To provide an informed comparison of free weights and selectorized machines it is first necessary to have an explicit understanding of their purpose – both what it is and how it is best accomplished. The reason for the numerous misconceptions that exist surrounding the subject is that the majority of people claiming expertise and making authoritative statements on the issue lack this understanding, as well as understanding of related concepts such as motor learning and skill transfer, and basic mechanical physics.
It is important to note that there are currently over fifty companies manufacturing a wide variety of selectorized machines, and the overwhelming majority of these are incorrectly designed. This article is only concerned with properly designed machines.
The purpose of performing an exercise is to fatigue or inroad the strength levels of the involved muscular structures to a meaningful degree to stimulate a growth mechanism. Ideally, this should occur within some minimum time frame to avoid using any more of the body’s limited metabolic resources than necessary, since those same resources are required for recovery and growth. There are a variety of factors which affect how efficiently an exercise accomplishes this. Some of these include how well the resistance is balanced to the users strength in different positions over the range of motion, whether the targeted musculature is loaded directly, and how well the exercise facilitates focus and concentration on the targeted musculature.
Variable Resistance power press machine manufacturers
A person’s usable strength changes from position to position during any movement due to the effects of friction, myofibril congestion, virtual cam effects resulting from the changes in tendon insertion angles and other factors, as well as changes in leverage resulting from the changing angles of the involved bones during multi-joint movements. If we graph these changes in usable strength over the full range of motion of any particular exercise, we have what is referred to as the strength curve for that exercise.
The resistance provided by free weights or machines can also vary over the range of motion of an exercise due to changes in leverage or torque. This change in resistance over the range of motion is what is referred to as the resistance curve for that exercise.
Ideally, this resistance should vary in proportion to the strength curve of the involved muscular structures, increasing in positions where they are stronger and decreasing in positions where they are weaker. This makes it possible to meaningfully load the muscles continuously throughout the exercise and achieve the desired level of inroad with little or no energy wasted. The less wasted during exercise, the more available to the body afterwards for recovery and growth.