Within the safety limitations all exercises should be as effective as possible. This has been always the primary goal in our equipment design. To achieve this David has participated and initiated research on various areas for a quarter of a century. The core of the know-how lies in biomechanical and physiological studies from the 80’s continuing in current studies in Finland, Germany and Austria. For us it is not enough that we think we have the right technology or concept, we also want to prove it. It is a long road, but eventually it pays off as the example with NICE guidelines in the UK shows. It took 17 years from the publication of a study with David back devices before it was used as in the guidelines of how to treat chronic and recurrent back pain.
The specificity of training
It is a well proven fact that human adaptation is highly specific. What you train is what you get. It is often claimed that training with machines with guided movement patterns are less effective than exercises, which develop also overall coordination, like training with free weights. There is a fundamental error in this thinking. Each and every exercise will teach exactly what that exercise requires, not much more. A bench press adapts the muscles and nervous system to do this movement as efficiently as possible but this specific skill provides few benefits in real life. A question to be asked is if this exercise is the best to effectively and safely train the chest, shoulder and arm muscles and the corresponding joints.
In fact there is a reverse correlation between coordination requirement and intensity of safe training. The more coordination is required; the lower is the training intensity of safe training. A good example of this is a squat exercise, which can be highly effective at high intensities but at the same time poses a high risk of injury. There is hardly anyone, who has not injured somehow their back with this exercise when young and enthusiastic.
For this reason we must ask ourselves what is the primary motivation for a particular exercise, is it to become skillful in this particular movement, like a bench press, or is it to strengthen the muscles and joints “the tools” in the body to manage in daily life or to practice sports of choice safely.
Our philosophy is the latter and therefore our sole motivation is provide safe, effective and comfortable exercises for all relevant muscle groups and joints in the human body.
Amount of coordination required in exercise
There is a very big difference between multi-joint exercises and single joint exercises when it comes to machine design. In a multi-joint exercise, like the leg press, the load regulation takes place automatically with the human skeletal leverage changes, whereas with a single joint movement, like the leg extension, all loading regulations must be designed, typically with an eccentric cam. For this reason, even without any regulation in the loading, a multi-joint training device can be reasonably good as long it functions without friction and has adequate seats, grips and footplates.
The same cannot be said about single joint movements. In fact most equipment on the market are so poorly designed that they should be banned. These are good examples, where the CE mark is obtained for technical safety, but no regard is placed in the biomechanical safety.
In a multi-joint movement the load is regulated by the changing “leverage arm” of the musculoskeletal system. In the beginning (red) the leverage is long, as legs are straightened (green) the leverage shortens and the load to muscles is reduced. In a single joint movement, the leverage arm remains the same in all positions and therefore the load must be regulated by the device.
There are several biomechanical and physiological principles to consider when the correct loading principles are designed. We will cover here three:
- Strength-length relationship of the muscle Sarcomere
- Stretch-shortening cycle
- Disproportionate fatigue
Strength – length relationship of the Sarcomere
The first basis for the loading design is the functioning principle of the smallest part of the human muscle, the “Sarcomere”. Sarcomere consists of two different elements, the ”myosin” and the “actin”. These elements slide within each other and are connected with “cross bridges”.
The number of active, interacting cross bridges determines the pulling strength of the sarcomere. The greatest number of interactions occurs at the midpoint of the length of the sacromere. When lengthened, many cross bridges are detached and when shortened, they cross over each other.
In short, purely concentric pulling force of the muscle is the greatest at the midpoint of the movement.
Stretch Shortening Cycle
This graph below shows the importance of eccentric / concentric cycle. Eccentric movement is when muscles are activated while lengthening and concentric movement when muscles are shortening. This diagram shows that muscles strength in the concentric phase is significantly higher when perceived by an eccentric phase (graph in the middle) compared to a situation where there is only concentric phase (graph on the left). This stored energy is lost if the eccentric phase is stopped for 0,9 seconds or more (graph on the right).
Electromyographic and force production characteristics of leg extensor muscles of elite weight lifters during isometric, concentric, and various stretch-shortening cycle exercises. Häkkinen K; Komi P V; Kauhanen H., 1989. International journal of sports medicine 1986;7(3):144-51.
This phenomenon is a critical characteristic of the muscles in real world situations where we have to face mass with inertia. Without even understanding this theory, we practice this in every-day situations. For example if we need to jump up, we go down first in a rapid movement before changing direction. This would seem stupid, since we have to first decelerate our body mass on the way down and again accelerate on the way up.
Artificial loading principles, like air pistons or electric motors can never imitate inertia in a proper way. This is a serious drawback, since inertia behaves totally differently during fast or slow movements and “stretch shortening cycle” is a critical component of muscle development. Combined with the David cam principle and natural loading with inertia, David can provide the most effective loading at any speed regardless of the training motivation.
Disproportionate muscle fatigue
Muscle fatigue causes the reduction of strength with each passing repetition. This is a very well known fact and can be felt by every exerciser.
What is not so well known is that the muscle does not fatigue in a proportional manner at all muscle lengths. In fact the muscle fatigues in a disproportional manner in shortened position.
The graph demonstrates that the “strength curve” produced by the muscle changes as the fatigue progresses.
This phenomenon is better understood if we do one single, maximum repetition. Since the load is very high, the movement is very slow. What happens is that during this one repetition, muscle fatigues. So at the end of the movement muscle is more fatigued than in the beginning, which is logical. In the same manner fatiguing happens also with a set of repetitions
Loading pattern design
This is a very typical situation with practically any other manufacturer on the market. The loading patterns are far from being accurate and there seems to be very little emphasis placed in the proper design of the cam shapes which provide loading curves.
It is very easy to test this with any product on the market. Put enough load to make even one repetition difficult. Perform then a few repetitions and you will quickly notice that it is difficult to complete the movement. What happens is that you have reached so called ”sticking point”, a point where the loading is higher than your respective strength in that knee angle.
When there is enough strength to make a few repetitions, it is possible to ”throw” the legs to full extension, but this will result in overloading in a critical area which can produce even knee pain and problems.
In order to provide precise and correct loading curve David cam rotates almost 1,4 times faster than the joint. This is a precondition to provide the rapid changes required in resistance. Disproportionate fatigue must be incorporated in the cam design in order to ensure that correct training is possible all the way to complete fatigue.