The Single-Leg Squat as a Screening Tool to Identify Excessive Trunk Lean during Pitching

Plummer HA, Oliver GD, Powers CM, Michener LA. Trunk lean during a single-leg squat is associated with trunk lean during pitching. Int J Sport Phyiscal Ther. 2018;13(1):58-65. doi:10.26603/ijspt20180058.

Background

The baseball pitching motion requires the transfer of energy from the lower body, through the trunk, to the shoulder, and down to the hand before ball release.  The hips and trunk contribute a large portion of the kinetic energy and force required in overhead sports.  Therefore, coordination and control of trunk movement and position is important for optimal pitching mechanics.  Previous studies have identified 10° of contralateral trunk lean as the ideal trunk position to minimize varus moments about the elbow at maximum shoulder external rotation1,2.

Baseball players exhibiting > 10° of contralateral trunk lean could potentially be at risk for shoulder and elbow injury.  Identifying baseball players who exhibit altered trunk control during pitching may help reduce this risk.  The single-leg squat (SLS) has been used as a screening test to assess trunk control during dynamic movement.  However, it is unclear if lateral trunk lean during the SLS is associated with lateral trunk lean during pitching.

Purpose

The purpose of this study was to determine the relationship between lateral trunk lean during the SLS and the amount of trunk lean during pitching and to determine if trunk lean during pitching can be predicted from the SLS.

Study Population

  • Youth baseball players (n=73) with a mean age of 11.4 years
  • Free from injury during the past 6 months

Research Methods

Kinematic data was collected in a laboratory setting while research participants performed the SLS on their lead leg.  The lead leg was selected because weight is transferred to this leg as the pitching motion progresses.  Participants were instructed to place their hands on their hips and squat as low as possible before returning to a full upright position.  The trunk position in the frontal plane at 45° of knee flexion was selected for analysis.  Next, after unlimited time to warm-up, kinematic data were collected during three  two-seam fastball pitches.  Negative lateral trunk lean values indicate lean away from the throwing arm.  The position of maximum shoulder external rotation was the point selected to measure trunk lean towards and away from the throwing arm.  A linear regression was performed to assess the predictive ability of lateral trunk lean during the SLS on lateral trunk lean during pitching.

Key Results

  • A positive correlation between trunk lean during the SLS and trunk lean during pitching was observed (r = 0.53; p<0.001)
  • Mean lateral trunk lean during pitching was -17.1° ± 13.0° (away from the throwing arm) and -5.8° ± 10.4° during the SLS.
  • Lateral trunk lean during the single-leg squat predicted the amount of lateral trunk lean during pitching (R2 = 0.28; p< 0.001).

Study Limitations

  • The results may not be generalized to other age groups or populations
  • Strength of individual muscles was not assessed so any explanation of hip or trunk weakness as the cause of trunk lean is purely speculative.
  • Motion analysis equipment and software is not available in most clinical settings. It is unclear if visual assessment would produce similar results.

Practical Implications & Additional Thoughts

Trunk lean during the SLS explained 28% of the variance in trunk lean during pitching.  The moderate correlation between trunk lean during the SLS and pitching motion suggests the SLS may have potential as a clinical screening tool.  Further research would be needed to confirm this hypothesis.

During pitching, contralateral trunk lean functions to position the shoulder in the proper arm slot (position of the shoulder relative to the trunk).  Contralateral trunk lean increases from foot contact to maximum shoulder external rotation. This phase of the pitching motion represents the late cocking phase.  Solomito et al.2 showed for every 10° increase in trunk lean there was an increase in elbow varus moment of 3.7 Nm and a 2.5 Nm increase in glenohumeral internal rotation moment.

The SLS has potential to help identify youth baseball pitchers with excessive trunk lean during pitching which may lead to elbow or shoulder injury.  However, there are many unanswered questions related to how the SLS can be employed clinically to identify risk in youth baseball players.   The next step is to track injuries during the season and examine the relationship between SLS performance and upper extremity injury.

Intervention programs targeting neuromuscular control and strength of the hip and core musculature could potentially improve SLS performance and trunk lean during pitching.  However, this has not been specifically investigated.  Such programs would likely involve training motor control, muscular strength and endurance.  In such programs, single –leg and split stance positions should be used with a particular emphasis on pelvis and trunk control.  Once these physical attributes are enhanced it is imperative that pitchers work with coaches to ensure carryover to proper pitching mechanics.

References

  1. Oyama S, Yu B, Blackburn JT, Padua DA, Li L, Myers JB. Effect of excessive contralateral trunk tilt on pitching biomechanics and performance in high school baseball pitchers. Am J Sports Med. 2013;41(10):2430-2438. doi:10.1177/0363546513496547.
  2. Solomito MJ, Garibay EJ, Woods JR, Nissen CW. Lateral trunk lean in pitchers affects both ball velocity and upper extremity joint moments. Am J Sports Med. 2015;43(5):1235-1240. doi:10.1177/0363546515574060.

 

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