Yesterday, I looked at a research article that suggested the improvements to vertical jumping caused by plyometrics were not due to any increase in stored elastic energy.
So what adaptations are plyometric exercises actually causing? Let’s find out.
An epic leap – photo courtesy of Hermitianta
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Some background
If you didn’t catch yesterday’s article, I strongly recomend you start there or this article might not as much sense as it should. If you’re really pressed for time, here’s the condensed introduction to plyometrics:
What are plyometrics? Plyometric movements are fast movements that combine a muscle lengthening phase followed by an explosive muscle shortening phase.
What is important about the lengthening phase? If a muscle is lengthened while loaded just prior to its contraction phase, it has been observed that it somehow produces greater force.
Does that phenomenon have a name? Yes, it’s called the stretch shortening cycle and it’s defined as “an active stretch followed by shortening of the same muscle.”
And as far as today’s article is concerned, the issues that really concern us are as follows:
Do we know how this cycle produces extra force? Not really. Research has suggested some possibilities, including the storage of elastic energy in the tendons, the increased threshold of the Golgi tendon organs, and the increased ability of the muscle fibres to fire in tandem.
How might plyometric training increase jumping? Well, there are two basic categories of possibility. Firstly, they could increase something peculiar to the stretch shortening cycle (which would be very exciting). Secondly, they could increase something about the muscle, as resistance training does.
Remind me how resistance training increases strength? Resistance training increases power by increasing the force generated by the muscle. It does this by increasing the number of muscle fibres recruited (recruitment), the size of those fibres (hypertrophy) and the number of fibres (hyperplasia), if you believe in that sort of thing.
Other possible adaptations There is one other possible category of adaptations to training that could lead to increased strength and/or power. These are other neuromuscular adaptations that are not specific to the prime-mover muscle (agonist), and include: decreased activity of antagonist muscles, better co-contraction of synergistic muscles and the increased excitability of the neurons.
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What’s the study?
It’s called Muscle power and fibre characteristics following an eight week period of plyometric training, by Jeffrey Potteiger, Robert Lockwood, Mark Haub, Brett Dolezal, Khalid Almuzaini, Jan Schroeder, and Carole Zebas, Journal of Strength and Conditioning Research, 1999.
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How was the study set up?
The researchers worked with 19 physically active men, who had been performing resistance training and aerobic exercise for 3 months prior to the study. The subjects were allocated to one of two groups.
- The first group performed 8 weeks of plyometric training, and
- The second group performed 8 weeks of plyometric training and aerobic training at 70% of max HR
Before and after the training period, the subjects were tested as follows:
- VO2-max
- Vertical jump
- Body mass
- Body composition
- Muscle fibre analysis
The plyometric training was performed 3 times a week and included:
- Squat jumps
- Tuck jumps
- Broad jumps
- One and two legged bounding
- Depth jumps from 40cm
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What happened?
The key variables were tested and gave the following results:
- VO2-max – no significant differences in VO2-max improvements were made between the groups but both groups improved their score after the training programme. Group one improved by 12% and group two by 14%.
- Vertical jump – both groups increased their vertical jump height, by 4.6% and 5.0% respectively.
- Body mass – body mass in both groups was not substantially increased, although it did rise slightly.
- Body composition – body composition was unchanged in both groups.
- Muscle fibre analysis – no significant changes in the proportion of fibre types (between types 1, 2a and 2b) was observed in either group. However, the cross-sectional area of the muscle fibres increased and the increase was slightly larger in the type 2 fibres.
Essentially, the main result of this study is that the plyometric training programme resulted in an increase in muscle size, which was strongly correlated with the improvement in the vertical jump ability.
The addition of the aerobic exercise in the second group had so little impact that is possible for the researchers to disregard it completely in their analysis of the results.
In addition, plyometric training was shown to be significantly more effective than aerobic training at 75% for improving VO2-max.
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So how does this tie in to the study on plyometrics and elastic energy?
Yesterday, I looked at A Comparison of Plyometric Training Techniques for Improving Vertical Jump Ability and Energy Production. The result of that study was that increases in vertical jump ability generated by plyometric training were not produced as a result of increases in stored elastic energy.
This study takes things one step further and shows that the increases in vertical jump ability caused by plyometric training are very closely correlated with increases in muscle hypertrophy. That was it. No neuromuscular voodoo.
This is very interesting, as Verkoshansky originally started using plyometric exercises as a replacement for resistance exercises that hurt his athletes and not as an additional training modality designed to achieve a different aim. Perhaps the overlap between resistance training and plyometrics is closer than people like to make out.
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And what about the other studies?
Recently, I’ve read quite a few studies on strength, power and speed. As always, there are a lot of conflicting results but some patterns start to emerge.
- In Strength and power predictors of sports speed, it was found that resistance training for maximum strength (so long as bodyweight does not increase) and plyometrics were good ways to improve sports speed. However, it was found that training repetition strength was not so effective.
- In Maximum Strength-Power-Performance Relationships in Collegiate Throwers, it was found that increasing maximum strength can increase power and the peak rate of force production. However, it was also noted that maximum strength, power, and specific sports performance variables do not appear to change at exactly the same rate and that increases in strength may continue after the changes in power or sport performance become asymptotic.
- In A Comparison of Strength and Power Characteristics Between Power Lifters, Olympic Lifters, and Sprinters, it was clear that sprinters had the best vertical jumps but whether this was because of genetic selection for their sport or because of their training programmes is unclear.
Reading those studies again in the light of this one makes me think that (within certain parameters) there is more in common between training limit strength and plyometrics than there is between training limit strength and repetition strength.
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What does this study imply for athletic training?
Well, I think there is a lot to say here.
- Plyometrics cause hypertrophy – let’s be clear here that plyometric training definitely makes people stronger and faster because of muscular hypertrophy.
- Plyometrics might cause other power adaptations too - the jury is still out as to whether muscular hypertrophy is the only factor responsible for the increase in vertical jump performance in this study. If it is the only factor (which is very unlikely), then plyometrics become basically interchangeable with more conservative forms of resistance training, to be used depending on their suitability and risk/reward ratio. This sounds odd, but just because there is a stretch shortening cycle phenomenon observable, doesn’t mean that the components of that cycle can be trained.
- Plyometrics increase VO2-max – the improvements in VO2-max achieved in this 8-week study were considerable and show the benefits of power-focused training over traditional aerobic training for improving cardiovascular fitness.
- Limited aerobic training doesn’t impede power training – I think that the researchers miss the opportunity to note that the group that carried out the additional aerobic training did not achieve worse results in the power-related tests than the other group. Many people think that if they do any conditioning at all, their strength training will suffer. I think this study shows that these concerns are overstated. Of course, that doesn’t mean that running 20 miles a week is a good idea, but let’s be sensible here.
I hope you found this review interesting. As always, if you have a study that sheds any more light on the matter, please do send let me know.
