Go to failure on isolation exercises?

[Ideuzee]

Let's say we're doing rear delts on a machine and I do 70kg x 10 max even tho my form is pretty much the same as me doing 50kg x10 I can't feel the muscle that has been worked. With the lower weight I feel the muscle a lot more and feel more of a burn. Why is this?

Thank you

 

[zazen5]

Let's say we're doing rear delts on a machine and I do 70kg x 10 max even tho my form is pretty much the same as me doing 50kg x10 I can't feel the muscle that has been worked. With the lower weight I feel the muscle a lot more and feel more of a burn. Why is this?

Thank you

Higher reps is anaerobic glycolysis with lactic acid. Heavier weights is atp-cp, anaerobic metabolism hasnt shifted to glycogen/blood glucose without oxygen(not enough time-has to be beyond 30 seconds). Keep in mind how the muscle "feels" doesnt equate with intensity. Although a 20 rep set feels more intense near the 20th rep due to lactic acid buildup, the reality is the intensity is lesser than lifting a really heavy weight and only a few reps, and the danger is also higher to the tendons and joints during heavier sets. Physics shows evidence of this: A tour de france cyclist may only make 60 watts of energy a day from cycling, the rest is dissipated as heat whereas one rep on a heavy weight does more watts because there is direct energy generated to move the heavy weight. The reason some people prefer higher reps is related to synaptic wiring. With a slow twitch unit the speed is like 50 m/sec, whereas a fast type IIB fiber is 400 m/second so the person with slow twitch muscles is just getting started at rep 10 due to spatial summation of engagement of the muscle fibers at the neurological level. It is much sooner that a type IIB fiber undergoes temporal summation and tetanus of the muscle hence the need for fewer reps.