A practical approach to leg strength
Investigating lower limb muscular relationships using common strength training exercises
There is a disconnect between the practise of strength training and the science behind it. Daniel Marshall examines more practical approaches for testing leg strength.
While the majority of science-based resistance training research has been performed on athletes from the elite level using laboratory equipment such as isokinetic dynamometers, the majority of athletes are of a sub-elite or amateur level and only have access to common strength training equipment. There is a practical disconnect between the wider strength training community and the science, to the extent that in the ‘real world’ a system of trial and error is the predominant method used for prescribing exercises and loads.
The modern-day strength and conditioning coach and fitness professional for the amateur athlete is faced with a stark lack of practical guidance with regards common muscular relationships, particularly in the lower limbs. Being able to strength test using key strength exercises, and prescribe loads from these results, may aid in more effective program design.
Orthodox common strength environment testing has focused on bilateral assessment (both sides of the body) using exercises such as the back squat and 1RM loads12. Not only is the back squat an accurate indicator of lower body strength2, training the back squat has been shown to have a direct relationship to performance measurements such as sprint velocity and jump height4,13 . However, for the unilaterally dominant athlete (stronger on one side of the body), assessing absolute (1RM) lower body strength with a bilateral exercise such as the back squat may not provide adequate levels of information about the unilateral strength profile of the player. Additionally, the American College of Sports Medicine (ACSM) recommends that unilateral strength exercises be performed at no less than 6RM.
A number of studies have investigated the relationship between the back squat and various common strength exercises.
The sports-specific demands of athletic activity can alter the strength profile of athletes and cause muscular strength asymmetry, where one limb is required to operate at a greater frequency and force to the opposing limb, otherwise known as limb dominance11 . Examples in the literature of limb dominance include the primary kicking leg in soccer or Australian Rules Football, the throwing arm in baseball, or the racquet playing arm in tennis. Training the athlete with an exercise that exhibits similarities to the function required during the activity can serve as a preventative action and help to eliminate the muscular strength asymmetry that exists between limbs8. However, traditional methods of muscular strength assessment discount this need for specificity.
In 1955, Steindler5 pioneered the notion that peak knee extension force should be greater than peak knee flexion force by a magnitude of 3:2. This relationship provided a theoretical foundation upon which a muscular rehabilitation regimen could be administered by physical therapists. Since that time, peak knee extension force to peak knee flexion force has been replaced by the term ‘hamstrings to quadriceps ratio’ and numerous studies have investigated its use in performance, prehabilitation and rehabilitation1,5,6 . The literature suggests that an ideal muscular relationship at the knee will reduce risks of injury from muscle imbalance and help to maintain peak performance6.
Traditional testing of hamstrings to quadriceps ratios is performed with an isokinetic dynamometer, which is able to capture peak muscle force at knee joint angle-specific speeds and moments. Isokinetic dynamometers, while accurate, are generally considered expensive, specialised and restricted in the sporting arena to use by elite-level athletes and teams3 . Testing on this type of equipment may provide ratios upon which strength goals can be established, however it is unlikely that clients who perform these strength exercises will utilise this equipment (isokinetic dynamometers) due to availability of the equipment and the impractical methods by which the equipment is required to be used effectively9 .
In an attempt to develop a more practical approach for the testing of hamstring to quadriceps ratios, Moss et al compared three different methods of muscular strength assessment10: isokinetic using a Cybex II dynamometer; isometric using a Nicholas manual muscle tester; isotonic using both Nautilus and Universal leg curl and leg extension machines. The researchers found that the strength values obtained from each device were significantly different to one another. Despite this, they were able to conclude that each device could be used to assess subject ratios as long as resultant strengthening programs were performed on the same device. This is an important finding, as it suggests that although common strength training environment equipment may produce varied results to isokinetic or isometric test equipment, the ratios produced are valid providing the strength programs performed utilise that same equipment.
|IMPLICATIONS FOR THE FITNESS PROFESSIONAL|
To develop this concept further, a number of studies have investigated the relationship between the 6RM back squat and various common strength exercises such as the lunge, hamstring curl, deadlift and step-up variations. Ebben et al investigated the presence of linear correlations using the Pearson product-moment correlation coefficient and linear regression between various hamstring exercises and the back squat in collegiate athletes. Twenty-one men and thirteen women from unspecified sporting backgrounds were tested for six repetition maximum lifts for the back squat, seated leg curl, single leg stiff-legged deadlift, stiff-legged deadlift and good morning lower limb strength exercises.
Despite finding a significant relationship between the back squat and each of the hamstring strength exercises in males, there was no significant relationship in the same exercises for the female subjects. Ebben simply stated that results should not be generalised between genders, offering no explanation as to why the females had highly varied back squat to hamstring exercise relationships compared to the male subjects. It is possible that the athletic background of the female subjects had a significant effect on the outcomes. In a subsequent study by Ebben et al, a similar research design was implemented to investigate the presence of linear correlations using the Pearson product-moment correlation coefficient between the back squat and various quadriceps exercises in recreationally active students that participated in lower body resistance training7 . The results of this study indicate that the training loads for the deadlift, lunge, step-up and leg extension can be formulated using the back squat as a predictor exercise. In order to quantify the back squat with two bilateral and two unilateral stepping movements of the lower body, Wong et al tested fourteen elite male karate athletes14 . Using the Pearson product-moment correlation coefficient, the authors reported strong linear correlations between the back half squat and deadlift, leg press, lunge and step-up, which supports the findings by Ebben.
Despite each of these studies testing only a small group of subjects, the findings are encouraging for future research involving 6RM loads and common bilateral and unilateral strength training exercises.
Daniel Marshall, BCom
Daniel completed his sport and exercise science honours degree in 2012 at the University of the Sunshine Coast, with the topic ‘Compare and quantify various lower body 6RM strength scores and resultant ratios in female football players and strength athletes’.