Back muscle fatigue of younger and older adults with and without chronic low back pain using two protocols: A case-control study

Abstract

The purpose of this study was to compare back muscle fatigue of younger and older participants with and without chronic low back pain (CLBP). Twenty participants without and 20 with nonspecific CLBP participated in this study. Each group contained 10 younger (50% males; mean age: 31 ± 6 yrs) and 10 older adults (50% males; age mean: 71 ± 7 yrs). Two isometric fatigue protocols were presented randomly: (1) to maintain the unsupported trunk at the horizontal position while on a 45° Roman chair for a minute, and (2) to maintain a 10% of body weight box close to the trunk in the upright position for a minute. Surface electromyography (EMG) signals from the back (multifidus and iliocostalis) and one hip (biceps femoris) muscles were recorded bilaterally, and the median frequency fatigue estimate from linear regression slopes of the EMG time-series was computed. There were no significant (P > 0.05) age effects, and group-by-age interaction in both isometric and functional fatigue tasks. However, the CLBP groups (both younger and old) displayed more back fatigue than people without CLBP in both fatigue protocols (P < 0.01; effect size varying of d = 0.17–0.32). This study was sensitive to discriminate that individuals with CLBP did present significantly more pronounced EMG back fatigue than people without CLBP, in both younger and older adults. These results have significant clinical implications for low back pain rehabilitation programs with regard to endurance assessment in both younger and older.

Introduction

Poor back muscle endurance has been associated with chronic low back pain (CLBP) (Biering-Sorensen, 1984, Enthoven et al., 2003). The excessive fatigue of back muscles in subjects with CLBP may be associated with a shift in muscle fiber proportion toward type II fibers and a reciprocal atrophy of lumbar muscles (Hides et al., 1994, Mannion et al., 2000). Trunk muscle fatigue may increase neuromuscular deficits, resulting in brief uncontrolled intervertebral movements and consequently may also increase spine instability resulting in tissue strain and in turn a cumulative effect of CLBP (Granata and Gottipati, 2008, Panjabi, 1992).

Aging also results in neuromuscular changes, such as decreased force generation capacity, altered muscle fiber-type proportion, and slower motor unit firing rate. Consequently, this contributes to weaker, less fatigue resilient muscles and increased rate of falls in older adults (Avin and Law, 2012, Dionne et al., 2006, Leboeuf-Yde et al., 2009, Rogers and Evans, 1993, Dutta, 1997, Doherty, 2003). The combination of CLBP and aging effects may help to explain why up to 84% of older adults suffer from back pain (Dionne et al., 2006, Leboeuf-Yde et al., 2009). Given the high prevalence of CLBP, understanding fatigue responses of younger and older adults with and without CLBP is important for the management and prevention of these disorders. Few studies have, however, compared the effects of back pain on muscle fatigue between younger and older adults.

Fatigue can be measured by the rate of decline of the median frequency (MF) of the muscle firing rate observed using surface electromyography (EMG) (Arab and Salavati, 2007, da Silva et al., 2005, da Silva et al., 2008, Larivière et al., 2011, Moreau et al., 2001, Kankaanpaa et al., 1998). The rate of decline of EMG-MF is a valid and reliable index of muscle fatigue during isometric and dynamic contractions (for the latter, when well controlled by slow motion velocity from a context where the EMG signal is not stationary) (Singh et al., 2011a, Biedermann et al., 1991, Larivière et al., 2002, da Silva et al., 2008, da Silva et al., 2005). The muscle fatigue-induced decrease in spectral indices has been related to the action potential conduction velocity propagation, which is believed to be a result of metabolic by-product accumulation, such as lactate and extracellular K+ (Brody et al., 1991, Hagg, 1992). EMG-MF has been suggested to be the most suitable parameter for measuring EMG spectral shifts toward lower frequencies, where MF is less sensitive to noise and more sensitive to muscle-fatigue-related physiological changes (especially action potential conduction velocity decreases) during sustained and dynamic contractions (Moritani et al., 1986, De Luca, 1993). This physiological tool could thus be of interest for comparison between old and younger individuals with and without CLBP.

Several fatiguing protocols have been used to measure back fatigue in people with and without CLBP (Arab and Salavati, 2007, da Silva et al., 2005, da Silva et al., 2008, Larivière et al., 2011, Moreau et al., 2001, Kankaanpaa et al., 1998). One protocol that is easy to perform is a prone back extension exercise on a Roman chair (Arab and Salavati, 2007, Larivière et al., 2011, Clark et al., 2003) or on an examination table – the Sorensen test (Biering-Sorensen, 1984, da Silva et al., 2005, Kankaanpaa et al., 1998). With the classical protocol, the participant lies on the examining table (or Roman chair) in the prone position with the upper edge of the iliac crests aligned with the edge of the table (Moreau et al., 2001). The lower body can be fixed to the table by three straps (not in a Roman chair), located around the pelvis, knees, and ankles. With the arms folded across the chest, the participant is asked to isometrically maintain the upper body in a horizontal position (Moreau et al., 2001). Overall, the time during which the participant keeps the upper body straight and horizontal is recorded, and for those who experience no difficulty in holding the position, the test is stopped after 240 s. This test in parallel can be measured by EMG estimates as a MF index (Kankaanpaa et al., 1998, da Silva et al., 2005, Champagne et al., 2009).

On the other hand, most studies have only compared older and younger individuals with this type of exercise, evaluating only a healthy sample and using the endurance time variable as the main outcome (Champagne et al., 2009, Parreira et al., 2014) A few other studies assessed muscle fatigue based on EMG techniques, particularly in older adults (Singh et al., 2011a). Furthermore, no study generalized this comparison for tasks representing the workplace and occupational conditions (prolonged static postures, lifting, trunk flexion, repetitive movements), which often expose workers to risk factors for musculoskeletal disorders such as back pain (da Costa and Vieira, 2010, Pope et al., 2002, Vieira and Kumar, 2004). The present study introduced a functional protocol related to workplace activity of the trunk muscles that would be of interest for this comparison (Bonato et al., 2002): a sustained upright posture task with a box-load at 10% of total body mass, to be safe for individuals with and without CLBP (Han et al., 2013), which is also often used during activities of daily living.

The purpose of this study was to compare back muscle fatigue of younger and older subjects with and without CLBP using two isometric fatigue protocols through EMG measurements: (1) to maintain the unsupported trunk in a horizontal position while on a 45° Roman chair for one minute, and (2) to maintain a 10% of body weight box close to the trunk in the upright position for one minute. First, we hypothesized that individuals with CLBP would present a higher rate of back fatigue than individuals without CLBP during a classical unsupported trunk protocol on a Roman chair. Second, we hypothesized that the results could be generalized for the functional protocol of standing and maintaining a box-weight. Finally, we hypothesized that older participants with CLBP would present poor endurance by EMG measurement compared to those without CLBP.