5.  Even when you can't complete another repetition, keep trying to complete this "impossible rep". Even though you aren't moving, metabolic work is being done, and a more thorough "inroad" to muscular failure is accomplished. "Pushing to failure" signals the body to upgrade its capabilities ("adaptive response")... something it will not do unless given a good reason to.
Bonds H. The politics of the male body in global sport - the Danish involvement. Oxon: Routledge; 2010.  Müller, like Checkley, took a firm stand against exercise machinery, stating they were unnecessary and harmful with advocates described as having ‘biceps or triceps … as their chief credentials.’15 Müller JP. My system. London: Link House; 1904. [Google Scholar] After ‘My System’ was published, Müller traveled throughout Europe with lectures and exercise demonstrations and settled in London in 1912, to establish the ‘Müller Institute’ in which he offered group and individual classes to the public.15,16 Müller JP. My system. London: Link House; 1904.
The goal in training competitive athletes is to provide training loads that are effective in improving performance. At some stages during the training process, athletes may experience an unexplainable decrease in performance. This might happen when prolonged excessive training takes place concurrent with other stressors and insufficient recovery. This unexplainable performance decrements can result in chronic maladaptations that can lead to the overtraining syndrome (OTS). A keyword in the recognition of OTS might be “prolonged maladaptation” not only of the athletic performance but also of several biological, neurochemical and hormonal regulation mechanisms. When athletes deliberately use a short-term period (eg, training camp) to increase training load, they can experience short-term performance decrement, without severe psychological or lasting other negative symptoms.1 2 This functional over reaching (FO) will eventually lead to an improvement in performance after recovery. However, when athletes do not sufficiently respect the balance between training and recovery, non-functional over-reaching (NFO) can occur.1 2 At this stage, the first signs and symptoms of prolonged maladaptation such as performance decrements, psychological disturbance (decreased vigour, increased fatigue) and hormonal disturbances are present, and the athlete will need weeks or months to recover. The distinction between NFO and OTS is very difficult and will depend on the clinical outcome and exclusion diagnosis.
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.