Recommended Strength Ratios – Mel Siff Classic

It is often claimed that the optimal ratio of quadriceps to hamstring strength is 60:40 and any significant deviation.frorn this value is to be regarded as a possible cause of hamstring injury. Russian experts, however, have found that this ratio “depends on the specific type of sport” (A. Vorobyev, A Textbook on Weightlifting, 1978). For example, they have determined that this ratio (measured when knee extension torque is greatest) should be nearer 80:20 for weightlifters and jumpers.  Moreover, if the ratio is measured during movement on a treadmill, the ratio for runners is approximately 50:50. Despite these findings, the traditional 60:40 recommendation is widely accepted and much rehabilitation is based on restoring this ratio. Another popular belief is that injuries are far more common if the difference in strength between right and left is more than 10%.

Recent Western research has corroborated the Russian findings. Stephens and Reid have shown that neither of these recommendations is supported by scientifically controlled experiments which avoid stretching the hamstrings and correct measured torque outputs for the effect of gravity (Canadian Journal of Sport Sciences 1988). In addition, the recommendation of a specific flexor/extensor ratio is vague, because this ratio varies throughout the range of joint motion, as shown in Figure 1. For example, the ratio for the knee at 80 degrees is about 75:25 at 36 degs/ sec, whereas it is 68:32 at 180 degs/ sec. The only stage at which the ratio is 60:40 occurs at an angle of approximately 50 degrees.

Not only does the ratio change with joint angle, but it also changes with velocity of measurement, so it is meaningless to prescribe an optimal ratio for any joint. If the ratio of quadriceps to hamstring strength were indeed constant, the graph would be a straight line passing through the vertical axis at 1.5 and running parallel to the horizontal axis. It may be seen that this ratio varies from as little as 0.5 for a small knee angle to as much as 3.0 for a larger angle.

The hamstrings are equal in strength to the quadriceps when the ratio is 1 at a knee angle of about 40 degrees. In other words, the hamstrings are stronger than the quadriceps for knee angles of less than 40 degrees, but the quadriceps are as much as three times stronger than the hamstrings at about 90 degrees for a slow knee extension velocity of 36 degrees per second.


Figure I. Ratio of isokinetic torque of knee extensors to flexors at 36 degs/sec, 108 degs/sec and 180 degs/sec. The knee extension data measured on a Cybex dynamometer were divided by the knee flexion data for the equivalent joint angle in each case. Joint angle varies between partial knee flexion in the seated position to full extension at 90 degrees.

If any analysis of strength ratios is to be made, then it would be more relevant to compare results with a characteristic curve describing the variation in ratio over the full range of joint movement for a given angular velocity (e. g. as given in Fig 1). It is apparent that recommendations to base rehabilitation or strengthening of any muscle group on specific ratios based on isokinetic tests should be applied more cautiously than has been the case until now.

Functional Anatomy

Another confounding factor is the influence of the angle of nearby joints on the torque that is produced by given muscles about the joint in question. For instance, knee extension torque increases with hip angle, a phenomenon that is of immense practical importance to weightlifters, jumpers and sprinters, in particular. These athletes are well aware of operating over optimal ranges of relative knee and hip angle.

Measurement of the relative disposition of these angles forms the basis of the cyclogram used by biomechanists to study gait efficiency. Therapists attempt to solve this problem by immobilizing the seated athlete’s hip using strong inextensible straps across the lower pelvis. This immediately creates non-functional conditions for evaluating the biomechanical characteristics of knee extension/ flexion in free space. The seated posture produces


Highly constrained and accurate conditions for measuring torque that is specific to the seated posture and not the posture exhibited during any actual sporting action.

Furthermore, seated isokinetic evaluation of knee motion is usually imprecisely controlled, since it is rarely combined with electromyography or muscle tension measurement (myotonometry) to ascertain the relative contributions made to joint torque by the different muscles comprising the quadriceps and hamstrings. Prescription of any exercise regime without knowing precisely which muscles are inadequately strong is just as haphazard with or without the aid of expensive isokinetic devices.

What complicates matters further is that the degree of lateral or medial rotation of the lower extremity has a significant effect on the relative involvement of vastus medìalis and lateralis, so that this variable also needs to be more accurately controlled if isokinetic testing is to become scientifically rigorous. Open-chain testing of the lower extremity with the sole of the foot not in contact with the ground immediately prevents popliteus from initiating knee flexion or gastrocnemius from facilitating knee flexion, two actions that are of immense importance in running, lifting or jumping.

In addition, seated testing does not take into consideration medial rotation of the knee by sartorius, gracilis, semimembranosus or semìtendinosus, or lateral rotation by biceps femoris. The role of these muscles in performance may be largely ignored for the average client, but certainly not in the case of competitive athletes, who place maximal demands on their bodies.

The entire system of PNP (proprioceptive neuromuscular facilitation) is based on the primacy of specific patterns of joint action and muscle recruitment in determining movement efficiency and safety. Yet the same therapists with their extensive knowledge of PNF unquestioningly accept results produced under the highly unnatural conditions imposed by isokinetic machines. They are fully aware that training in a particular way produces neural changes which become part of the central program that determines the efficiency of movement; they religiously apply the precise kinesiological patterns which they learn at medical school, but they are compelled to ignore this knowledge at the behest of the manufacturers of isokinetic machines.