Elsevier

Manual Therapy

Volume 17, Issue 5, October 2012, Pages 411-415
Manual Therapy

Original article
The role of experimentally-induced subacromial pain on shoulder strength and throwing accuracy

https://doi.org/10.1016/j.math.2012.03.008Get rights and content

Abstract

Shoulder injuries often comprise two separate yet related components, structural tissue damage and pain. The role of each of these components on shoulder function is difficult to ascertain. Experimental pain models allow the assessment of consequences of localized pain when applied to healthy individuals. By understanding the role of pain on shoulder function, clinicians will be able to more efficiently assess and treat shoulder injuries. The objective of the study was to evaluate the role of experimentally-induced sub-acromial pain on shoulder isokinetic rotational strength and throwing accuracy. This was a block counterbalanced, crossover, repeated measures study design utilizing 20 individuals without self-reported shoulder or cervical pathology. Shoulder function was measured with and without experimental pain injection (2 mL of 5% hypertonic saline) in the sub-acromial space. Functional tasks consisted of shoulder rotational strength utilizing isokinetic testing and throwing accuracy via the functional throwing performance index. The hypertonic saline induced moderate pain levels in all participants (4.3–5.1/10). Normalized shoulder internal (t = 3.76, p = 0.001) and external (t = 3.12, p = 0.006) rotation strength were both diminished in the painful condition compared to the pain free condition. Throwing accuracy was also reduced while the participants experienced pain (t = 3.99, p = 0.001). Moderate levels of experimental shoulder pain were sufficient to negatively influence shoulder strength and throwing accuracy in participants without shoulder pathology.

Introduction

Subacromial pathology is generally believed to comprise structural damage and/or inflammation to one or more sub-acromial tissues (supraspinatus tendon, sub-acromial bursa). Individuals with sub-acromial pathology commonly present with impairments related to shoulder pain, shoulder muscle function, and neuromuscular control (Lukasiewicz et al., 1999; Ludewig and Cook, 2000; Reddy et al., 2000; Myers et al., 2006). Specific functional complaints often deal with an inability to perform overhead reaching or high demand activities performed against large resistance or at high velocities. The reason why individuals with sub-acromial pathology exhibit functional deficits, however, is not clear. The functional deficits related to sub-acromial pathology appear to be associated with pain provocation more than structural tissue damage given the prevalence of pathological findings using diagnostic testing in asymptomatic individuals (Milgrom et al., 1995; Tempelhof et al., 1999; Kim et al., 2009). Non-specific shoulder pain has been shown to alter force production, muscle coordination, and activities of daily living (Itoi et al., 1997; Ginn and Cohen, 2005; Park et al., 2005; Diederichsen et al., 2009) To date the roles of pain and structural damage on functional deficits are not sufficiently understood (Lewis, 2011).

The aims of shoulder rehabilitation generally aim in restoration of strength, range of motion, activities of daily living, functional or occupational deficits, and alleviation of pain. Successful treatment regimens for sub-acromial pathology typically address many factors related to the dysfunction with the anticipation that some or all will assist in recovery of function (Kibler et al., 2001; Myers et al., 2006). While this method has been shown to be advantageous, there may be more efficient means to treat these conditions if more is known about the roles of pain and structural damage. Understanding the specific deficits related to shoulder pain, independent of structural damage, will allow clinicians to appreciate the role pain plays on their patients' function.

Pain is the most common clinical complaint of individuals seeking medical treatment for musculoskeletal injury (Raofi and Schappert, 2006). Experimental pain models allow assessment of the influence of pain only, when administered to individuals without tissue injury. Hypertonic saline injection into muscle or periarticular structures creates transient local or referred pain and has been shown to be an appropriate model for simulating musculoskeletal pain (Staahl and Drewes, 2004). Hypertonic saline mimics moderate musculoskeletal pain yet the nociceptive response is short lived, lasting only several minutes (Capra and Ro, 2004). Undue harm is thus minimized on research participants, yet valuable information on the role of pain on functional performance is able to be determined (Staahl and Drewes, 2004). Prior studies assessing the role of experimental pain and strength have shown deleterious effects when simulating anterior knee and low back pain (Arendt-Nielsen et al., 1996; Hodges et al., 2003; Bennell et al., 2004; Henriksen et al., 2010). Similarly at the shoulder, experimental pain has demonstrated altered muscle activity of several humeral and scapular muscles when introduced in the muscles and periarticular structures (Falla et al., 2007; Diederichsen et al., 2009). Yet to date the role of experimentally-induced sub-acromial pain on shoulder rotational strength and other functional tasks has not been described. Thus, the aim of this study was to evaluate the role of experimentally-induced sub-acromial pain on shoulder isokinetic rotational strength and throwing accuracy. The information gleaned from this study may help clinicians to more efficiently treat patients with sub-acromial pathology by delineating where functional deficits originate. The authors hypothesized that both strength and accuracy measures would be diminished while participants were experiencing pain (Falla et al., 2007; Diederichsen et al., 2009; Vuillerme and Pinsault, 2009; Henriksen et al., 2010; Cagnie et al., 2011).

Section snippets

Participants

A sample of convenience consisting of 20 participants (10 males, 10 females) participated in the study. Participants were all aged between 18 and 31 years. This age group was specifically chosen to decrease the likelihood of age related degeneration of the rotator cuff (Milgrom et al., 1995). Participants were considered healthy using the following criteria: denied any history seeking medical care for shoulder or neck injuries and reported no current shoulder or neck pain. All testing was

Results

Twenty (20) participants completed the study. Their demographics were as follows: mean age (range) 22.3 years (18–31), mass 71.6 kg (52–107), and height 1.72 m (1.78–1.87). The hypertonic saline injection caused moderate shoulder pain in all patients, lasting approximately 15 min. Participants reported pain at 5.1 ± 2.6 cm (range 1.5–8.5 cm) immediately following the throwing task and 4.3 ± 2.8 cm (range 1.4–8.5 cm) immediately after the isokinetic evaluation. No pain was reported for any

Discussion

To the authors' knowledge this is the first study to assess the role of experimental pain on throwing accuracy. Throwing accuracy has been shown to be adversely influenced with local muscle fatigue and following the application of cryotherapy to the shoulder (Suzuki et al., 2006; Wassinger et al., 2007; Freeston et al., 2010). Both the fatigue condition and the ice application were hypothesized to disrupt motor control either through decreased afferent input (proprioception) or neuromuscular

Conclusion

As hypothesized, this study demonstrated that injection of hypertonic saline into the sub-acromial space produces moderate shoulder pain associated with decreased throwing accuracy and muscle strength in healthy individuals. Understanding the causes of decreased functional performance allows clinicians to focus on treating the aspects of shoulder injury which directly contribute to functional decline. Clinicians working with patients should be mindful of the influence of pain on shoulder

Funding

This study was funded by the Emerging Investigator Award in the School of Physiotherapy at the University of Otago, New Zealand.

Acknowledgments

The authors would like to thank Malcolm Gollan for his assistance with data collection and analysis.

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