Although there is a requirement for proprioceptive training and objective testing amongst athletes and in the clinical setting, the assessment of proprioception in the shoulder has traditionally been a challenge [1]. To date there have been wide variations in methodology and populations sampled with poor psychometric properties reported in tests [2]. Emerging field based tests often lack reliability and validity [3], however laboratory assessment methods with greater precision and psychometric properties are often not available for the everyday therapist wanting to assess proprioception.
Proprioception includes joint position sense (positional awareness of body parts and angles), kinesthesia (awareness of movement) and sensation of force [2, 4] and occurs through the stimulation of mechanosensory neurons (termed proprioceptors) located in the tissues providing afferent feedback [1, 4, 5]. Through the complex integration of peripheral and central systems [1, 6], the sum of these proprioceptors is important for both reactive and preparatory control as well as regulating muscle stiffness [4] in order to achieve movement coordination, timing and joint stability [7]. Being the most mobile and inherently unstable joint in the human body [8], the requirement for a strong and coordinated active stabilizing system in the shoulder joint is imperative. Consequently, any event negatively impacting neuro, muscular and skeletal systems has the potential to decrease proprioception [7], whether that be due to pain, effusion, trauma or fatigue [4]. Studies of the shoulder have shown a reduction in proprioception amongst subjects with joint laxity [9], in athletes undertaking overhead sports [10], in conjunction with shoulder pain [11], rotator cuff injuries [12] and amongst athletes who are fatigued [7, 13–16]. Although the role of proprioception in musculoskeletal disorders has been extensively researched in the lumbar and cervical spine [17–19] comparatively, there is limited research within the shoulder complex.
With the handball shoulder exposed to on average 48,000 throwing motions per season [20], 3–12 shots on goal and up to 180 ball contacts per match [21, 22], the effect of fatigue on proprioception has a good theoretical base. The extent to which this can be explained through central or peripheral mechanisms however requires further investigation. Peripherally, concentric contraction can lead to fatigue through the depletion of metabolic factors with eccentric contractions resulting in both metabolic fatigue and muscle damage [23]. It has also been proposed that fatigue increases the threshold of muscle spindle discharge effecting afferent feedback [24]. Within the central nervous system, central fatigue can potentially inhibit proprioception through diminished motor control and muscle stabilizing activity [24]. The effect of central and peripheral fatigue on shoulder proprioception has been investigated [22, 25] and although this research offers further insight into proprioception, it paradoxically highlights the complexity of peripheral and central mechanisms underpinning a local response.
Contemporary research [3, 26] investigating joint position sense (JPS) has included the use of laser pens allowing the assessment of athletes with a field-based test. Balke et al [3] initially used laser technology to evaluate the proprioceptive deficit in patients with shoulder instability through the use of an active JPS test however did not evaluate the reliability and validity of the test. Glendon and Hood [5] subsequently amended the methodology and showed no difference between dominant and non-dominant arms in healthy subjects but also recommended further studies to establish inter and intra-rater reliability. This concept was subsequently developed further by Vafadar et al. [26] who investigated the inter-rater, intra-rater reliability and validity of an active JPS test. They concluded inter-rater and intra-rater intraclass correlation coefficients were .86 and .78 respectively for this assessment method and the test had good validity when validated against a Vicon motion-capture system (Vicon, Oxford Metrics Ltd, Oxford, UK). The barrier for clinicians using this method, however, is that although the collection of data is efficient, the calculation of angle error afterwards remains time consuming. The aim of this study was to evaluate the test-retest reliability of an active JPS test using a laser pen and calibrated two-dimensional (2D) target for swift and feasible calculation of reposition error. Another aim was to investigate the effect of local fatigue, due to repeated throwing, on JPS in amateur male handball players.