilates, and strength and balance training), no conclusive relationship has been proven between its intensity and such improvements. Recently, Pau et al. [14] reported that spatiotemporal gait parameters and sit-to-stand performance significantly improve through vigorous (but not light) exercises, thus suggesting that higher levels of intensity might be more suitable in generally improving static and dynamic daily motor tasks.