Assessment and screening of athletes using a single leg squat provides valuable information for coaches and player management staff. Historically, biomechanical information obtained from a single leg (SL) squat has been used to determine the risk of injury. The likelihood of injury in tasks such as landing and cutting may be identified through biomechanical deficiencies in the SL squat (i.e. knee valgus). The performance of a single leg squat also highlights areas of mobility restrictions and strength imbalances that may be challenging the athlete . For these reasons, the single leg squat features on many screening tools and is used frequently as an objective clinical assessment . Therefore, this article aims to review the validity of the single leg squat for injury risk assessment and guiding corrective interventions.
‘Valgus’ refers to the position of the knee when it moves towards midline in relation to the hip and ankle – as seen in Figure 1. Knee valgus involves a cascade of many kinematic abnormalities occurring over multiple joints. Knee valgus is due to a combination of movements resulting in reduced control of the knee, including ; trunk flexion, lateral flexion and rotation, pelvic rotation, tilt and obliquity, hip adduction and internal rotation and medial knee displacement. Such positions create increased forces at the knee, hip and pelvis during athletic tasks, in which these movements are more apparent in female sporting populations compared to males, suggesting that these combination of movements are influential on injury incidence [2-5]. Because knee valgus movements increases the loading on the ACL , this is thought to contribute to females being 4-6 times more likely to injure this structure of the knee [7, 8]. Therefore, athletes should aim to adopt a landing technique whereby the knee is in a position under the hip and over the ankle, as this has shown to reduce the risk of injury as a consequence  – see Figure 2.
Figure 1: Knee valgus during a landing activity.
Figure 2: Correction of mechanics during landing.
Single Leg Squat Assessment: What Does It Tell Us?
Due to the significance and prevalence of ACL injuries, research has primarily investigated the use of single leg squats to determine the relationship between biomechanical variables and ACL injury risk factors. Specifically, researchers have primarily sought to investigate the biomechanical events that contribute to knee valgus in order to guide interventions to reduce the incidence of injury.
Numerous studies have associated reduced gluteus muscle activity with greater knee valgus [2, 4, 9-12], specifically the gluteus medius, as this muscle predominantly acts to abduct the hip and therefore corrects the alignment of the knee in the frontal plane. A systematic review analysed 7 studies investigating the role of the external rotators on knee valgus, of the seven studies only one study suggested that weakness of hip external rotators influenced knee valgus . Greater knee valgus during a horizontal jump is associated with greater lateral quadriceps and hamstrings activity, whereby increased medial quadriceps activation reduced knee valgus . The medial gastrocnemius has also been associated to reduce knee valgus movements .
Limited research has investigated the role of joint ranges of motion and the subsequent effects on muscle activation patterns during a single leg squat. As knee valgus occurs as a result of a cascade of events occurring over multiple joints , joint range of motion could be hypothesised to be responsible for knee valgus . Previous research has linked tightness of muscles and structures such as the IT band , hip flexors , adductors and internal rotators  to cause increased hip internal rotation, leading to increased knee valgus. In contrast, tightness of the lateral gastrocnemius and peroneals may also contribute to tibial external rotation, thus promoting knee valgus . Additionally, talar mobility effects ankle dorsiflexion, whereby limited dorsiflexion has been shown to increase knee valgus during functional tasks  .
With findings inconclusive, Mauntel et al. (2013) investigated lower limb muscle activation and passive range of motion to determine the effects on performance of a single leg squat . Researchers assessed ten measures of passive range of motion (PROM) of the lower limb and collected electromyographical data of eight lower limb muscles during the performance of five single leg squats. There was a significant reduction of muscle co-activation patterns in individuals who displayed a knee valgus, specifically reduced gluteal activity (medius and maximus) and increased hip adduction activity were shown . Additionally, participants with medial knee displacement demonstrated reduced PROM of dorsiflexion both with the knee extended (targeting the gastrocnemius) and knee flexed (targeting soleus) and also a reduced posterior talus glide . A posterior talus glide is used to assess the capsular tightness of ankle dorsiflexion  – see Figure 3.
Therefore, an approach to reducing knee valgus should be multifaceted, addressing the strength and range of motion of key muscle groups, as mentioned above. However, improving an athlete’s biomechanical efficiency should be a key focus for coaches wanting to prevent injuries. Athletes of all levels should be taught the correct technique, with progressions specific to each athlete’s need. For example, a netballer could be taught proper landing mechanics using a horizontal jump in a closed environment (beginner), then continually be progressed to game simulated drills (open environment) performing single leg lands under the pressure of an opponent (advanced). This systematic increase in task-representation based on skill level will help the athlete transfer this learning to competition.
Figure 3: Posterior talus glide measurement with inclinometer .
Assessing Single Leg Squat Performance
3D biomechanical analysis is the gold standard test to determine kinematics of a single leg squat performance , however barriers include access to facilities, financial costs and the need for trained assessors. Fortunately the 2D assessment techniques are a valid method for identifying and quantifying the biomechanical inefficiencies [20, 21]. Using a 2D assessment to assess knee valgus in the frontal plane has been shown to be representative of the outcomes that can be measured in 3D biomechanical, although 2D assessments do not always show a constant relationship compared with 3D analysis . However, the appearance of knee valgus may be exaggerated in a 2D assessment , therefore the reliability of the results are dependent on the assessor’s ability to identify the biomechanical dysfunctions. Given the availability of high-resolution cameras built into phones and tablets, coaches and clinicians would benefit from using specific apps such as ‘HUDL Technique’ to quantify and assess movements (slow down movements, adds notes, assess joint angles) in their assessment of a single legged squat.
Coaches and player management staff should ensure that testing procedures are carefully followed to maximise validity and reliability of testing – the meaningfulness of testing and reassessment results. Unfortunately, protocols to assess a single leg squat vary, therefore below is some suggested procedures to assess an athlete’s performance:
- Instructions – clearly explain testing procedure to the athlete
- Warm-up – general aerobic, neuromuscular exercises and mobility exercises
- Practice trials – prescribe a set number of practice trials for the athlete
- Performance – prescribe the athlete 3–5 SL squats to allow quantitative/qualitative assessment and determine technique change with fatigue
- Feedback – provide the athlete with the results of the assessment
- Review & plan – identify specific areas for improvement for the athlete to be included as part of the training plan
- Squat depth – options include using a full or ½ squat or using a box at a specified height so that buttocks touch
- Squat technique – varies within the literature as participants can have; hands on hips, hands behind back, hands straight out, opposite leg placed forward or backward, opposite leg at 90° hip flexion
- Assessment plane – frontal plane details most information
- Knee valgus criteria – use of a scoring system for qualitative assessment to scale the severity of altered kinematics at each joint (1-3)
- General assessment – could include comment on balance/stability, fatigability
- SL squat assessment – using a methodological order = head > trunk > hips > knee > ankle
- Assessor – experience and use of same assessor over multiple repeated assessments
- Reliability – using the same protocols for reassessment to ensure testing sessions are the conducted in the same way
Overall, the SL squat is a reputable tool to assess athletic movement competency. Implementing a SL squat assessment to screen athletes provides coaches, management staff and athletes with valuable information when conducted in a robust manner, using experienced assessors and analysing movement both qualitatively and quantitatively. In isolation, the SL squat can be used as an effective tool to identify biomechanical faults. This can help to predict future risk factors for lower limb injuries. This would allow for interventions to be aimed to address the strength, mobility and motor control deficiencies that may underpin the highlighted biomechanical faults. Coaches and player management staff should compliment the SL squat assessment with a variety of additional assessment tools. This would allow for a more comprehensive screening of the athlete.
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