A balanced approach to leg strength

Knee injuries are common in athletes, and effective strengthening programmes are often required to improve functional strength and stability. In particular, a deficit in hamstring muscle (rear of the thigh) strength compared to the quadriceps muscle (frontal thigh) is a precursor to lower extremity injuries^{J Sci Med Sport, 2008, Sep, 11, 5, 452–459}. The aim of this article is to explain the importance of achieving balanced leg strength, and how to attain this during conditioning or rehabilitation after injury.

What is H/Q ratio?

The hamstring to quadriceps ratio abbreviated H/Q ratio refers to the ratio of strength between the two muscle groups. Let’s take some examples to try and explain what this involves. During the kicking action in football, as the ball is struck,the hamstring muscles are required to lengthen under load which is referred to as eccentric muscle action (see figure 1). At the same moment, the quadriceps muscles are required to shorten, which is referred to as a concentric muscle action (figure 1).

Figure 1: Definition of muscle actions

Now consider a rower. During the rowing action as the seat moves backwards, the hamstrings are lengthening as the quadriceps shorten (and vice-versa as the seat moves forwards). A further example is during cycling; while the pedal is being pushed through the downward phase, the quadriceps shorten as the hamstrings lengthen.

During the upward phase, the hamstrings are shortening while the quadriceps are lengthening. Due to this reciprocal relationship between the hamstrings and quadriceps, it turns out that the ratio of hamstring to quadriceps strength is important, affecting the function of the knee joint and the risk of injury.

Measuring H/Q ratio

Measuring H/Q ratios is most commonly done by using an isokinetic dynamometer (see later). These are very expensive pieces of equipment used in high performance facilities, which provide a vast amount of data with regards to forces applied at certain speeds and at different joint angles.

An athlete may be fortunate enough to gain access to one of these when rehabilitating a knee or thigh injury. If this option isn’t available, other means such as a leg extension and seated curl machines used in a gym setting can also be used to determine individual muscle strength (see figure 2). This can be done using a 1 rep maximum (1RM) test, which is the maximum weight that can be lifted under control, through the full range of movement, and without compromising technique. It’s not possible using these machines to obtain the same data as an isokinetic dynamometer but it does allow for any strength deficits to be identified.

Figure 2: Measuring H/Q ratio

What is a normal H/Q ratio?

One of the key questions is what is considered as a desirable H/Q ratio for a healthy knee. The consensus of research suggests that a 50-80% (ratio of 0.5-0.8) is acceptable, but greater than 60% is desirable^{Phys Thera in Sport, 2010, 11, 12–17}. To help explain, this is the percentage of strength of the hamstring muscles in comparison to the strength of the quadriceps. So for example, if this ratio was 80%, the hamstrings would be able to lift a maximum of 80kg in comparison to a 100kg quadriceps maximum.

This matters because research also has shown that higher H/Q ratios of around 100% (ie hamstring strength the same as the quadriceps throughout full range of movement) may help to reduce the risk of a hamstring strain^{J of Human Kinetics, 2012, 33, 63-71}. A ratio of 100% also reduces the risk of the knee partially dislocating in those who have been unfortunate enough to suffer a previous anterior cruciate ligament (ACL) injury^{J Athl Train. 2012, Aug, 47, 4, 396–405}.

The use of an isokinetic dynamometer is helpful in this context because it can provide far more specific details such as strength and force capacity of hamstrings and quadriceps at different knee joint angles. It is particularly productive during pre-season training or at a medical to provide data of an athlete’s normal H/Q ratio, so in the event of a knee injury previous ratios can be attained.

Figure 3: Dynamometer for evaluating H/Q ratio

Anterior cruciate ligament (ACL) rehabilitation

The ACL is frequently injured in sportsmen and women. The hamstring muscles function to stabilise the knee and support the ACL by preventing the shin bone from shifting forwards as the knee straightens. Therefore, during a rapid extension action such as kicking, the hamstrings are acting eccentrically (muscle lengthening) to help prevent the shin bone from shifting forwards, placing excessive loading onto the ACL^{J of Human Kinetics, 2012, 33, 63-71}. Figure 4 illustrates the concentric (muscle shortening) action of the quadriceps during a kicking action in comparison to the eccentric (muscle lengthening) action of the hamstrings. You can hopefully see why ACL rehabilitation needs to promote hamstring strength and provide the knee joint with additional control.