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