Muscle fibers work all day, every day shortening and lengthening to make the body move. But not everyone can be a smooth operator— at least not all the time. When those muscles start shaking mid-exercise, research suggests a few things could be to blame, ranging from fatigue to dehydration Muscle fatigue: what, why and how it influences muscle function. Enoka, R.M., Duchateau, J. Department of Integrative Physiology, University of Colorado, Boulder, CO. Journal of Physiology. 2008 Jan 1;586(1):11-23. Muscle K , Na , and Cl disturbances and Na -Kpump inactivation: implications for fatigue. McKenna, M.J., Bangsbo, J., Renaud, J.M. Muscle, Ions and Exercise Group, School of Human Movement, Recreation and Performance, Centre for Ageing, Rehabilitation, Exercise and Sport, Victoria University, Melbourne, Victoria, Australia. Journal of Applied Physiology. 2008 Jan;104(1):288-95. .

Shake, Rattle, and Roll — The Need-to-Know

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In one study, researchers traced unsteady muscles to exhaustion or fatigue, particularly from more intense exercise (like sprinting or resistance training), which relies on fast muscle contraction Muscle fatigue: what, why and how it influences muscle function. Enoka, R.M., Duchateau, J. Department of Integrative Physiology, University of Colorado, Boulder, CO. Journal of Physiology. 2008 Jan 1;586(1):11-23. Differentiation and fiber type-specific activity of a muscle creatine kinase intronic enhancer Ta,i P.W., Fisher-Aylor, K..I, Himeda, .CL., et al. Department of Biochemistry, University of Washington, Seattle, WA. Skeletal Muscle. 2011 Jul 7;1:25. . But why the shake-up? These muscles are actually smarter than they look, divvying up the workload between fibers (some work while others rest, and then “switch!”) The influence of contraction amplitude and firing history on spike-triggered averaged trapezius motor unit potentials Westad, C. Westgaard, R.H. Department of Industrial Economics and Technology Management, Norwegian University of Science and Technology, Trondheim, Norway. Journal of Physiology. 2005 Feb 1;562(Pt 3):965-75. . But as they’re challenged more and more, this job-swapping can get a little ragged, causing muscles to lose their steady motion Muscle fatigue: what, why and how it influences muscle function. Enoka, R.M., Duchateau, J. Department of Integrative Physiology, University of Colorado, Boulder, CO. Journal of Physiology. 2008 Jan 1;586(1):11-23. . The shaking? Just the body’s signal that it might be quittin’ time The neurobiology of muscle fatigue: 15 years later. Barry, B.K, Enoka, R.M. Department of Integrative Physiology, University of Colorado at Boulder, CO. Integrative and Comparative Biology. 2007 Oct;47(4):465-73. Epub 2007 Jun 6. .

Muscle endurance can also be a factor. If those hammies haven’t completely recuperated from yesterday’s killer workout, the muscles are forced to work harder than usual to keep up. Trying a new exercise can also cause fatigue to hit faster, making the muscle shake its groove thing— and not in a good way Neural influences on sprint running: training adaptations and acute responses. Ross, A., Leveritt, M., Riek, S. School of Human Movement Studies, University of Queensland, Brisbane, Australia. Sports Medicine. 2001;31(6):409-25. Measurement of voluntary activation of fresh and fatigued human muscles using transcranial magnetic stimulation Todd G., Taylor JL., Gandevia SC. Prince of Wales Medical Research Institute and the University of New South Wales, Sydney Australia. Journal of Physiology. 2003 Sep 1;551(Pt 2):661-71. Epub 2003 Aug 8. .

And then there’s dehydration. When the body gets low on fluids (which can also cause electrolytes to get off kilter) the connective tissue has trouble doing its job— including transmitting signals from the brain down to the muscle fibers. And with a garbled message about when to contract, the firing sequence can get noticeably out of whack Muscle K , Na , and Cl disturbances and Na -Kpump inactivation: implications for fatigue. McKenna, M.J., Bangsbo, J., Renaud, J.M. Muscle, Ions and Exercise Group, School of Human Movement, Recreation and Performance, Centre for Ageing, Rehabilitation, Exercise and Sport, Victoria University, Melbourne, Victoria, Australia. Journal of Applied Physiology. 2008 Jan;104(1):288-95. . Hence, the Harlem Shake.

All Shook Up — Your Action Plan

For some, shaking muscles can be like a badge of honor marking an intense workout. Still, there’s no solid evidence suggesting this is something exercisers should try to work through. Since shakiness can be especially common toward the end of an exercise, the muscle may be close to exhaustion, signaling it might just be time to call it a day Measurement of voluntary activation of fresh and fatigued human muscles using transcranial magnetic stimulation Todd G., Taylor JL., Gandevia SC. Prince of Wales Medical Research Institute and the University of New South Wales, Sydney Australia. Journal of Physiology. 2003 Sep 1;551(Pt 2):661-71. Epub 2003 Aug 8. . A solid warm up and stretch session might also keep shaking at bay by helping to reduce muscle fatigue from the get-go A review of the acute effects of static and dynamic stretching on performance. Behm, D.G., Chaouach,i A. School of Human Kinetics and Recreation, Memorial University of Newfoundland, St. John’s, NF, Canada. Europena Journal of Applied Physiology. 2011 Mar 4. .

But if the muscle starts to shake toward the beginning of the exercise, it may be more of a warning sign that the exercise is too difficult for the body to handle, so scaling back on the intensity or lightening up on the weights may be necessary.

Generally, as muscles get stronger and become accustomed to an exercise, shaking will happen less, but everyone responds differently to exertion. So it’s possible for some to reduce the effects of shaking by improving muscle endurance, while others may experience shaking muscles with every workout, regardless of how experienced they are in the weight room Stretch-shortening cycle exercises: an effective training paradigm to enhance power output of human single muscle fibers. Malisoux, L., Francaux, M., Nielen,s H., et al. Département d’Education Physique et de Réadaptation, Faculté de Médecine, Université Catholique de Louvain, Louvain-La-Neuve, Belgium. Journal of Applied Physiology. 2006 Mar;100(3):771-9. Functional adaptability of muscle fibers to long-term resistance exercise Shoep,e T.C., Stelzer, J.E., Garner, D.P., et al. Department of Exercise and Sport Science, Oregon State University, Corvallis, USA. Medicine and Science in Sports Exercise. 2003 Jun;35(6):944-51. .

No matter how hard the exercise routine, be sure to maintain good nutrition and hydration to help counteract shaking due to factors like dehydration and electrolyte balance. Getting enough fluids throughout the workout will help keep the tissues happy and hydrated, so the signal from the brain to the muscle fibers won’t get lost in translation.