It is well accepted that success in most sports depends on the ability of the athletes to exert high levels of strength and power (Stone et al., 1994). For this reason, training programs are nowadays directed to improve those two characteristics no matter what sport the athlete is preparing for. Technology in sport has quickly developed and nowadays many tools are availble to assess an athlete’s strength and power capabilities. Moreover, recent dynamometers provide a form of biofeedback to be able to guide the athlete during his/her strength training program just like heart rate monitors provide guidance during running, rowing and cycling sessions.
Before discussing how to measure strength and power and how to use those measurements to change an athlete’s training program, we need to review some very simple biomechanical concepts.

First of all, strength relates to the ability of an individual to produce force. Force is measured in Newtons (N) and is what is required to stop, start or alter motion. According to Newton’s 2nd law, Force can be calculated as follows:

F = m x a

Where m is the mass and a the acceleration.

Acceleration can be defined as the rate of change in velocity of an object and is calculated as follows:

a = Dv/Dt

Where v is velocity and t is time. Acceleration is measured in m/s2.
When an object experiences a change in position (displacement) it has been displaced and motion has occurred. Since motion has spatial and temporal elements it is important to know the speed and/or the velocity of the object. Specifically, speed defines how fast an object is moving whereas velocity tells both how fast and in what direction the object was moving.
Velocity is calculated as follows:
v= Ds/Dt

Where s is the distance travelled by the object and t is the time needed to cover the distance. Velocity and speed are measured in m/s.

In many short duration events (i.e. sprinting, vertical jump, Olympic lifts) the rate at which muscles can produce mechanical work is often the variable limiting performance. For this reason, knowing how much power an athlete is producing in a given motion could be important to assess the effectiveness of a training program. Power production is measured as the amount of work done per unit of time.

Since mechanical work is calculated as the product of the displacement experienced by an object and the component of force acting in the direction of its displacement divided by the time needed to complete the displacement, it can be calculated in the following ways:

F x s/Dt = F x v

Power is measured in watts (W).

All of the above parameters can be calculated as an average over a range of motion or as an instantaneous value occurring at a particular instant during the displacement of an object.
Enough biomechanics! Let’s start to use it to understand how many commercial systems are using biomechanics to measure Force, Power and Velocity during movements.
Let’s say we want to know how much Force, Power and Velocity a particular athlete is producing while performing a simple weight lifting exercise like the Bench Press. What do we need to do? First of all, we need to know how much is the mass the athlete will be lifting. This will be important as it is a necessary parameter for calculating Force. Secondly, we will need to know how much does the barbell travel and how long it takes to complete the full bench press motion in order to calculate the velocity of the barbell. Finally, since we know the force and velocity, we will be able to calculate the mechanical power produced.

The most common systems currently on the market use very simple technology. The Ballistic Measurement System (BMS, http://www.innervations.com/), Muscle Lab (http://www.ergotest.com/), Real Power (http://www.globusitalia.com/) and others base all the calculations on potentiometers, linear and/or rotary encoders (distance transducers) and accelerometers. Those transducers are attached to the bar (or weight stack track the displacement of the bar or weight stack (See Image) and by inputting the mass of the object tracked, the dedicated software calculates the relevant biomechanical parameters. The information can be provided in real time and stored for analysis.

All the devices used to measure force, velocity and power while performing weight lifting exercises (isotonic or isoinertial movements) are termed isoinertial dynamometers.
How can isoinertial dynamometers help us for designing a training program? In many ways. We can use isoinertial dynamometers to establish the strength and power characteristics of an athlete and compare his/her values with elite athletes and track his/her progression at different phases of the training program. Furthermore, we can use it to assign the training load and also as a biofeedback system to make sure the athlete is moving the weight as fast as he/she can. Also, due to the possibility of applying the position transducers to every moving object, we can test the athletes in open and closed chain type of exercises (i.e. Leg Press vs. Leg Extension). Furthermore, we can measure differences in Force, Velocity and then Power production between limbs.
Let’s do some examples. If we want to establish the strength and power characteristics of an athlete in the upper body, we can measure him/her while performing the bench press with 5 increasing loads (see Figure 2).

For each load we will measure Force, Velocity and Power. The information will be used to plot the Force/Velocity and Power/Velocity relationships (See Figure 3). The curves can be used to identify the lifting load needed to improve strength and power performance. In particular, if we want to identify the training load that maximises the athlete’s power output, we will chose the one that corresponds to his/her peak power. We know that in order to enhance explosive power performance, the load at which the maximal power is obtained must be used for training and the velocity of execution of the exercise used must be always maximal. This approach to strength training has been shown to produce remarkable enhancement of mechanical power (Berger, 1963; Hakkinen, 1994; Kaneko et al., 1983; Moritani et al., 1987).

Following few weeks of training, we can repeat the testing procedure and verify the effectiveness of our training programme and how the athlete is adapting. If the Force/Velocity and Power/Velocity curves are shifted to the right, we have made our athlete stronger and faster (green line, Figure 4), if the curve is the same as before, no change occurred, and if the curve moved to the left, it means the athlete is weaker and slower (red line, Figure4).

Isoinertial dynamometers can be used to detect also overtraining and overreaching, since a decrease in strength and power could be due also to an excess of training load and intensity.
The recent generation of isoinertial dynamometers provides realtime feedback during training. So, just like heart rate monitors providing information and biofeedback about the level of effort exerted by your cardiovascular system, iso-inertial dynamometers can be used to gauge your neuromuscular system during a weight training session. We can measure in real time power output (or how fast the athlete is lifting the weight) and provide feedback to the athlete to move it faster if the speed is too low. Furthermore, we could use the real time monitoring to decide how many repetitions the athlete should perform if we see any sign of fatigue and/or drop in power output. The following is an example of two different athletes performing 10 repetitions with a load equal to 70% of their 1RM. They were both asked to perform the lifts as fast as possible in the concentric phase. As you can see, Athlete A can maintain power output without significant drops for all the 10 repetitions, however Athlete B starts producing less power after the 5th repetition (Figure 5).

How can we use this information? If the goal of the session is to improve power output, then the athlete should perform a number of repetitions in which power output (and velocity of movement) is high, lifting the weight slowly in fact does not help in getting him/her faster and more powerful. Therefore, we can use isoinertial dynamometers not only for testing purposes but also to monitor single training sessions and make sure that the athlete is lifting fast and is producing high levels of power.

Are isoinertial dynamometers valid and reliable? Yes they are, provided that if you are using a linear encoder or another type of position transducer the path of the barbell, dumbbell or weight stack is linear. Few studies have been conducted comparing the parameters calculated by encoder-based systems with the same parameters measured with force plates. During Squat exercises manyh authors have found the measures to be valid and also highly reliable (i.e. Bosco et al., 1995; Rahmani et al., 2001). Also, since the best way to measure your athlete’s progression is to measure Force and Power in the exercises routinely used in training

In conclusion, if you want to measure Force, Speed, Velocity and Power in order to evaluate your athlete’s progression and to identify the proper training load, isoinertial dynamometers are your best option since they allow you to measure every lifting exercise (Barbells, Dumbbells and/or isotonic weight stack machines) with a wide range of loads.
For non linear movements (such as Olympic lifts) it is better to use accelerometers to be able to directly measure the acceleration of the barbell, dumbbell and weight stack and then calculate Force and Power.
However some principles need to be applied when measuring your athletes. If you are planning to repeat the measurements on your athletes, make sure you use the exact same range of motion used in previous measurements, use the same loads used in previous measurements and make sure the mass lifted by the athlete is exactly the same as in previous measurements to avoid that your athletes results in faster scores only because he/she is lifting a lighter barbell!

References:
Berger, R. (1963). Effect of dynamic and static training on vertical jumping. Research Quarterly, 34: 419-424

Hakkinen, K. (1994). Neuromuscular adaptation during strength training, aging, detraining and immobilization. Crit. Rev. Phys. Rehab. Med., 6: 161-198

Kaneko, M., Fuchimoto, T., Toji, H., & Suel, K. (1983). Training effect of differing loads on the force velocità relationship and mechanical power output in human muscle. Scandinavian Journal Sport Science, 5: 50-55

Moritani, T., Muro, M., Ishida, K., & Taguchi, S. (1987). Electrophysiological analyses of the effects of muscle power training. Research Journal of Physical Education, 1. 23-32

Bosco C, Belli A, Astrua M et al. (1995) A dynamometer for evaluation of dynamic muscle work European Journal of Applied Physiology 70: 379–86
Rahmani A, Viale F, Dalleau G, Lacour JR. (2001) Force/velocity and power/velocity relationships in squat exercise. European Journal of Applied Physiology 84(3):227-32