Although your mind’s powers are highly limited, thinking alone can strengthen your body.

Mental Imagery

—that’s the key concept here: the conscious mental rehearsal of movement. If used as a regular practice, mental imagery can improve your physical strength even without performing any actual movement. You can literally sit at home on your couch and grow stronger by merely thinking about lifting weights.

Experimental Studies

Most studies on motor imagery follow an experimental design like this:

The subjects are healthy college students, randomly grouped into:
- a motor imagery group (those who think about performing a movement)
- a physical exercise group (those who actually perform the movement)
- a control group (those who don’t do anything)
Subjects practice regularly for several weeks.
Physical strength is systematically measured either as muscle contraction or as force exerted on a machine.

As the results showed, muscle strength increased significantly in the motor imagery group as compared to the control group and to a similar extent as in the physical exercise group for various motion patterns:

ankle dorsiflexor torque (Sidaway & Trzaska 2005)
ankle plantar flexor torque (Zijdewind et al. 2003)
bench pressing (Reiser 2005)
bench pressing, leg pressing, triceps extension, and calf raising (Reiser et al. 2011)
elbow flexion (Yao et al. 2013, Bahari et al. 2011, Ranganathan et al. 2004)
finger motion (Helm et al. 2015, Stinear et al. 2006, Ranganathan et al. 2004, Smith et al. 2003, Yue & Cole 1992)
hip flexion (Shackell & Standing 2007)
quadriceps muscle contraction (Cornwall et al. 1991)

Further findings:

The more vividly subjects imagined contracting their muscles, the more strength they gained (Reiser et al. 2011).
The more effortful the movement, the stronger the results (Helm et al. 2015).
Kinesthetic imagery produced significant effects but not external/visual imagery (Yao et al. 2013, Stinear et al. 2006, Reiser 2005): Only subjects who mentally simulated the feeling of performing the movement from inside their bodies increased their strength, while those who visualized themselves moving from a 3rd person perspective (like watching themselves in a video) did not.

How does this work?

The basic principle of how strength was increased in these experiments was neural adaptation: The motor imagery training strengthened the brain-to-muscle command (bodybuilders call this the “mind-muscle connection”), which improved the recruitment of motor units. This led to higher muscle output, which resulted in greater measured strength.

In this causal chain, improved motor unit recruitment could mean various things:

recruitment of additional (hitherto inactive) motor units
higher activation level of participating motor units
more effective inter-muscular coordination, which means that the neuromuscular junctions at the antagonist muscle fire less when the agonist contracts (Folland & Williams 2007)
reduced inhibitory input to the muscle’s motor-neuron pool (Ranganathan et al. 2004)

This is how muscle strength can increase due to adaptations in the nervous system without any metabolic or morphological changes in muscle tissue (i.e., no hypertrophy). Yet how can mere mental imagery lead to motor adaptations in the nervous system?

What is the basis of the “mind-muscle connection”?

This brings us to the heart of the matter, which is the idea that mental imagery uses the same brain processes as action execution (Ietswaart et al. 2015). When you think about picking up the trash, your brain activity is equivalent to when you’re actually picking it up, as well as to when you see someone else picking it up—that’s what your mirror neuron system is all about.

So what’s the difference then between seeing, thinking, and acting if the neural processes are equivalent?—According to simulation theory, imagined actions are simply actions that are not executed (see Cognitive Psychology: Mind and Brain, Chapter 11). This suggests that mental imagery of a movement and execution of the movement are similar in neural terms, while only for the latter the motor output is not inhibited. As of today, however, the role of inhibition in mental imagery requires further research.

“Does this mean I don’t need to go to the gym?”

No!

Pure neural adaptation means that you gain strength without building muscle mass.
All the many health and mood benefits that come from physical exercise won’t come from mental imagery.
It’s a lot harder to get motivated to regularly practice mental imagery than it is to work out or play a sport, which is way more fun.
The scientific studies lasted only a few weeks, and neural adaptation to any movement is strongest at the beginning: The contribution of neural factors to strength gains drops dramatically over the course of the first sixteen weeks, while hypertrophy factors become increasingly important.

Let’s keep this clear, there’s absolutely no reason to replace actual movement with imagined movement! Why would you even want to make yourself a prisoner of your mind, imagining how it would be if you did something, rather than doing it?

Nevertheless, mental imagery does have some useful applications:

Neurological rehabilitation (e.g., Helm et al. 2015)
Support of chronic pain treatment (e.g., Hidalgo-Peréz et al. 2015)
Strength maintenance during injury and immobilization (e.g., Meugnot et al. 2015, Clark et al. 2014)
Maximum athletic performance (e.g., Munzert et al. 2009)
Muscle relaxation (e.g., Kato et al. 2015)

“Can you be more specific? I need concrete practical guidelines!”

Unfortunately, from a scientific point of view, the research that has been conducted so far doesn’t allow me to give any clinical advice. Even though the neuroimaging data seems to be robust, the behavioral evidence and actual efficacy of mental imagery are currently too weakly investigated to conclude anything from it (cf. Ietswaart et al. 2015).

In case you need some practical advice, check out the Mastery section of mindcoolness.com! Furthermore, new studies are being conducted as I write (and as you read). If you want to stay informed about the most recent findings on this topic, subscribe to my newsletter:

Resources

Bahari, S. M., Damirchi, A., Rahmaninia, F., & Salehian, M. H. (2011). The Effects of Mental Practice on Strength Gain and Electromyographic Changes in Elbow Flexor Muscles. Annals of Biological Research 2(6), pp. 198-207.

Clark, B. C., Mahato, N. K., Nakazawa, M., Law, T. D., Thomas, J. S. (2014). The power of the mind: the cortex as a critical determinant of muscle strength/weakness. Journal of Neurophysiology 112(12), pp. 3219-3226.

Cornwall, M. W., Bruscato, M. P., & Barry, S. (1991). Effect of Mental Practice on Isometric Muscular Strength. The Journal of Orthopaedic and Sports Physical Therapy 13(5), pp. 231-234.

Folland, J. P. & Wiiams, A. G. (2007). The adaptations to strength training: morphological and neurological contributions to increased strength. Sports Medicine 37(2), pp. 145-168.

Helm, F., Marinovic, W., Krüger, B., Munzert, J., Riek, S. (2015). Corticospinal excitability during imagined and observed dynamic force production tasks: effortfulness matters. Neuroscience 290, pp. 398-405, doi: 10.1016/j.neuroscience.2015.01.050.

Hidalgo-Peréz, A., Fernández-García, Á., López-de-Uralde-Villanueva, I., Gil-Martínez, A., Paris-Alemany, A., Fernández-Carnero, J., La Touche, R. (2015). Effectiveness of a motor control therapeutic exercise program combined with motor imagery on the sensorimotor function of the cervical spine: a randomized controlled trial. The International Journal of Sports Physical Therapy 10(6), pp. 877-893.

Ietswaart, M., Butler, A. J., Jackson, P. L., Edwards, M. G. (2015). Editorial: Mental practice: clinical and experimental research in imagery and action observation. Frontiers in Human Neuroscience 9(573), doi: 10.3389/fnhum.2015.00573.

Kato, K., Watanabe, J., Muraoka, T., Kanosue, K. (2015). Motor imagery of voluntary muscle relaxation induces temporal reduction of corticospinal excitability. Neuroscience Research 92, pp. 39-45, doi: 10.1016/j.neures.2014.10.013.

Meugnot, A., Agbangla, N. F., Almecija, Y., Toussaint, L. (2015). Motor imagery practice may compensate for the slowdown of sensorimotor processes induced by short-term upper-limb immobilization. Psychological Research 79(3), pp. 489-499, doi: 10.1007/s00426-014-0577-1.

Munzert, J., Lorey, B., Zentgraf, K. (2009) Cognitive motor processes: the role of motor imagery in the study of motor representations. Brain Research Reviews 60, pp. 306-326.

Ranganathan, V. K., Siemionow, V., Liu, J. Z., Sahgal, V., & Yue, G. H. (2004). Form mental power to muscle power—gaining strength by using the mind. Neuropsychologica 42, pp. 944-956.

Reiser, Mathias (2005). Kraftgewinne durch Vorstellung maximaler Muskelkontraktionen. Zeitschrift für Sportpsychologie 12(1), pp. 11-21.

Reiser, M., Büsch, D., Munzert, J. (2011). Strength gains by motor imagery with different ratios of physical to mental practice. Frontiers in Psychology 2(194), doi: 10.3389/fpsyg.2011.00194.

Shackell, Erin M. & Standing, Lionel G. (2007). Mind Over Matter: Mental Training Increases Physical Strength. North American Journal of Psychology 9(1), pp. 189-200.

Sidaway, Ben & Trzaska, Amy R. (2005). Can Mental Practice Increase Ankle Dorsiflexor Torque? Physical Therapy 85, pp. 1053-1060.

Smith, D., Collins, D., & Holmes, P. (2003). Impact and mechanism of mental practice effects on strength. International Journal of Sport and Exercise Psychology 1, pp. 293-306.

Smith, E. E. & Kosslyn, S. M. (2006). Cognitive Psychology: Mind and Brain. 1st Edition. Publisher: Pearson.

Stinear, C. M., Byblow, W. D., Steyvers, M., Levin, O., Swinnen, S. P. (2006). Kinesthetic, but not visual, motor imagery modulates corticomotor excitability. Experimental Brain Research 168(1), pp. 157-164.

Zijdewind, I., Toering, S. T., Bessem, B., van der Laan, O., & Diercks, R. L. (2003). Effects of Imagery Motor Training on Torque Production of Ankle Plantar Flexor Muscles. Muscle Nerve 28, pp. 168-173.

Yao, W. X., Ranganathan, V. K., Allexandre, D., Siemionow, V., & Yue, G. H. (2013). Kinesthetic imagery training of forceful muscle contractions increases brain signal and muscle strength. Frontiers in Human Neuroscience 7(561), doi: 10.3389/fnhum.2013.00561.

Yue, G. & Cole, K. J. (1992). Strength increases from the motor program: comparison of training with maximal voluntary and imagined muscle contractions. Journal of Neurophysiology 67(5), pp. 1114-1123.