News & Comment / Training


Frederick C Hatfield PhD

International Sports Sciences Association, Santa Barbara, California 93101. Email:

Sportscience 3(1),, 1999 (762 words)

Reviewed by: William J Kraemer PhD, Human Performance Laboratory, Ball State University, Muncie, Indiana  47306; Steven S Plisk, MS CSCS, Director of Sports Conditioning, Yale University, New Haven, Connecticut 06520


A new strength-training machine allows the user to instantly change resistance at any point in the exercise movement. The machine has the potential to improve strength by optimizing the time each targeted muscle spends under maximum stress. Whether it is more effective than other machines or free weights remains to be seen. Reprint · Help


KEYWORDS: gym equipment, strength training
Reviewers' Comments


Those of you who, like me, have spent nearly 50 years in theweight room will recall all manner of equipment that promised torevolutionize training. In my opinion only a small handful ofinnovative devices have been introduced, and most of those during thelast two or three years. Whether they have improved the effectivenessof strength training is often hotly debated. The new machine I reviewhere may be one of the good ones.

For years, strength-training machines were built with no specialintent beyond making muscles bigger and stronger by allowing the userto work against resistance. Then a few of the more thoughtfulathletes and trainers realized that the ability of a muscle torapidly generate maximum force involves a time/rate dependency thattheir current training technology was unable to address adequately.An explosion of innovations ensued. Eventually four categories ofdevice could be discerned (Hatfield et al.,1999):

  • constant-resistance--resistance does not increase or decrease during the course of exercise;
  • variable-resistance--devices that increase or decrease the resistance during a movement to match changes in joint leverage;
  • accommodating-resistance--devices designed to allow you to exert maximum resistance throughout the full range of movement by controlling the speed of movement;
  • static-resistance--devices that prevent movement altogether.

Most sport scientists agree that machines are generally inferiorto the constant resistance provided by free weights. Nevertheless,derivative technologies continue to flood the marketplace. Recently,Scott Naidus has patented and produced a cam-operated selectorized(weight-stack) machine (Figure 1).

Figure 1: The new weights machine for providing dynamically controlled resistance.

The machine has an electric motor that drives two sprocketsengaged by a cam designed to increase or decrease the angle of motionof a weighted lever arm. This innovation allows the user to adjustthe resistance at any instant during a movement. The inventor usesthe term dynamically controlled resistance (DCR) to describethe way the machine works (see his commercial site claims the following unique benefits for his machine:

  • The user can change either the positive or negative resistance instantly at any moment during a set or repetition.
  • If the user learns how to manipulate the resistance properly, s/he will be able to achieve a higher work load during each rep and set.
  • Casual fitness enthusiasts will therefore be able "…to work out twice as hard and be out of the gym in half the time."
  • Protocols can be fine tuned to make the training stress more closely match the user's sport-specific requirements.

Let's look at this technology from the standpoint of current gympractice. One training method in particular comes to mind: forcedreps. An experienced and educated spotter can be helpful in ensuringthat the lifter is working at the highest level of stress simply byadding to or taking away some of the weight in the lifter's hands.But such gifted spotters are rare. With the new machine, a lifter canalter the amount of resistance at any time, thereby accounting formomentary fatigue or a sticking point in a given movement. As usual,the chief drawback is the level of training wisdom of the lifter inknowing when to alter the resistance, and by how much.

In a personal communication to me, Naidus wrote:

I believe the features of my machines are excellent for permitting the exerciser to engage in different and unique routines in every workout, thereby serving to stimulate the muscles and impose upon them new patterns of workloads. This should ultimately result in constant adaptation, and better strength gains.

I do not disagree. This, from an old gym rat: it's certainly wortha try! Just don't throw away your dumbbells and barbells.

Hatfield FC, KreisEJ, Hatfield II FC (1999). Sports conditioning: the complete guide.Santa Barbara, California: International Sports SciencesAssociation

Annotated by the Editor

Stephen Plisk pointed out that the machine is a modified versionof those designed to exercise muscle groups in isolation. He assertedthat such exercises are non-functional. The author thought thisassessment was "a bit harsh", because they "work fine at producingoverload".

The reviewer also downplayed the importance of "time understress". He emphasized the importance of "impulse and motor control"in the development of strength, and he thought that "this type ofresistance-training technology does little to account for them". Inreply, the author agreed with the need to lift weights more slowly atfirst, and that the importance of motor control is why "...we alllike free weights more than machines.."

William Kraemer was a little more positive than Stephen Plisk, buthe raised several issues. I quote with minor editing:

  • Optimizing force over the range of motion has been a quest for many lifters seeking a unique resistance exercise machine. The rationale for this quest is the fact that each movement has its own strength curve and the fact that each individual has a different body make-up (limb lengths, tendon insertions, etc.).
  • The key concept here is that the machine allows one to modulate the resistance over the range of motion rather than having it set beforehand. In theory, therefore, force can be optimized over the range of motion. The assumption of such a theory is that the body will be able to tolerate such forces and benefit from their use. However, it has been shown recently that when you maximally stimulate eccentric phases of the movement in a set the body compensates by producing less force concentrically even in that same set. [So the athlete may lose on the swings what s/he gained on the roundabouts.]
  • But what is optimal? If force maximized through a complete range of motion is optimal, this device may be better than one that simply limits resistance to the weakest point in the strength curve.
  • It is not clear how the lifter will optimally increase the resistance over the range of motion. The necessary neural feedback would appear to be faster than the lifter's ability to respond with more or less resistance, especially at high velocities of movement.
  • Some exercises are bounded by an impact point at the end of a joint's range of motion (e.g., knee extension). The loading and activation of muscle is related to protection of the joint--hence the deceleration through certain parts of the range of motion at higher speeds. How will the lifter know when to change the amount of resistance due to deceleration of the limb as it reaches its terminal limits?
  • This equipment is an example of technological capability that goes beyond our understanding of how muscle adapts to overload. It may also go beyond our ability to withstand the forces.
  • Nevertheless, increasing the intensity of load over the range of motion has always been a dream of many lifters. The question now is can the lifter engage such training purely on perceptual cues? This piece of equipment will certainly take some "smarts" to use.
  • [Technical issues] How well does the machine work for people at the extremes of body size? Are all pieces of equipment for different body parts the same? Limb speed affects force production, so is limb speed calculated and displayed?

The author thought that this response was fair. He agreed thatonly time will tell whether the new tech is better than what has gonebefore.

I also gave Scott Naidus an opportunity to respond to thisarticle. He said that it was not his intention to replace freeweights with his machines. His objective is to give athletes anothertool to maximize performance.

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Published March 1999