Below is a research study looking at the effects of either strength training or skill work aiming to improve the single leg pistol squat.
Improving single-legged–squat performance- comparing 2 training methods with potential implications for injury prevention, by Dawson & Herrington, in Journal of Athletic Training (2015)
Non-contact anterior cruciate ligament (ACL) injury is a very common and quite serious injury that occurs frequently in popular team sports, particularly those which involve lateral pivoting, jump landing or quick decelerations during sprint running. Previous research has identified that ACL injuries occur under two predominant loading patterns: knee valgus collapse or anterior tibial shear.
While there are advocates and adversaries of both mechanisms, studies have shown that female athletes tend to display greater knee valgus than males and greater ACL injury incidence. Researchers have also found that the tendency to display greater knee valgus is a good predictor of ACL injury risk. In general, the term “valgus” refers to the outward angling of the distal segment of a bone. Where a joint or a neighboring joint has more than one degree of freedom, this outward angling can involve movements in transverse, sagittal or frontal planes. Indeed, research indicates that knee valgus (also called valgus collapse or medial knee displacement) actually arises as a result of hip joint or foot joint actions. Knee valgus in stance is seen in conjunction with a combination of hip joint adduction and hip joint internal rotation and occurs most frequently in positions of hip joint flexion. Additionally, foot pronation is commonly observed where knee valgus occurs. The joint actions at hip and foot appear to lead to a medial displacement of the knee, which is accompanied by knee joint abduction and knee joint external rotation.
Various explanations have been proposed as causes of knee valgus. One popular proposal is that individuals have inadequate hip external rotator and hip abductor strength and/or short, tight or overactive hip adductors and/or hip internal rotations, leading to a tendency for the hips to move into hip adduction and hip internal rotation. Another very popular proposal is that ankle dorsiflexion mobility is poor or that the lower leg muscles (soleus, gastrocnemius, and anterior tibialis) are short, tight or overactive, which prevents the tibia and knee from moving forwards and causes the foot to compensate by pronating. This inward foot movement leads to mirrored movement at the hip (hip internal rotation and hip adduction) and therefore knee valgus. Two less popular proposals are that a lack of either vastus medialis obliquus strength or a lack of medial hamstrings strength leads to poor knee stabilization and consequently allows the knee to track inward. Changes of direction during running (cutting) are common in many popular team sports and the ability to perform them quickly is a key attribute of successful athletes. However, cutting maneuvers are also associated with knee valgus and consequently an increased risk of non-contact ACL injury.
OBJECTIVE: To compare the effects of a long-term program of either hip muscle strengthening or skill acquisition on single-leg squat joint angle movements that are thought to be predictive of increased injury risk, specifically the frontal-plane projection angle (FPPA) and hip abduction angle (HADD) (as measured by video and rated using the qualitative analysis of single-leg squat tool (QASLS), which is a previously validated qualitative analysis tool for measuring FPPA and HADD). In addition, the researchers measured maximum voluntary isometric contraction (MVIC) hip external rotation force in the clam exercise and MVIC hip abduction force in side-lying using a hand-held dynamometer.
POPULATION: 17 physically active subjects (9 females, aged 27.9 ± 3.1 years, and 8 males, aged 30.4 ± 6.4 years), allocated to either a hip muscle strengthening program (HIP, 9 subjects), or to a skill- acquisition program (SKILL, 8 subjects).
INTERVENTION: The subjects in the HIP group performed 3 workouts per week for 6 weeks, comprising 4 exercises for 3 sets of 12 repetitions. The exercises were intended to bring about hip extensor strengthening and comprised the side-lying hip abduction, quadruped hip extension, the clam exercise, and front step-up. The subjects in the SKILL group were given handouts with instructions on the correct technique for a single-leg squat and performed single-leg squats in front of a mirror for 3 sets of 12 repetitions.
Changes in FPPA and HADD
The researchers found that FPPA was significantly reduced in both HIP and SKILL groups after 6 weeks of training and these improvements were maintained after a further 6 weeks of no training. However, there was no difference between groups. Additionally, they found that HADD was significantly reduced in both HIP and SKILL groups after 6 weeks of training and these improvements were maintained after a further 6 weeks of no training. However, there was no difference between groups.
Changes in hip muscle strength
The researchers found that hip external rotation MVIC force in the clam position and side-lying hip abduction MVIC force both increased in the HIP group but not in the SKILL group.
What did the researchers conclude?
The researchers concluded that both HIP and SKILL training programs can produce improvements in the biomechanics of the single-leg squat, including reductions in FPPA and HADD. However, only the HIP program produced increases in hip external rotation and hip abduction strength.
The study was limited as no loaded resistance training exercises were used to develop hip muscle strength.