Isokinetic muscle strength testing
Knee
Purpose
Isokinetic testing has several purposes: to obtain objective records, to screen athletes,
to establish a database, to quantify objective information, to obtain objective serial
reassessments, to develop normative data, to correlate isokinetic torque curves with
pathological conditions, and to use the shape of the curve to adjust the rehabilitation
programme to a specific athlete’s needs.
Isokinetic assessment allows the clinician to assess muscular performance objectively
in a way that is both safe and reliable. It produces projective criteria for the clinician and
provides reproducible data for assessing and monitoring an athlete’s status. Isokinetic
testing has been demonstrated to be reliable and valid.
Contraindications
As with any methodology in medicine, absolute and relative contraindications for testing
and using isokinetics in rehabilitation must be established.
Examples of such
contraindications are soft tissue healing constraints, acute strains and sprains and,
occasionally, sub-acute conditions. A standard test protocol should be established to
enhance the reliability of the test. There are numerous considerations, including

educating the athlete regarding the particular requirements of the test;

testing the uninvolved side first to establish a baseline and to demonstrate the
requirements so that the athlete’s apprehension is decreased;

providing appropriate warm-ups at each speed;

having consistent verbal commands for instructions to the athlete;

having consistency for testing different joints;

having properly calibrated equipment; and

providing proper stabilization.
A standard orthopaedic testing protocol should be followed during isokinetic testing.
Isokinetic testing allows for a variety of testing protocols ranging from power to
endurance tests. Our primary recommendation is to perform velocity spectrum testing
so that the test will assess the muscles’ capabilities at different speeds. For example,
athletes with a patellofemoral problem often has more power deficits at slow speeds,
whereas athletes who have had surgical procedures of the knee will have deficits at
high velocity.
Advantages of isokinetic testing

Efficiency: It is the only way to load a dynamically contracting muscle to its
maximum capability at all points throughout the ROM.

Safety: An individual will never meet more resistance than he or she can handle,
because the resistance is equal to the force applied.

Accommodating resistance: Accommodating resistance occurs predicated on
changes in the musculotendinous length-to-tension ratio or changes in the skeletal
leverage (due to biomechanics, fatigue or pain).

Physiologic overflow through the velocity spectrum: When an athlete exercises at
a particular speed, there is a specificity response with the greatest power gains
occurring at the speed of training; however, a concomitant increase in power gain
occurs at other speeds as well.
The majority of studies indicate that this
phenomenon occurs at the slower speeds, although the same research
demonstrates an overflow in both directions from the training speed.

Velocity spectrum training: Because of the various velocities at which functional
and sporting activities are performed and the specificity of training, the ability to
perform at various functional velocities is critical. It is important to train the muscle
neurophysiologically to develop a normal motor recruitment pattern of neural
contraction of the muscle.

Minimal post-exercise soreness with concentric isokinetic contractions

Validity of the equipment

Reliability of the equipment

Reproducibility of the physiologic testing

Development of muscle recruitment quickness

Objective documentation of testing

Computer feedback provided so that an athlete can train at sub-maximal or
maximal levels
Limitations

Isolated joint or muscle testing

Non-functional patterns of movement

Limited velocities to replicate the actual speeds of sports performance

Increased compressive forces at slower speeds

Increased tibial translation at slow speeds without proximal pad placement
Criteria for interpreting isokinetic test results

Bilateral comparison: Comparing the involved to the uninvolved extremity is
probably the most common evaluation. Bilateral differences of 10% to 15% are
considered to represent significant asymmetry. However, the single parameter by
itself has limitations.

Unilateral ratios: Comparing the relationship between the agonist and antagonist
muscles may identify particular weaknesses in certain muscle groups.
parameter is
This
particularly important to assess with velocity spectrum testing,
because, in many muscle groups, the percentage relationships of the muscles
change with changing speeds.

Torque-to-body weight relationship: Comparing the torques to body weight adds
another dimension in test results. The torque-to-body weight relationship is often
altered, even when bilateral symmetry and normal unilateral ratios are present.

Comparison to normative data: Although the use of normative data is
controversial, it can provide guidelines for testing or rehabilitation if properly used
relative to a specific population of athletes.
Procedure for knee flexion/extension isokinetic testing
The patient positions him-/herself on the cybex chair in a 90° back flexion position.
Strap the patient to the chair with the belt provided. The upper leg should also be
strapped to the chair. The dynamometer should be in line with the lateral epicondyle (of
the femur) of the knee to be tested. The length adapter should then be strapped to the
ankle.
Set up the computer.
Prepare the patient for the test by explaining the
procedure. The patient will now do 4 sub-maximal trials of knee flexion and extension.
The test, during which five repetitions of maximum strength will be done, will start after
the trials.
The results will give the quadriceps-to-hamstring ratio, the absolute power and the
relative power of the quadriceps and the hamstrings.
Quadriceps:Hamstring
Quadriceps (L) + Hamstring (L) = x
Quadriceps (L)/x X 100 = Quadriceps %
Hamstring (L)/x X 100 = Hamstring %
A quadriceps-to-hamstring ratio of 10:6 is accepted as sufficient for the non-sporting
populations.
Absolute power = Highest repetition
Relative power = % Body weight/100
These values should be compared to the norms.
Normative data for knee flexion/extension isokinetic testing
Concentric and eccentric torque adjusted for body mass (Adapted from
Highgenboten et al., 1995)
Concentric
Group
Hamstrings
Mean
SD
Eccentric
Quadriceps
Mean
Hamstrings
SD
Quadriceps
Mean
SD
Mean
SD
Average torque
Males (15-24)
0.85
0.29
1.78
0.42
1.02
0.31
1.87
0.62
Males (25-34)
0.73
0.18
1.48
0.45
0.95
0.26
1.71
0.57
Females (15-24)
0.59
0.12
1.26
0.30
0.70
0.22
1.31
0.53
Females (25-34)
0.58
0.12
1.22
0.37
0.72
0.21
1.38
0.51
Peak torque
Males (15-24)
1.21
0.24
2.98
0.57
1.44
0.33
3.09
0.88
Males (25-34)
1.08
0.28
2.49
0.66
1.37
0.32
2.67
0.82
Females (15-24)
0.87
0.16
2.19
0.51
1.06
0.26
2.37
0.90
Females (25-34)
0.85
0.18
1.98
0.49
1.11
0.28
2.36
0.77
All values are Nm/kg BM. Testing was performed at 50/sec. The quadriceps was
tested with the athlete seated and the hamstrings with the athlete prone.
Peak torque outputs of the knee: comparative figures from Hong Kong squash
athletes and other national sportsmen (Adapted from Chin et al., 1995)
Sports
n
Extension
Flexion
Flexion-extension
ratio
Squash Hong Kong team
10
3.1
1.87
62
Squash Australian U20
12
2.58
1.63
60
Tennis Australian team
9
2.7
1.91
70
3.42
1.89
56
Badminton
Hong Kong 11
team
Soccer Hong Kong team
24
2.72
1.65
60
Cycling Hong Kong team
10
3.27
1.88
60
n = number of athletes. Data presented for the dominant knee. Values in Nm/kg BM.
Test velocity 60/sec.