In extreme instances, over-exercising induces serious performance loss. Unaccustomed overexertion of muscles leads to rhabdomyolysis (damage to muscle) most often seen in new army recruits.[88] Another danger is overtraining, in which the intensity or volume of training exceeds the body's capacity to recover between bouts. One result of detrimental overtraining is suppressed immune function, with an increased incidence of upper respiratory tract infection (URTI). An increased incidence of URTIs is also associated with high volume/intensity training, as well as with excessive exercise (EE), such as in a marathon.[89] Marathon training requires the runner to build their intensity week to week which makes them more susceptible to injury the more they increase their mileage. A study shows that in the last 10–15 years up to 90% of marathon runners have suffered a physical injury from their training.[90]
6.  If an exercise can be done for more than 90 seconds, increase the resistance so that momentary muscular failure occurs within 45 - 90 seconds (this is considered "high-intensity" exercise). If you can do sit-ups for ten minutes, the intensity is insufficient to cross that threshold mentioned above, and you're just wasting valuable physiological resources. If you can't do even one rep, reduce the resistance (i.e. if doing a push-up, change from being on your toes to on your knees, or start from the top and slowly lower yourself; if using a machine, choose a lower setting; if using free-weights, pick a lower weight; if doing a chin-up, use a chair to boost yourself up to the top, then take your feet off the chair and slowly lower yourself).
Barre workouts require minimal equipment. You’ll need a free-standing or wall mounted bar and a mat. Sometimes a soft exercise ball may be used during leg workouts. If you are taking classes in a studio, the required equipment will most likely be provided for you. If you are working out at home, bars can be purchased for home use. You may prefer to be barefoot or purchase socks with grips on the bottom. As with all other workouts, having a water bottle and towel nearby is helpful.

"CrossFit differentiates itself by being constantly varied in both movements and time domains," Mandelbaum says. "You might have a day in the box with a four-minute sprint workout one day, and then come in the next day for a 15-minute moderate-to-fast-paced workout featuring three movements that need to be repeated in a cycle or round until the time clock runs out."
Both groups performed an equal proportion of exercise sessions alone (MCT: 50%, HIIT: 49.6%) and together with others (MCT: 50%, HIIT: 50.4%). In both groups, women had a significantly higher proportion of sessions together with others compared to men (56% vs. 44%, p < 0.01). The HIIT group had a significantly higher proportion of sessions organized by Generation 100 compared to the MCT group (8.1% vs. 5.9%, p < 0.01).
Regular aerobic exercise improves symptoms associated with a variety of central nervous system disorders and may be used as an adjunct therapy for these disorders. There is clear evidence of exercise treatment efficacy for major depressive disorder and attention deficit hyperactivity disorder.[43][48][52][53][54][55] The American Academy of Neurology's clinical practice guideline for mild cognitive impairment indicates that clinicians should recommend regular exercise (two times per week) to individuals who have been diagnosed with this condition.[56] Reviews of clinical evidence also support the use of exercise as an adjunct therapy for certain neurodegenerative disorders, particularly Alzheimer’s disease and Parkinson's disease.[57][58][59][60][61][62] Regular exercise is also associated with a lower risk of developing neurodegenerative disorders.[60][63] A large body of preclinical evidence and emerging clinical evidence supports the use of exercise therapy for treating and preventing the development of drug addictions.[64][65][66][67][68] Regular exercise has also been proposed as an adjunct therapy for brain cancers.[69]

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.