Functional hamstring to quadriceps ratio in football players

Injuries and therefore injury prevention is a big topic in football, especially with regards to knee injuries (10, 34 - see references). The (fast and repeated) knee extensions during kicking (also shown in its correlation with isokinetic measurements (26)), seem to have an effect on the muscles influencing the knee joint. However, also the antagonist (flexion the knee - the hamstring muscle perform a lot of work during motions in football such as sprinting (24, 33, 39). The activity of the hamstring muscles during kicking was also correlated with isokinetic strength measurements (26) showing the importance of the knee flexors as well.

So how does football and its training influence the knee joint and possible injuries with regards to the quadriceps and hamstring muscles?

Generaly, there are several muscular relationships that can be “created” from the different contraction modes and muscle surrounding the knee joint. The combinations are:

• Eccentric hamstring to eccentric quadriceps
• Concentric hamstring to concentric quadriceps (conventional ratio)
• Eccentric hamstring to concentric quadriceps (functional H/Q ratio)
• Concentric hamstring to eccentric quadriceps

However, the following paragraphs will focus on the functional H/Q ratios as it received great attention throughout the football literature.

The functional hamstring to quadriceps ratio (also called H/Q ratio) is a calculation in which the strength (peak torque) of the hamstring muscles in eccentric motion is divided by the strength of the quadriceps in a concentric motion (2).

So basically, in both movements, the knee gets extended. In the hamstring measurement, the (hamstring) muscle is lengthened (whilst working) while in the quadriceps measurement the (quadriceps) muscle is shortened (whilst working). This functional ratio/relationship is thought to describe the muscular relationship at the knee joint (3) and generally increases with velocity (32).

An isokinetic dynamometer is used to assess the H/Q ratio.

The following figure provides some information from publications regarding H/Q ratio in different speed of testing for the preferred and the non-preferred kicking leg (from an un-injured population as it seems).

It was concluded that a potential (above) 1:1 strength relationship was observed at knee extension indicating a functional capacity of the hamstring muscles, which provide stability at the knee joint (3). However, it seems also confirmed that the ratios increase with increasing speed (6, 21).

Values of female footballer showed similar ratios compared to males. A ratio of 0.84 and 1.58 for starters and 0.89 and 1.42 for non-starters in 60 and 240 degrees/second speed in NCAA division I womens soccer (25) were given. Starters also showed a sharper increase of H/Q ratio with increasing speed compared to non-starters (25). Ratios from a different study were 0.74, 1.10, and 1.23 for angular velocities of 60, 120 and 240 degrees/second from another NCAA division I womens soccer team (22). Data from New Zealand National female footballers showed a non-significant increase from U17 to U19 and Senior players (29). H/Q ratios were 0.9, 1.03 and 1.04 respectively for isokinetic measurements with 60 degrees/second.

However, considering the true speed of movement during kicking (28) testing at (EVEN) higher velocities seemed to make sense. 12-18 rad/second were observed for ankle movement (28), which can be re-calculated to 855 degrees/second (if 15 rad/second is multiplied with 57 degrees/second – 1 rad/second = ~57 degrees per second), while the highest testing velocity in the figure above showed values of 300 degrees/second in football players.

Asymmetries/dysbalances in the functional H/Q ratio was shown to significantly impact injury incidence (5, 8, 16). For example, it was reported that muscular dysbalances/asymmetries at the knee joint are shown to influence injury occurrence and players with untreated strength imbalances showed 4.66 times greater risk for injuries (8).

As a result of injuries (and/or insufficient rehabilitation), different ratios were obtained from injured vs. un-injured national and international French footballers (11). players. While 0.8 was shown for uninjured players, 0.6 was measured (at a 60 degrees/second speed) in injured legs.
Is there a prevalent link in youth football players
It seems that age had an influence on asymmetries/dysbalances as older players (>21 years) had a greater ratio compared to younger players (<21 years) in the dominant knee at 300 degrees/second (21).

Information in youth players in both genders are also present throughout the literature (4), suggesting greater development for quadriceps muscle compared to hamstring for girls after menarche and therefore a higher risk for ACL injuries. As a result, hamstring strengthening should be instituted for girls mid/after menarche (4).

Other research investigating youth/developing players reported the effect of football training and additional resistance training in youth footballers. For example. a significant greater H/Q ratio in youth players from an English professional football league club, compared to age-matched controls (23).

Furthermore, resistance trained football players showed greater H/Q ratios in both the preferred and the non-preferred kicking leg compared to conventional trained youth footballers (23).

It was shown that a warm-up (10) as well as fatigue (13, 20) influences the H/Q ratio in male professional (13, 18, 20), semi-professional (35), amateur (31), university (15) and female amateur (14) football players.

With fatigue, the ratio dropped from ~1.24 to ~0.99 (~11% -17% (18, 35) and 8 (non-preferred) to 14% (preferred kicking leg) (14) with an effect size (ES) of 0.9 (13) and showed that eccentric muscle motion is more affected (compared to the concentric quadriceps motion). This would also explain, why injuries are more likely to happen in a fatigue state and possibly at the end of the first and the second half of a football match (13, 35).

Opposing these statements, the H/Q-ratio (measured at 120 degrees/second) increased (0.88 – 1.08) due to fatigue in recreational football players (38).

The table below formed the base for the figure above and shows in greater detail the actual H/Q ratios and the reference from which it was retrieved.

Interestingly, other strength measures (than the maximal torque), such as the rate of force development (for the Q/R ratio) were also investigated, as explosive movements in football do not permit full strength development due to the required time (1). However, the research (19, 41) utilized the conventional ratio and limited knowledge makes any defined conclusions impossible.

It seems that investigating the functional Q/H ratio of football players is a useful tool to identify players that might be at risk with regards to injuries. A “not-at-risk” player seems to maintain/increase a “good” ratio with increasing velocities.

Age (and training experience) and (football) training (in general) has been proven to affect the H/Q ratio. In addition, resistance training seems to be an appropriate tool to manipulate an “unhealthy ratio”.  In particular eccentric strength training seemed to be a promising tool to change the force – length relationship and furthermore to reduce injury risk in football players.

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