It is generally accepted that testing muscle performance isokinetically is not directly related to function. It would seem obvious that movements performed at constant angular velocity are completely unrelated to those seen in most activities particularly sports where rapid periods of acceleration are followed by high speed limb movements which can not be reproduced by today’s dynamometers! Further to this most movements in the real world occur due to the interactions of multiple joint and muscle systems unlike most isokinetic tests which occur at single joints it’s that simple.
It has to be said that different activities require different levels of aptitude in many areas e.g. a power lifter who has to lower a bar to their chest and the push it back to it’s starting position only one time depends mainly on strength. Whereas a long distance runner relies on their body’s ability to remove waste products from their muscles efficiently whilst delivering the optimum amount of oxygen and energy. So we wouldn’t test these two sets of athletes in the same way, as the requirements of their individual sports are very different, and temporary factors like sleep can change the requirements of the athlete. It would then seem obvious that isokinetic testing would not be appropriate to all populations and further more the populations that did suit isokinetic testing would not necessarily be tested in the same way.
To look further at our example we could perform a closed chain bench press isokinetic test on most modern dynamometers and this could be used to monitor the athletes’ progress and maybe even for training purposes if that was deemed to be appropriate. But we are aware that the bench press is an exercise that involves multiple joint systems; it could then be appropriate for us to look at the contribution of the individual joints in that system to performing that movement. With this information we could look to see if any one individual part of the system was more important than another within an athletic group (do international level athletes have much stronger triceps than regional level athletes?) i.e. is the strength of the triceps more critical than the strength of the shoulder horizontal adductors. Thought would then have to be given to the relative length of the levers between the athletic groups along with handgrip width and protrusion of the thoracic cage i.e. how far does the bar have to travel in total. To perform a study of this nature would involve a lot of time and effort not to mention mathematical ability so in the ‘real’ world of research we don’t do this we take a group of local level athletes and a group of national level athletes of mixed age, ability and training status and look to see if there is a difference between their triceps strength. Then we try to work out, using very reliable statistics, how strong someone should be if their triceps were certain strength. Then we test that athlete and when they are not able to perform to that level (they are either higher or lower) we conclude that the isokinetic test does not show a large correlation to the activity (as the research protocol can not be at fault!).
At the elite level Wrigley (2000) puts performance characteristics in perspective by describing the individual physiological and biomechanical differences amongst athletes as being relatively small. Often other problems such as the insensitivity of the protocol selected or a requirement for only a certain level of strength, beyond which actual performance will not increase, can lead to a lack of correlation between isokinetic strength assessment and performance.
Many studies have reported high correlation between isokinetic measurements and athletic performance. Next is a list all of those which found correlation coefficients higher than 0.5.
| Alexander (1989) | Sprinting |
| Appling and Weiss (1993) | Jump performance |
| Ashley and Weiss (1994) | Jump performance |
| Bartlett, Storey and Simmons (1989) | Throwing speed |
| Blazevich and Jenkins (1998) | Sprinting |
| Cabri et al. (1988) | Football |
| Chan and Maffulli (1996) | Badminton, tennis, gymnastics, swimming, wheelchair racing, triathlon, table tennis, bowling and canoeing |
| Ciccone and Lyons (1987) | Swimming |
| Delecluse et al. (1995) | Sprinting |
| Deproft et al. (1988) | Football |
| Fleck et al. (1992) | Handball |
| Fry and Morton (1991) | Kayaking |
| Greenberger and Paterno (1995) | Hopping |
| Guskiewicz, Lephart & Burkholder (1993) | Sprinting |
| Inbar, Kaiser and Tesch (1981) | Sprinting |
| Kano et al. (1997) | Sprinting |
| Klentrou and Montpetit (1991) | Swimming |
| Cohen et al. (1994) | Tennis |
| Mascaro, Seaver and Swanson (1992) | Ice skating |
| Mognoni et al. (1994) | Football |
| Mookerjee et al. (1995) | Swimming |
| Narici, Sirtori and Mognoni (1988) | Football |
| Nesser et al. (1996) | Sprinting |
| Newberry et al. (1997) | Quadriceps strength |
| Oddsson and Westin (1991) | Jump height |
| Olbrecht et al. (1992) | Swimming |
| Patton and Duggan (1987) | Sprinting |
| Pawlowski and Perrin (1989) | Baseball |
| Pedegana et al. (1982) | Throwing speed |
| Pincivero, Lephart & Karunakara (1997) | Jump height |
| Podolsky et al. (1990) | Ice skating |
| Poulmedis et al. (1988) | Football |
| Roetert et al. (1996) | Tennis |
| Weiss et al. (1997) | Jump height |
There are relatively few studies, which have not shown a correlation between isokinetic strength and performance in the shoulder. Ellenbecker (1991) failed to find a correlation between arm strength and performance among highly skilled tennis players which was later refuted by Cohen et al. (1994). Mikesky et al. (1995) also failed to find a correlation between throwing velocity and isokinetic arm strength values in collegiate baseball players. However, Mikesky et al. (1995) did not look at as many performance variables as Pawlowski and Perrin (1989) who had earlier demonstrated such a relationship.
On a more mathematical note it has been shown that isokinetic dynamometry results show high reliability (Brown, 2000, Chan and Maffulli, 1996 and Dvir, 1995) whereas the reliability of athletic performances, which are used to compare isokinetic strength, results to have been few and far between. This can affect the magnitude of the Pearson Product Moment Correlation Coefficient. For example, a javelin thrower may throw 10 times with a test-retest reliability of .80 (on a good day) but the same javelin thrower may produce isokinetic shoulder internal/external rotation correlations greater than .95. Any figure greater than .91 would not show high correlation, as the 2 measures are too far distinct from one another. Athletic performances, such as cycling, running, swimming and golf, have been shown to have dubious reliability (Hopkins and Hewson, 1993). Using a single measure of strength and correlating this with a measure of athletic performance may not be appropriate as the athletic event and its success performance-wise is more often than not dependent on multi-factorial elements. To compensate for this multiple muscle strengths at multiple joints through many positions and angular velocities compared to athletic performance may be more appropriate.
Multiple regression studies, although becoming more popular, are still in very short supply. There are multiple regression studies demonstrating correlation between sprinting performance and multiple isokinetic measurements (Alexander, 1989, Anderson et al., 1991 and Berg, Miller and Stevens, 1986).
To follow an ideal model for the assessment of isokinetic strength measures compared to athletic performance many measurements would have to be taken. Isokinetically concentric and eccentric actions would have to be assessed with and without isometric pre-activation.
Factors, which would have to be measured, would be:
Peak torque
Peak torque to body weight
Power
Angle of peak torque
Acceleration energy
Appropriate relationships between opposing muscle actions and opposing muscle groups would have to be obtained.
Then the measurements would have to be taken across multiple joints with appropriate anatomical variances in position e.g. shoulder internal/external rotation would have to be performed in several different planes of the scapula with many different positions of shoulder abduction and shoulder horizontal abduction/adduction. To further compound this many different angular velocities would have to be performed.
Unfortunately, comparing the above measures to actual athletic performance can be further complicated when we look at the work of Delecluse et al. (1995). They actually found that sprinting, a sport traditionally thought to be not multi-factorial, was in fact just that. Throughout the sprint they were able to demonstrate how different isokinetic performance parameters related to different measures of athletic performance compared the total distance sprinted. So individual athletic events would have to be broken down into component elements e.g. whilst pulling through during a freestyle shoulder extension from flexion the isokinetics performance variables which correlated to the sub-sections of the movement may change. To elaborate, when the shoulder first reaches a point perpendicular to the body the acceleration energy of the shoulder flexors may be the most appropriate measure to that point in the motion. As the shoulder is extended to the mid point the power may be most important. At the mid point (where there is the greatest lever length, and hence a mechanical disadvantage) the peak torque at that angle may be most appropriate, followed once more by the power in the second portion of the movement. As the swimmer progresses through the distance they have to travel these variables may change e.g. the swimmer may loose strength at the mid point leading to reduced efficiency. Within populations the swimming technique may vary and so different positions of the scapula and arm would have to be incorporated, indeed the individuals may change their technique throughout an event as they tire. Measuring all these variables may help us to further correlate athletic performance to laboratory testing.
So the next question must be can isokinetics discriminate between competition levels within individual sports? This has been demonstrated by:
| Abe et al. (1992) | Skiing |
| Atwater et al. (1991) | Synchronised swimming |
| Brown and Wilkinson (1983) | Skiing |
| Callister et al. (1991) | Judo |
| Cisar et al. (1987) | Wrestling |
| Davis, Brewer and Atkin (1992) | Football |
| Farrar and Thorland (1987) | Sprinting |
| Fry and Morton (1991) | Kayaking |
| Gillian et al. (1979) | Football |
| Gleim (1984) | Football |
| Jackson and Nyland (1990) | Lacrosse |
| Kirkendall (1995) | Football |
| Kraemer, Morrow and Leger (1993) | Rowing |
| Larsson and Forsberg (1980) | Rowing |
| McHugh et al. (1993) | Football |
| Minkoff (1982) | Hockey |
| Morrow et al (1979) | Volleyball |
| Sapega et al. (1978) | Fencing |
| Sapega et al. (1984) | Fencing |
| Smith et al. (1981) | Ice hockey |
| Zakas et al. (1995) | Basketball and soccer |
Compared to the number above there are very few studies at the shoulder, which have not shown competition level differences.
| Fry et al. (1991) | Volleyball |
| Reilly et al. (1990) | Swimming |
| Reizebos (1983) | Basketball |
Reilly et al. (1990) did not find a difference in isokinetic shoulder strength between the fastest and slower male collegiate freestyle swimmers. Sharp (1986) had already proposed an interesting reason for this. Sharp (1986) tested an elite group of swimmers (unfortunately not using isokinetics) and surprisingly found a large variance in power figures. Speculating that certain body types created increased drag forces, as did inefficient swimmers, he surmised that swimmers with either an inefficient stroke or body type would require more power to compete at a similar level with those who were more efficient. This would make athletes at the same competitive level in the same event, which was limited by strength or power, stronger or weaker dependent on their body type and the resultant relative efficiency of their technique.
It has been said by Foster, Thompson and Snyder (1993) that there are certain prerequisites for any performance measure. As isokinetic dynamometry is an absolute performance measure there are certain performance variables it would have to assess accurately. Detecting longitudinal changes in athletes is one of the fundamental tasks of a performance measure. There are many studies to suggest that seasonal changes in strength can be detected using isokinetics. This ability allows an athlete to review the effectiveness of their training competitive timetable and gives them the ability to detect over-training. Below is a list of the studies that show this correlation between isokinetics and longitudinal changes in strength.
| Alexander (1991) | Rhythmic gymnastics |
| Callister et al. (1990) | Judo |
| Eckerson et al. (1994) | Wrestling |
| Foster and Thompson (1990) | Speed skating |
| Fry et al. (1994) | Weight training |
| Hoffman et al. (1991) | Basketball |
| Hagerman and Staron (1983) | Rowing |
| Housh et al. (1988) | Wrestling |
| Johansson et al. (1988) | Orienteering |
| Koutedakis et al. (1992) | Skiing |
| Koutedakis (1994) | Rowing |
| Martin et al. (1994) | Cycling |
| Posch, Haglund & Ericsson (1989) | Ice hockey |
| Roemmich and Sinning (1996) | Wrestling |
It has often been said that there is a strong correlation between muscle fiber type and athletic performance (McArdle, Katch and Katch, 1986). There are many studies which demonstrate a relationship between the predominance of one muscle fiber type within a muscular group and isokinetic testing. This is a necessity because an athletes performance within a particular discipline will, more often than not, be substantially affected by the muscle fiber types within the groups of muscles used most in that event. For example, a 100 meter sprinter will show a propensity for type 2b muscle fibers within their quadriceps and gluteal muscles, whereas a long distance runner will show a larger number of type 1 muscle fibers. Below is a list of some of the studies which have shown a good relationship between isokinetic performance and muscle fiber cross-sectional type.
| Borges and Essen-Gustavsson (1989) | Age and torque development |
| Clarkson, Kroll and Melchionda (1982) | Paddlers |
| Gerard et al. (1986) | Swimmers |
| Inbar, Kaiser and Tesch (1981) | Lower limb performance |
| Ivy et al. (1981) | Knee extension |
| Johansson et al. (1987) | Sprinting/marathon running |
| Larsson and Forsberg (1980) | Rowers |
| Larsson, Grimby and Karlsson (1979) | Ageing |
| Nygaard et al. (1983) | Elbow flexion performance |
| Sleivert, Backus and Wenger (1995) | Volleyball, middle distance running |
| Suter et al. (1993) | Knee extension strength and endurance |
| Thorstensson, Grimby & Karlsson (1976) | Knee extension |
There are some studies which have not shown strong correlation between fibre type and isokinetic testing (Clarkson et al., 1982, Froese and Houston, 1985, Sale et al., 1987 and Schantz et al., 1983). However, these are much fewer in number than those which have shown a positive correlation.
The apparent relationship between athletic performance would suggest that even though athletic activities do not resemble the isokinetic test in terms of velocity and consistency of velocity a relationship still exists between the two. In fact the weight of evidence produced in peer review scientific journals would seem to suggest the apparent face validity previously attributed to the lack of a relationship between isokinetic testing and performance was unfounded. This has been said by Wrigley (2000) to be due to a lack of understanding of joint kinetics. For example, in the knee a maximum unloaded peak velocity is in the region of 500-700 degrees per second (Bobber, Putnam and Woodworth, 1987). When this is compared to sporting activities the knee can reach speeds of up to 2000 degrees per second (Putnam, 1983). These joint velocities recorded usually involve a large multi-joint chain movement with large acceleration phases across a large range of motion and typically occur during ballistic movements. To break this down further the movement occurs after a large burst of agonist muscle activity, which is more often than not part of an eccentric/concentric cycle allowing for isometric reactivation. Or in other words, the limb is accelerated using the muscles concentrically after a brief transition where the velocity of movement equalled zero (the length of the transition determines the amount of stored energy for the concentric cycle). This then means the muscle is already in a high state of preactivation and bursts concentrically aver a range of motion, which occurs relatively at slow velocities. The high velocity measured is typically further into the range of motion, thus, the concentric muscle action that allowed for the high speed joint motion occurred briefly whilst the joint rotated at a slower speed. To compound the problem, to use an isokinetic dynamometer there must be an external resistance applied i.e. the lever system and limb attachment. Brown et al. (1995) surprisingly found that none of their elite male tennis players were able to rotate their shoulder internally or externally fast enough to reach a speed of 450 degrees per second. Cook et al. (1987) even more surprisingly found that baseball pitchers who could internally and externally rotate their shoulder at speeds of over 6000 degrees per second during the pitch could not achieve isolated shoulder flexion/extension or internal/external rotation movements at a speed of 300 degrees per second during isokinetic testing. Further to this Cook et al. (1987) actually found that subjects found a speed of 300 degrees per second so uncomfortable they developed a motor pattern which attempted to decelerate the arm near the anticipated end of range of motion of the test, which could be interpreted to suggest that even highly trained athletes have the need to protect the should joint structures at high testing speeds.