Is what you measure relevant to muscle performance?
Isokinetic dynamometry has content validity with respect to specific aspects of muscle performance. For instance, maximum isokinetic derived power (during plantar flexion) is reached at the functional velocity equal to that of toe off during walking.
Or as velocity increases force generation decreases but average power increases (Fugl-Meyer et al. 1982).
Think of this as looking for a series of relationships with factors that are well established. Some are covered below.
Gender differences. Many isokinetic studies have shown that men are significantly and consistently stronger than women (this is often documented as an obvious observation concerning gender differences).
Effect of Age. Backman and Oberg (1989) demonstrated consistent strength increases in the dorsiflexors through the ages of 6, 9 and 12 in girls and boys. (Up to this stage strength was equal). Girls then plateaud and boys continued to get stronger.
Strength normally reaches its peak in the third decade and there after declines moderately with age until the seventh decade where there is a steeper decline. Timm (1988) and Borges (1989) found these variations using isokinetic knee and trunk testing.
Silder A, Heiderscheit B, Thelen DG (2008) found hip strength became more vital as you aged and could compensate for poor ankle strength
Activity Level. Fugl-Meyer (1981) found isokinetic ankle plantar and dorsiflexor strength differences between trained versus sedentary subjects.
Body weight. This is a more difficult subject as in normal individuals muscle mass rises proportionately with body weight. Hence heavier subjects produce higher isokinetic moments. However, this relationship is not linear and is one of the reasons for normalising strength to body weight using Newton meter per kilogram body weight (seen recently in research as Newton meter per kilogram of lean body mass).
Muscle characteristics. As shown by Backman and Oberg (1989) the slope of moment angular velocity curve changes with age in that children cannot utilise stretch shortening cycles as adults can, possibly because of softer muscles (in other words they are more flexible). This is seen in isokinetics especially in adolescents who generate more moment at slower speeds. The issue of fiber type has yet to be resolved as Thorstensson et al. (1976) described muscle performance as expressed by maximal contraction speed and the ability to generate force. They said that performance at 180 degrees/second correlated with the percentage of relative area of fast twitch fibers. This was not substantiated by Fugl-Meyer (1989) when using isokinetics. However, he did not use sporting subjects. There is some correlation between isometric moment and isokinetic moment. This is described as significant, however, static strength is so vague and poorly researched that associations should be carefully made.
Nordgren et al. (1983) demonstrated this best when they found that local anesthetic injected into the knee joint resulted in normalisation of an otherwise depressed moment curve in patients with OA.
Pienimäki TT, Siira PT, Vanharanta H. (2002) showed isokinetic strength directly related to grip and elbow dysfunction. Whilst isokinetic treatment could reduce grip and elbow tendon problems.
Criterion Referenced Validity
This is split into 2 parts - Concurrent or whether what you find isokinetically can be related to another activity, and predictive, or is the isokinetic measure able to forecast a future event.
These have to considered as specific examples.
Gerdl et al. (1986) found a strong correlation between isokinetic endurance testing during repeated plantarflexion contractions and the maximal functional walking capacity in patients with peripheral arterial insufficiency and intermittent claudication. That means if a patient has a low score on this endurance test their walking capacity will be correspondingly limited (this test may be more suitable as it involves less risk to the patient).
Pain sensation scores during knee testing have a high correlation with PF pain syndrome, particularly at lower test angular velocities (Dvir 1995). If after therapeutic intervention slower tests give less pain then this could be a measurable outcome.
The examples of concurrent validity are extensive for every major joint of the body including the spine
This is an area of great debate and there are 2 main categories. The power to predict dysfunction from testing and the power to predict progress in rehabilitation.
Prediction of injury. The prediction of injury has been studied by Grace et al. (1984) and Eriksson (1991) who both found no relationship between pre-season isokinetic testing and rate of injury in season. However, Knapik et al. (1981) found in season injury rates higher in women if they had knee flexors 15% (or more) stronger than the contralateral side at 180 degrees/second. (Their results are questionable.)
Prediction of injury has probably been best studied by Gilliam et al (2002). They looked at two different companies’ isokinetically screening prospective employees. One was an airline and the other was a large parcel delivery company. They were able to reduce injuries by 75% using isokinetic screening pedicting who would get injured.
Prediction of progress. It is likely that repeated tests