Shoulder Joint Position Sense Improves with External Load
David N. Suprak, PhDLouis R. Osternig, PhD
Paul van Donkelaar, PhDAndrew 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
Methods
θθ
Absolute Error Calculation
Target Positions
20/30
35/50
50/7020/70
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
4
5
6
7
8
9
NR 10% 20% 30% 40%
Resistance (% above baseline torque)
Vec
tor
Err
or (
Deg
)
0
Results
Plane Error by ResistanceResults
Elevation Error by ResistanceResults
4
5
6
7
8
9
NR 10% 20% 30% 40%
Resistance (% above baseline torque)
Vec
tor
Err
or (
Deg
)
0
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
Thank you