Shoulder Joint Position Sense Improves with External Load David N. Suprak, PhD Louis R. Osternig, PhD Paul van Donkelaar, PhD Andrew R. Karduna, PhD

Introduction Joint stability –Functional activity –Contributing factors Proprioception –Feedback from limbs to CNS ( Sherrington, 1906 ) Joint position sense (JPS) and kinesthesia ( Dover et al, 2003 ) –Muscle stiffness, coordination –Importance for shoulder function

Mechanoreceptors Capsuloligamentous Pacinian Ruffini Golgi tendon organ-like End range ( Salo and Tatton, 1993; Vangsness, 1995; Steinbeck, 2003 ) Musculotendinous Muscle spindles Golgi tendon organs Throughout ROM ( Rymer and D'Almeida, 1980 ) Spindle sensitivity modulation ( Burke, 1978; Durbaba, 2001; Jami, 1980 ) Introduction

Muscle Spindle Alpha-gamma linkage ( Edin and Vallbo, 1990 ) Increased stimulation and sensitivity w/ muscle contraction ( Burke, 1978; Durbaba, 2001; Jami, 1980 ) Introduction

JPS and Muscle Activation Active JPS more accurate than passive ( Aydin et al, 2001; Edmonds et al, 2003; Potzl et al, 2004 ) Knee JPS with weight-bearing ( Bullock-Saxton, 2001 ) Introduction

Effect of Elevation Angle Decreased error approaching 90 deg ( Suprak et al, 2006 ) –Torque Muscle spindle sensitivity Did not assess effect of muscle activation directly Introduction

Purpose Effect of altering external load at constant position on repositioning error Hypothesis –Linear decrease in error with increased load Introduction

Subjects 24 healthy subjects –(10 M, 14 F) –Inclusion No hx of shoulder injury requiring surgery or rehabilitation –Exclusion Limited elevation ROM Diagnosed neuromuscular disorders Methods

Experimental Set-up Load Head-mounted display Thoracic receiver Humeral cuff and receiver Methods

Load Calculation Baseline shoulder torque –Arm length –Body mass –Segment mass, COM estimates ( Dempster, 1955 ) 10%, 20%, 30%, and 40% above baseline torque 5 Loading conditions Methods

Absolute Error Calculation

Target Positions 20/30 35/50 50/70 20/70 5 trials: 2 trials at 35/50 1 trial at distracters Methods

Statistical Analysis Planned linear contrast One-way repeated measures ANOVA –IV: External resistance (5 levels) –DV: Vector error Analysis repeated for plane and elevation error α < 0.05 Methods

Vector Error by Resistance Results

Plane Error by Resistance Results

Elevation Error by Resistance Results

Discussion Non-linear pattern –Internal representation –Sampling variability

Discussion JPS increases with external load –Muscle activation – receptor sensitivity ( Durbaba, 2001 ) Only in direction of load Role of musculotendinous receptors Injury prevention under high forces

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