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A partnership between Australian Fitness Network, the University of the Sunshine Coast and the Australian Institute of Fitness, Fitness Research studies the populations, communities and environments related to the fitness industry, with the mission of improving the health of Australians through an improved body of fitness knowledge.

Research paper: Can a single 6RM squat score predict strength ratios in female football players?

Research team: Daniel P Marshall, Brendan J Burkett PhD, Anthony G Boutagy and Mark R McKean PhD; University of the Sunshine Coast.

Published: Journal of Fitness Research; Dec 2013, No.2.2

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Introduction: Soccer is played in over 200 countries with approximately 26 million females participating in competition from the ages of six to 45. Soccer requires athletes to perform a large range of coordinated non-contact lower limb movements such as kicking, jumping, sprinting, cutting and braking. Although the skills required to play soccer are similar between genders, the physiological capacities of each gender may result in different game structures. A clear finding is that strength capacity is important for performance in soccer players. Developing maximal strength increases the force available to perform important actions of kicking, and specific movements of running, jumping, cutting and tackling.

Studies have shown that female soccer players have a higher risk of suffering both hamstring and anterior cruciate ligament (ACL) injury than male players. Suggested reasons from a conditioning perspective include decreased functional or conventional hamstring to quadriceps ratios, and muscular asymmetry. Further to this, inadequate joint range of motion and prior injury can predispose the athlete to future injury, particularly where rehabilitation protocols have been inadequate. Despite an extensive coverage of the hamstrings to quadriceps ratio in the literature, there is little agreement as to what is acceptable in both rehabilitation and performance environments.

The purpose of this study was to determine if a single 6RM squat score can predict strength ratios in female soccer players. The influence of joint range of motion of the hip and knee, injury history, training background and training frequency on this score was also determined.

Methods: Fifteen state-level female soccer players aged 26.3 (±6.4) years, weighing 64.4 (±10.5) kg, height 165.9 (±6.9) cm; and fifteen recreational female strength athletes (who regularly performed lower body resistance training) aged 29.1 (±4.5) years, weighing 63.59 (±5.2) kg and height of 165.6 (±4.4) cm volunteered for the study. All participants were free of musculoskeletal injury. Participants also provided training and injury-related information during the pre-exercise interview. Bodyweight, standing height and hip and knee joint range of motion were assessed prior to strength testing. Strength testing included 6RM measures for a parallel back squat exercise, stationary lunge, single leg standing hamstring curl, and prone hamstring curl.

On testing day one, participants performed the back squat and single leg standing hamstring curl exercises. On day two, (between two and seven days later) participants performed the prone hamstring curl and stationary lunge exercises. Participants performed warm up sets for each exercise by performing six repetitions progressively at 50, 85 and 95 per cent, with two minutes between sets, before commencing the first 6RM attempt. Rest of five minutes was provided between 6RM attempts sets to allow sufficient recovery. Although there were no significant anthropometric differences, data was analysed collectively and by mode of athletic performance, i.e. soccer or strength group.

Results: Mean 6RM strength scores for the back squat, single leg standing hamstring curl, prone hamstring curl and stationary lunge are shown in Table 1. The mean 6RM values for all of the strength exercises tested were higher in the strength training group: stationary lunge, 25 per cent (right) and 22 per cent (left); squat, 18 per cent; standing hamstring curl, 13 per cent (right), seven per cent (left); and prone hamstring curl, seven per cent. There were significant differences in strength scores between the two groups in the stationary lunge for both limbs (left P = .049, right P = .05). For the soccer group, the R² values (the square of the sample correlation coefficient between the outcomes and their predicted values) were

P = .63 (left) and P = .67 (right), indicating that the stationary lunge can account for 63 per cent and 67 per cent variance in the back squat respectively. For the strength group, the R² values were less significant at P = .59 (left) and P = .54 (right).

Discussion: Despite both groups having experience in the common strength training environment, the strength training group achieved higher scores for all 6RM tests. Higher mean strength scores in the strength training group may be due to a significantly different training frequency, when compared to the soccer group. Although soccer players train the squat movement, they may not necessarily do so for consistent strength gains, whereas strength athletes tend to train movements specifically to increase the weight moved for that exercise.

Leg strength symmetry was achieved for both groups with similar strength scores between limbs and no statistically significant difference. Ebben and colleagues tested recreational collegiate athletes and also found similar lunge scores between legs as in the current study. In teenagers, a difference of less than 15 per cent was considered to be the norm for symmetry of strength and power. Accordingly, both groups achieved muscular symmetry, with differences between legs being less than 3 per cent, which is well within this range and can be considered normal.


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The current study showed that soccer players have similar bilateral strength profiles to strength athletes, even though they may have different training goals, such as balance and muscle symmetry. Differences in the standing hamstring curl strength between limbs was -0.6 per cent in the soccer group and 2.72 per cent in the strength group. This suggests the competitive level of the athlete may contribute to the level of asymmetry. The increased time an athlete trains and plays their chosen sport may induce a higher level of sports specific adaptation leading to asymmetry at the elite level, but not at the amateur level.

The current study identified two separate bilateral to unilateral ratios for the lower limbs. The first ratio was calculated for the knee flexors, using the back squat to single leg standing hamstring curl. The ratio of back squat to single leg standing hamstring curl for all females was 26.5 per cent (22.8 – 30.3 per cent). The second ratio compared two compound movements, the back squat and the stationary lunge and the ratio was 59.7 per cent (55.0 – 64.4 per cent). The back squat had a weak correlation to the exercises that involved the hamstrings as the primary mover (R2 range .01 – .28). This indicates that hamstring strength can only explain a small amount of variance in the squat scores between subjects.

Conclusions: The findings of this study indicate that in amateur female soccer players and strength athletes, two bilateral to unilateral ratios using 6RM strength scores of the bilateral back squat and unilateral stationary lunge and single leg standing hamstring curl exercises may be effective measures of symmetry and strength. Having these ratios may aid the strength and exercise professional in taking the guesswork out of program and load prescription, ensuring their methods conform to best practice.

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