Strength Training Improves Fatigue Resistance and Self-Rated Health in Workers with Chronic Pain: A Randomized Controlled Trial

Abstract

Chronic musculoskeletal pain is widespread in the working population and leads to muscular fatigue, reduced work capacity, and fear of movement. While ergonomic intervention is the traditional approach to the problem, physical exercise may be an alternative strategy. This secondary analysis of a randomized controlled trial investigates the effect of strength training on muscular fatigue resistance and self-rated health among workers with chronic pain. Sixty-six slaughterhouse workers with chronic upper limb pain and work disability were randomly allocated to 10 weeks of strength training or usual care ergonomic training (control). At baseline and follow-up, participants performed a handgrip muscular fatigue test (time above 50% of maximal voluntary contraction force) with simultaneous recording of electromyography. Additionally, participants replied to a questionnaire regarding self-rated health and pain. Time to fatigue, muscle strength, hand/wrist pain, and self-rated health improved significantly more following strength training than usual care (all P < 0.05). Time to fatigue increased by 97% following strength training and this change was correlated to the reduction in fear avoidance (Spearman's rho = -0.40; P = 0.01). In conclusion, specific strength training improves muscular fatigue resistance and self-rated health and reduces pain of the hand/wrist in manual workers with chronic upper limb pain. This trial is registered with ClinicalTrials.gov NCT01671267.

Figure 1

Participant flow. ^∗Missing data (i.e., participants with no data on fatigue test at baseline and follow-up) was present from 6 and 2 participants in the strength training and in the usual care ergonomic group, respectively. Therefore, 27 and 31 participants were included in the analyses of the fatigue test from the strength training group and usual care ergonomic group, respectively.

Figure 2

A representative illustration of the force output during the handgrip fatigue test in a strength trained person at follow-up. Individuals were to press as hard as possible throughout the entire fatigue test, and when the force output decreased to below 50% of MVC the test was finished. Peak force (normalized to MVC), start time (T _o), and time at fatigue (T _fatigue) are illustrated on the figure.

Figure 3

Radar-chart summarizing the interventional changes (% change) for the main variables following strength training (blue covered area) and ergonomic usual care (red covered area). The black broken line represents no change. Variables arranged clockwise: (1) pain intensity of the hand/wrist, (2) fear avoidance, (3) self-rated health, (4) handgrip strength, (5) extensor EMG during handgrip MVC, (6) flexor EMG during handgrip MVC, (7) time to fatigue, (8) extensor EMG during fatigue test, (9) flexor EMG during fatigue test. ^∗ P < 0.05.