Y-Balance Test Scores are Associated with Stride Length in College Baseball Pitchers

Culiver A, Garrison JC, Creed KM, Conway JE, Goto S, Werner S. Correlation among Y-Balance Test – lower quarter composite scores, hip musculoskeletal characteristics, and pitching kinematics in NCAA Division I baseball pitchers. J Sport Rehabil. 2019;28:432-437.

Background

The baseball pitching motion requires coordination and utilization of the entire kinetic chain starting at the hips and trunk extending to the distal upper extremity.  Adequate range of motion (ROM), strength, neuromuscular control, and balance is required to efficiently transfer energy up the kinetic chain.  Decreased hip ROM and strength have been linked to reduced stride length and arm injuries.  Greater stride length has also been associated with greater ball velocities.  Increasing stride length is believed to increase forward trunk velocity thus reducing the arm’s contribution to ball velocity.

The Y-Balance Test–Lower Quarter (YBT-LQ) is a clinical measure to assess lower-body strength, neuromuscular control, and dynamic balance.   Impairments in any of these measures may influence a pitcher’s utilization of the kinetic chain.  Consequently, a pitcher’s strength, balance, and ROM may influence the sequencing of the kinetic chain and impact performance and injury risk.

Purpose

The purpose of this study was to evaluate the relationship between pitching kinematics, YBT-LQ, hip ROM, and hip strength.

Study Population

  •  Healthy male D-1 College baseball pitchers (n = 19)
  • Ages 18 to 22 years
  • Mean 9.21 years of pitching experience
  • No shoulder or elbow injury in the past 6 months

Research Methods

All research participants underwent the same testing procedures consisting of hip passive ROM, isometric hip strength, YBT-LQ, and three-dimensional (3D) motion capture analysis of pitching mechanics.  The same physical therapist measured hip internal and external rotation passive ROM in the prone position with a bubble goniometer.  Isometric hip extension, external rotation, and abduction strength was measured with a handheld dynamometer.

The YBT-LQ composite score was calculated following administration of the 3 test reach directions: anterior, posteromedial, and posterolateral.   Finally, a 3D motion capture system analyzed the following measures during pitching: stride length, time to maximal humerus velocity, time to maximal thorax velocity, and time from stride foot contact to maximal knee flexion.  Pearson correlation coefficients were calculated to determine relationships between throwing kinematic data and clinical measures.

Key Results

  • Stride length demonstrated a moderate positive correlation with dominant limb YBT-LQ composite score (r = .524, P = .02).
  • Stride length demonstrated a moderate positive correlation with nondominant limb YBT-LQ composite score (r = .550, P = .01).
  • Stride length demonstrated a weak positive correlation with normalized time to maximal humerus velocity (r = .458, P = .04).
  • Stride length had a moderate negative correlation with normalized time to maximal thorax velocity (r = −.522, P = .02).
  • Stride length had a moderate negative correlation with dominant hip total ROM (r = −.660, P = .002)
  • Stride length had a strong negative correlation with normalized time from stride foot contact to maximal knee flexion (r = −.722, P < .001).
  • Dominant limb YBT-LQ composite score had a weak negative correlation with hip abduction strength difference (r = −.459, P = .04)
  • Dominant limb YBT-LQ composite score had a weak negative correlation with normalized time to maximal thorax velocity (r = −.468, P = .04).
  • Dominant limb YBT-LQ composite score had a moderate negative correlation with dominant hip total ROM (r = −.160, P = .004).
  • Nondominant limb YBT-LQ composite score demonstrated a weak negative correlation with normalized time to maximal thorax velocity (r = −.450, P = .046).
  • Nondominant limb YBT-LQ composite score demonstrated a moderate negative correlation with dominant hip total ROM (r = −.668, P = .001).
  • Dominant hip total ROM had a moderate positive correlation with normalized time to maximal thorax velocity (r = .484, P = .03)

Study Limitations

  •  Small sample size (n=19) with only healthy pain-free college pitchers assessed during the pre-season
  • Findings can only suggest a correlation, no cause-and-effect relationship can be established

Final Thoughts and Practical Implications

The results of this study support the concept of assessment and treatment of the entire kinetic chain in baseball pitchers.  This applies to pitchers rehabilitating from injury and healthy players seeking to improve performance.  College baseball pitchers with higher YBT-LQ composite scores on the dominant and/or non-dominant side demonstrate greater stride length during the pitching motion.  Pitching with an increased stride length may have a protective effect on the shoulder and elbow.  The fact that stride length was negatively correlated with time to maximal thorax velocity and positively correlated with time to humerus velocity supports this hypothesis.

Earlier initiation of forward trunk flexion may facilitate the efficient transfer of energy from the stance limb to the trunk and upper body as the pitcher strides and accelerates towards home plate.  A greater stride length is also associated with achieving peak humeral velocity later during the pitching motion.  This may lead to a greater energy transfer from the arm to the ball at release.  Pitchers who throw with less kinetic chain involvement may need to achieve maximal humerus velocity earlier, through upper body contributions, to compensate for decreased lower body contribution.

Stance limb YBT-LQ composite scores were negatively correlated with hip abduction strength differences.  This suggests that hip abduction strength asymmetries may play a role in stance limb balance during the throwing motion.  The hip abductors play an important role in stabilizing the pelvis, hip, and lower leg during single-leg stance.  Pitchers also demonstrated greater hip total rotational passive ROM on the dominant versus non-dominant side.  The increased mobility on the dominant limb may necessitate even greater neuromuscular demands to stabilize and control the kinetic chain during pitching.

The results of this study support the clinical utility of assessing hip rotational passive ROM, hip abductor strength symmetry, and balance using the YBT-LQ in baseball pitchers.  Designing rehabilitation and performance programs that address identified impairments in these areas may have positive effects throughout the kinetic chain during the pitching motion.  In particular, improvements in stride length may be achieved.  This may have protective effects on the shoulder and arm potentially reducing injury risk.  Further research would be needed to confirm this hypothesis.

 

 

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