ORIGINAL ARTICLE Variation of force amplitude and its effects on local fatigue Marcus Yung • Svend Erik Mathiassen • Richard P. Wells Received: 3 November 2011 / Accepted: 28 February 2012 / Published online: 10 March 2012 Ó Springer-Verlag 2012 Abstract Trends in industry are leaning toward stereo- typed jobs with low workloads. Physical variation is an intervention to reduce fatigue and potentially musculo- skeletal disorders in such jobs. Controlled laboratory studies have provided some insight into the effectiveness of physical variation, but very few have been devoted to force variation without muscular rest as a component. This study was undertaken to determine multiple physiological responses to five isometric elbow extension protocols with the same mean amplitude (15 % maximum voluntary contraction, MVC), cycle time (6 s), and duty cycle (50 %). Sustained (15 %Sus) and intermittent contractions including zero force (0–30 %Int) differed significantly in 19 of 27 response variables. Contractions varying by half the mean force (7.5–22.5 %Int) led to 8 and 7 measured responses that were significantly different from 0–30 %Int and 15 %Sus, respectively. A sinusoidal condition (0–30 %Sine) resulted in 2 variables that were significantly different from 0–30 %Int, and 16 different from 15 %Sus. Finally, ten response variables suggested that varying forces with 1 % as the lower contraction level was sig- nificantly less fatiguing than 15 %Sus, while no responses were significantly different from 0–30 %Int. Sustained contractions led to decreased twitch force 24-h post-exer- cise, whereas recovery was complete within 60 min after intermittent contractions. This suggests that time-varying force may be a useful intervention to reduce local fatigue in workers performing low-load tasks, and also that rest per se did not seem to cause any extraordinary effects beyond those predictable from the force variation amplitude. Keywords Physical variation Á Intermittent exercise Á Work breaks Á Recovery Á Ergonomics Introduction Current trends in industry are leaning toward specialized production systems (Sato and Coury 2009), outsourcing (Mathiassen 2006), automation of work processes (de Looze et al. 2009), and sedentary work tasks (Straker and Mathiassen 2009). This development implies more ste- reotyped jobs, a reduction in restorative work breaks, and pauses within the job (Wells et al. 2007). Thus, in many contemporary occupations, a main intervention focus should not only be on the reduction in peak loads and extreme postures, but also on the challenge of introducing more exposure variation (Mathiassen 2006; Wells et al. 2007). Variation in biomechanical exposures (physical work- load) has been described as ‘‘the change in exposure over time’’ (Mathiassen 2006) and is thus associated with the quantity and frequency of exposure changes that occur across time in occupational settings. Initiatives to increase variation include between-task schemes such as job Communicated by Toshio Moritani. M. Yung (&) Á R. P. Wells Department of Kinesiology, University of Waterloo, 200 University Avenue, W., Waterloo, ON, Canada e-mail: m4yung@uwaterloo.ca S. E. Mathiassen Department of Occupational and Public Health Sciences, Centre for Musculoskeletal Research, University of Ga ¨vle, 801 76 Ga ¨vle, Sweden R. P. Wells Centre of Research Expertise for the Prevention of Musculoskeletal Disorders (CRE-MSD), University of Waterloo, 200 University Avenue, W., Waterloo, ON, Canada 123 Eur J Appl Physiol (2012) 112:3865–3879 DOI 10.1007/s00421-012-2375-z