The aim of this study was to investigate the effect of the FIFA 11+ and HarmoKnee on CSR, DCR and FSR in young male professional soccer players. The results of the present study showed that the CSR of the players after 8 weeks of training using the 11+ and HarmoKnee were lower than the average values at various angular speeds, below 0.61, 0.72, and 0.78 at 60°.s−1
. These drops in strength ratio of soccer players have been reported by previous researchers 
. Similarly, this phenomenon is also seen in other sports. Pieter et al. 
studied the isokinetic strength ratio of professional American martial arts athletes and found their CSRs were below average 
In the present study, the 11+ program significantly increased the CSR between pre- and post-test in the non-dominant leg at 60°.s−1
by 8%. The 11+ group also indicated significant differences in the dominant leg and non-dominant leg in DCR and increased by 8% for FSR in the quadriceps of the non-dominant leg. However, no significant differences were shown in the HarmoKnee group and control group when pre- and post-test data were compared. These results support the proposal that an 8-week FIFA 11+ program is superior in improving the strength ratio. These results are in agreement with Brito et al. 
. They investigated the 11+ program on young male soccer players and reported significant increase in the CSR at 60°.s−1
by 14.8% and the DCR by 13.8% only in the non-dominant leg 
In the 11+ group, there was a significant increase between pre-test and post-test by 8% for FSR in the quadriceps of the non-dominant leg. However, no significant differences were shown between pre- and post-test in the HarmoKnee and control groups. Furthermore, the results did not indicate significant differences in FSR between the 11+, HarmoKnee and control groups in both legs and muscles. To our knowledge, there is only one study that investigated FSR on male soccer players. The study indicated no significant differences in FSR in the quadriceps and hamstrings of both legs in male soccer players after a 90-min soccer-specific intermittent program 
. The present results support this proposal that more training elements should be added in both programs to increase the strength of the quadriceps and hamstrings.
Comparison between groups showed significant main effect in CSR only at 60°.s−1
in the non-dominant leg. The previous studies support the notion that this produces the highest torque at the slow speed (60°.s−1
. The time available for contact between actin and myosin filaments decreases with increasing velocity of concentric activity (Huxley model); thus the period of the contact phase reduces in the overall cycle 
. Cross-bridges have to be re-released shortly after their connection without enough time to produce power, so the proportion of combined bridges in the muscle declines, and produces lower strength 
. More motor units can be recruited at slower speeds than higher speeds allowing more torque production 
. We believe that for a comprehensive assessment of strength ratio, measurement at slow angular velocity should be included since it produces the highest net peak torque.
In both experimental groups the DCR declined after the 8 weeks of training. This shows that in both groups the quadriceps strength increased more than that of the hamstring. In other words, certain components of the intervention programs have higher impact on the quadriceps’ isokinetic strength than on that of the hamstring. Further modification of both programs may be required to fully realise the normal DCR. The hamstring strength exercises, such as the Nordic hamstring 
, hamstring curl 
, single leg eccentric hamstring windmills and prone leg drops 
, which has been shown to increase hamstring muscle strength can be added to the programs 
. The quadriceps strength has been shown to be highly correlated to ball kicking speed 
, hence it is advantageous to have a higher value. Nevertheless, a disproportionate increment in quadriceps strength against that of the hamstring will increase the anterior tibia shearing of ACL and also predispose to hamstring strain. It is imperative that more hamstring exercises are added to the 11+ and HarmoKnee programs to improve the functional balance of the players. The hamstring plays a protective role during extension of the knee particularly in terminal swing of a sprint 
The DCR of both groups was less than the normal average (<1.0). Athletes with DCR less than 1.0 when measured at higher velocities (>120°.s−1
) are predisposed to knee injuries 
. DCR provides more suitable measure relating to knee function such as kicking, acceleration, deceleration etc. during play 
. The quadriceps strength would be more improved than that of the hamstring due to the nature of demands of running and kicking in soccer. Playing soccer requires activities such as kicking, jumping vertically and heading the ball in which there is weight-bearing in taking-off and landing. These demands may change the strength ratio in young professional soccer players 
. Tourny-Chollet et al. 
reported that the DCR of the soccer players is significantly lower than sedentary subjects at 60°.s−1
(0.80 vs. 0.93) and at 120°.s−1
(0.88 vs. 1.03) for both the dominant and non-dominant legs 
. Therefore, the results of our study support the proposal that the young professional soccer players are exposed to higher knee injury risks than other sports. The knee injury risk in professional soccer is high and occurs frequently by non-contact mechanisms 
. One of the causes for high knee injuries in professional soccer players is strength imbalance between hamstring and quadriceps.
It was found that there were no significant differences between the dominant and non-dominant legs in the strength ratios measured. Generally in soccer the dominant leg is used to kick the ball, while the non-dominant leg has the main role of providing postural support. This definition of footedness is commonly accepted by researchers 
. However, professional soccer players can perform kicking of the ball bilaterally and prefer to use both legs in different situations. This could be the possible cause of lack of differences observed between dominant and non-dominant legs in professional soccer players.
There is a limitation in this study that should be addressed. It must be noted that isokinetic dynamometer evaluates the net torque (output) of force of knee flexion and extension, not exactly the moment of hamstring or quadriceps muscles create.
The main finding of this research is that there are different degrees of changes in CSR, DCR and FSR following the 11+ and HarmoKnee programs. The 11+ showed significant improvement in CSR and FSR, but the HarmoKnee program did not demonstrate any improvement. Interestingly CSR in both programs showed the greatest increase in strength at slow speed (60°.s−1) than at fast speed (300°.s−1). This study also revealed that the changes gained in all the three ratios following the 11+ and HarmoKnee programs did not differ between the dominant or non-dominant leg. Further emphasis on certain exercises to improve the ratios or new additions of exercises may help to enrich the two injury prevention programs.
What does this Article Add?
Most professional young soccer players have higher predisposition of getting knee injuries. In comparison the FIFA 11+ program seems to show better improvement in CSR and FSR than the HarmoKnee program in the non-dominant leg. Both programs are lacking in exercise components that would improve hamstring strength which in turn would improve the balance in CSR and DCR. Measurement of CSR at low speed (60°.s−1) would allow a comprehensive assessment of CSR.