DESIGN PRINCIPLES

The end-user experience is the ultimate driving force for us at David when we design new products. Naturally we have to think about costs, maintenance and other client (facility owner) related matters, but they should never supersede the end user needs. This philosophy is perhaps not the fastest way to high sales volumes but in the long run it has built us a reputation few companies in the world can match. There are three main requirements for each design and they are always implemented in this order:

One of the main reasons for injury and discomfort in training is poorly designed resistance curves. Good example of this are the leg extension machines in the market. With way too light resistance in the first part of the movement, these devices tempt users to accelerate the movement and reach an area of overloading at the extended position by momentum creating a real danger especially in the eccentric portion with extended legs. Another example is the commonly used rubber band with internal and external rotation of the glenohumeral joint. The resistance of the rubber band increases when stretched while the strength actually decreases during the movement. This causes strain in the joint while the training effect is very poor.

Safety for muscles and joints can be secured by using correct resistance curves, which perfectly match muscles’ capacity at every point of the movement and take into consideration other factors like the stretch-shortening-cycle and disproportionate fatigue, which are covered more closely in the “Efficiency” section.

One would think that joint alignment is a pretty straightforward matter, but this is not the case. A common mistake in device design is to align the joint visually where the joint is, but there is much more to it. In a dynamic movement the center point of the joint can be far away from the static joint location.

A good example of this is the horizontal abduction or adduction (peck deck) of the shoulder joint. Most devices in the market position the axis points at the shoulder width, but in reality the axis points should be close to 10 cm narrower to each other. The simple reason for this is that in this exercise also the shoulder joints themselves make an arch during the exercise and the “virtual joint” is between the shoulder joint and the medial connection of the collar bone. 

By carefully analyzing the correct “virtual joint” positions of each device, David has been able to create dynamically safe and comfortable exercises enabling training for people with various joint and muscle related problems.

There is a considerable difference in the safe movement range with healthy persons and people with joint and muscle problems. A safe device is designed to serve both. With David the range of motion is controlled in two different ways: 

(1). RMA (range of motion adjustment) adjusts the starting point of the movement so that there is no load beyond a certain point. 

(2). RMC (range of motion control) in which the colorful and clear EVE display shows both ends of the movement with green zones and the aim is to keep the movement between these zone and exceeding the zone results in red alarm.

With the EVE system it is possible not only to train within the safe movement range prescribed, but also to learn to control the movement. However, if the control fails, the RMA mechanisms will make sure that the movement will stay within a safe range.

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.

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.

There are several biomechanical and physiological principles to consider when the correct loading principles are designed. We will cover here three:

(1). Strength-length relationship of the muscle Sarcomere 

(2). Stretch-shortening cycle

(3). Disproportionate fatigue

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.

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).

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.

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 above 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

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.

DAVID has succeeded in solving this challenge by using smart technical solutions. Actuators take care of major adjustments based on retrieved position information from the cloud. Minor adjustments are automatic using either gas springs or linearly sliding cushions. Body fixations are done using mostly lockable gas springs which provide easy, quick and firm isolation of the target joint. All elements users need to touch are yellow for easy recognition. All DAVID devices can be patient operated without assistance and positions are always correct.

The other challenging part in the use of rehabilitation devices is to ensure correct training. Training includes several variables that need to be fine tuned individually and followed by the patient. Variables like loading, start and end positions of the movement, speed of movement, duration, number of sets and loading and range progressions are critical for correct “dosage”. Mistakes can easily happen with any of these variables and even with close supervision by therapists does not guarantee correct compliance. 

With DAVID’s clear and colourful terminals for visual guidance and intelligent, cloud software which provides up-to-date, location independent individualised data, these variables are always in control and correct. Use of the devices and following programs without therapists assistance is easy and empowering and automatically collected data provides the level of compliance in real time.