In Chapter Four of Nautilus Training Principles: Bulletin Number One titled, Indirect Effect, Nautilus inventor Arthur Jones wrote,
“Throw a stone into a pool of water, and it will make a splash—and a wave will run to the far end of the pool; the larger the stone, the larger the splash—and the larger the wave. A very similar effect results from any form of exercise—I have named this “indirect effect”. When one muscle grows in response to exercise, the entire muscular structure of the body grows to a lesser degree—even muscles that are not being exercised at all; and the larger the muscle that is growing—or the greater the degree of growth—the greater this indirect effect will be.”
Jones believed performing exercises like barbell squats involving several large muscle groups would “indirectly” stimulate growth elsewhere in the body, although to a lesser degree. He thought you could not produce significant increases in muscular strength and size in some body areas without growth in others because there was a “…limit to the degree of such disproportionate development that the body will permit…” While Jones admitted the cause was unknown at the time it was eventually assumed to be the result of increases in anabolic hormones stimulated by intense exercise. It turns out this is not the case, however, and “…acute post-exercise systemic hormonal rises are not related to or in any way indicative of RT-mediated gains in muscle mass or strength.”(1, 2, 3)
It is possible to increase a muscle’s strength indirectly by performing exercise for it’s contralateral match,(4, 5, 6) however the effect is muscle-specific rather than general and primarily due to neural adaptations and not hypertrophy.(7) This contralateral effect would not explain the general indirect effect Jones wrote about, although other neural factors might be involved.
The more likely answer is a much simpler one; that the effect being observed is not indirect at all, but the result of those other muscles also being worked at least moderately hard during the exercise.
Although the barbell squat primarily targets the muscles of the hips and thighs, as the weight used becomes heavier the muscles of the arms and back must also work hard enough to control the bar to benefit from the exercise. This effect can also be seen in exercises like the barbell row and standing press. The barbell row and standing press primarily target the muscles of the upper body, but as the weights used become heavier the muscles of the trunk, hips, and thighs must work harder to maintain proper body positioning and would also be affected by the exercise.
When a leg press machine, compound row machine, or shoulder press machine are used instead the same effect is not seen because the bracing these machines provide tends to minimize the involvement of those other muscle groups. No demanding muscular work, no stimulus for growth.
Just because these stabilizing muscles are involved in free weight exercises does not mean they are worked effectively by them, though. How intensely a muscle is worked determines how effectively it is stimulated to grow, and these muscles are not being worked as effectively as possible in stabilizing roles. This is why it is important for your workouts to include one or two exercises for all muscle groups. While a small number of compound exercises can effectively target all the big muscle groups, if you want overall well-balanced and proportional muscular development you need to work everything and not depend on the indirect effect for optimal development of muscle groups like your abs, calves, forearms, and neck.
- Morton, R. W., Oikawa, S. Y., Wavell, C. G., Mazara, N., Mcglory, C., Quadrilatero, J., . . . Phillips, S. M. (2016). Neither load nor systemic hormones determine resistance training-mediated hypertrophy or strength gains in resistance-trained young men. Journal of Applied Physiology J Appl Physiol. doi:10.1152/japplphysiol.00154.2016
- West DW, Burd NA, Tang JE, Moore DR, Staples AW, Holwerda AM, Baker SK, and Phillips SM. Elevations in ostensibly anabolic hormones with resistance exercise enhance neither training-induced muscle hypertrophy nor strength of the elbow flexors. J Appl Physiol (1985) 108: 60-67, 2010.
- West DW, Kujbida GW, Moore DR, Atherton P, Burd NA, Padzik JP, De Lisio M, Tang JE, Parise G, Rennie MJ, Baker SK, and Phillips SM. Resistance exercise-induced increases in putative anabolic hormones do not enhance muscle protein synthesis or intracellular signalling in young men. J Physiol 587: 5239-5247, 2009.
- Zhou S (2000) Chronic neural adaptations to unilateral exercise: mechanisms of cross education. Exerc Sport Sci Rev 28: 177–184 [PubMed]
- Munn J, Herbert RD, Gandevia SC (2004) Contralateral effects of unilateral resistance training: a meta-analysis. J Appl Physiol 96: 1861–1866 [PubMed]
- Carroll TJ, Herbert RD, Munn J, Lee M, Gandevia SC (2006) Contralateral effects of unilateral strength training: evidence and possible mechanisms. J Appl Physiol 101: 1514–1522 [PubMed]
- Lee M, Carroll TJ (2007) Cross education: possible mechanisms for the contralateral effects of unilateral resistance training. Sports Med 37: 1–14