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The discussion as to whether free weights are more or less effective than resistance machines has raged on for years now, with the accepted wisdom being that whilst free weights are better for muscle growth, machines are safer and more accessible.
Probably the most famous recent study to look into this is the one by Shaner et al (2014) that compared barbell squats to the leg press.
The study found that barbell squats produced greater results in total work, strength, and produced a greater hormonal response . But what does the science say?
This article will take a look at the benefits and negatives of each to discover which is best.
Whilst this may seem to be the most important aspect of the debate, it is important to discuss this in the proper context. Because both free weight and resistance machine exercises are significantly safer than most exercises, particularly running and competitive sports .
In Bret Contreras' article on the subject he states that free weight exercises have higher rates of acute injury whilst machine exercises have higher rates of chronic injury . But as mentioned before, the injury rates for both are so low that it hardly bares mentioning.
Perform either with excellent form, and patience and you will remain injury free.
Not often talked about, but the nervous system has a crucial role in strength training.
In his article on machines versus free weights, McBride states that free weight exercises stimulate the nervous system more, which would lead to lifters being able to lift heavier weights .
In the previously mentioned study by Shaner et al (2014) they not only looked at the strength response of free weights compared to machine weights, they also looked at the hormonal response .
They found that free weights increased Human Growth Hormone (HGH) and Testosterone (T) significantly more than machine weights.
Another study by Budnar et al (2014) found that kettlebell swings also increased testosterone and HGH .
So why is the hormonal response to exercise important? Basically because testosterone and HGH can improve strength  increase muscle protein synthesis  and help increase muscle hypertrophy.
Another thing to look at is how many muscle fibres are recruited during an exercise, more muscle activity per exercise leads to more strength and hypertrophy increases.
Schick et al (2010) found that there was improved muscle activation during the bench press when compared to the Smith Machine .
Another study by McCaw & Friday (1994) produced similar results  whilst a study looking into the difference in muscle activation in free weights squats compared to Smith machine squats also found in favour of the free weight version .
Strength & Power
Finally we will be comparing the effects of free weights and machines on strength and power.
Jones et al (2008) found that maximum strength and average power were significantly higher during free weight power cleans when compared to machine power cleans .
In a study comparing fixed resistance machines (FR) and free-form (FF) resistance equipment it was found that FF improved strength 58% more than FR and improved balance 196%. FF participants also reported lower overall pain levels compared to FR .
So when looking at the evidence it becomes clear that there is only one winner when comparing the two, free weights have an improved effect on the nervous system, produce a greater hormonal response, produce increased muscle activity, and significantly improve strength and power compared to machines.
But why must your program have an either/or mentality? Yes there are more advantages to training with free weights but that doesn't mean that combining the two won't produce superior results.
If there is one takeaway from this article it should be that you mustn't prioritise machines over free weights. But look for ways to incorporate some machine exercises into your free-weight strength program.
 Shaner, A., Vingren, J., Hatfield, D., Budnar, R., Duplanty, A., Hill, D. 2014. The Acute Hormonal response to free weight and machine weight resistance exercise. Journal of Strength & Conditioning Research 28(4): 1032-40
 Requa, R., DeAvilla, L., Garrick, J. 1993. Injuries in recreational adult fitness activities. The American Journal of Sports Medicine 21(3): 461-7
 Jones, B., Cowan, D., Knapik J. 1994. Exercise, training and injuries. Sports Medicine 18(3): 202-14
 Contreras, B. 2013. Machines vs. Free Weights: More Research is Needed [Online]. Available at: https://bretcontreras.com/machines-vs-free-weights-more-research-is-needed/ [Accessed 04-May-2016]
 McBride, J. Machine Versus Free Weights [Online pdf.] NSCA-Lift.org Available at: http://www.elitesc.co.uk/files/machine_versus_free_weights.pdf [Accessed 04-May-2016]
 Budnar, R., Duplanty, A., Hill, D., McFarlin, B., Vingren, J. 2014. The Acute Hormonal Response to the Kettlebell Swing Exercise. Journal of Strength & Conditioning Research 28(10): 2793-800
 Tavares, A., Micmacher, E., Biesek, S., Assumpcao, R., Redorat, R., Veloso, U., Vaisman, M., Farinatti, Conceicao, F. 2013. Effects of Growth Hormone Administration on Muscle Strength in Men over 50 Years Old. International Journal of Endocrinology, vol. 2013, Article ID 942030, 6 pages, 2013. doi:10.1155/2013/942030
 Griggs, R., Kingston, W., Jozefowicz, R., Herr, B., Forbes, G., Halliday, D. 1989. Effect of Testosterone on muscle mass and muscle protein synthesis. Journal of Applied Physiology 66(1): 498-503
 Schick, E., Coburn, J., Brown, L., Judelson, D., Khamoui, A., Tran, T., Uribe, B. 2010. A comparison of muscle activation between a Smith machine and free weight bench press. Journal of Strength & Conditioning Research 24(3): 779-84
 McCaw, S., Friday, J. 1994. A Comparison of Muscle Activity Between a Free Weight and Machine Bench Press. Journal of Strength & Conditioning Research 8(4): 259-264
 Schwanbeck, S., Chilibeck, P., Binsted, G. 2009. A comparison of free weight squat to Smith machine squat using electromyography. Journal of Strength & Conditioning Research 23(9): 2588-91
 Jones, R., Fry, A., Weiss, L., Kinzey, S., Moore, C. 2008. Kinetic comparison of free weight and machine power cleans. Journal of Strength & Conditioning Research 22(6): 1785-9
 Spennewyn, K. 2008. Strength outcomes in fixed versus free-form resistance equipment. Journal of Strength & Conditioning Research 22(1): 75-81