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PLYOMETRIC VS. ISOMETRIC TRAINING INFLUENCES ON TENDON PROPERTIES AND MUSCLE OUTPUT

Burgess, K.E., Connick, M.J., Graham-Smith, P. and Pearson, S.J., 2007. Plyometric vs. isometric training influences on tendon properties and muscle output. The Journal of Strength & Conditioning Research, 21(3), pp.986-989.

In today’s letter

• Overview of the difference between plyometric & isometric training in tendon performance

• Rapid Results = Both plyometric & explosive isometric training can improve tendon & muscle performance

• 3 resources to check out to further you knowledge about implementing isometrics into tendinopathy rehab/training

• Meme of the week: Origins & insertions 😄 

Aim of the study

The aim of the study was to compare the effects of isometric and plyometric training on tendon stiffness, rate of force development, and jump height to determine how these training methods influence muscle and tendon performance.

Did you know?

• Previous research shows that the rate of force development is closely linked to performance in sports that require high explosiveness

• During explosive movements in sports, there's limited time to produce force, making the speed at which force is generated very important.

• If a tendon is less stiff, it takes longer to develop force.

  Why? Because a less stiff tendon stretches more under the same force, delaying the transfer of force from the muscle to the bone.

• Studies have found that rate of force development can improve after just 6 weeks of training.

• This improvement is believed to be due to increased neural drive, which helps muscles contract more quickly.

• Tendon stiffness might also change during this period of training.

• When tendons become stiffer, rate of force development tends to improve because the force is transmitted from muscle to bone more efficiently.

• Various training methods like plyometrics, resistance training, and isometric training have been used to enhance rate of force development.

• However, no studies have directly compared how effective plyometric and isometric training are in improving both rate of force development and tendon stiffness.

 Methods

The experimental approach

• Participants were randomly divided into 2 groups and trained for 6 weeks, with 1 group doing isometric exercises and the other doing plyometric exercises.

• Tests were conducted both before and after the 6-week training period to measure the effects.

• The key factors measured in this study were tendon stiffness in the medial gastrocnemius muscle, jump height, and the rate of force development

The subjects

• 13 men, with an average age of 23 years, height of about 180 cm, and body weight around 77 kg, took part in the study and were placed into either the plyometric or isometric training group.

• Participants did not engage in any additional training outside of the study.

• Before being selected, all participants were checked for past lower limb injuries and their usual physical activity levels.

The procedures

Training

• The isometric training involved performing explosive, one-legged isometric plantar flexions (calf raises), while the plyometric training involved doing one-legged straight-legged drop jumps.

• The training volume gradually increased over the 6 weeks, starting with 2 sessions a week (3 sets of 15 reps) and ending with 3 sessions a week (4 sets of 20 reps).

• Before each training session, participants properly warmed up by doing several sub-maximal repetitions of their assigned exercise

Testing

Participants were tested before and after the 6-week training period using three one-legged tests:

(a) a maximum straight-legged concentric jump

(b) an explosive maximum isometric plantar flexion (calf raise)

(c) a graded isometric plantar flexion to measure tendon stiffness.

• Before testing, participants warmed up by doing several practice runs at lower intensity.

• For the first 2 tests (jump and explosive calf raise), participants were told to generate force as quickly as possible.

Tendon elongation & stiffness

• Tendon elongation measurements were taken during the graded isometric plantar flexion test using ultrasonography

Single leg jump

• The participant stood on the portable force plate on 1 straight leg with his hands on his hips and jumped straight upward (no countermovement) by plantar flexing

Results

Tendon stiffness

• Image below compares the force-elongation curves for the plyometric and isometric groups before and after the training period.

• Tendon stiffness in the gastrocnemius muscle increased significantly for both groups:

29.4% for the plyometric group and 61.6% for the isometric group.

• However, there were no significant differences between the two groups in terms of the relative changes in tendon stiffness from before to after training.

Relationship Between Tendon Stiffness and Concentric Jump Height

• The isometric training group showed a near-significant increase in single leg calf jump of 64.3%.

• The plyometric group had a significant increase of 58.6% from before to after training.

• Improved performance in concentric jumps was significantly linked to increased tendon stiffness.

Rate of Force Development in Concentric and Isometric Movements

• Both the plyometric and isometric training groups showed improvements in concentric rate of force development from before to after the training period.

• However, these increases were not statistically significant for either group.

• The plyometric group saw an 18.9% increase in concentric rate of force development, while the isometric group had a 16.7% increase.

• There were no significant differences between the 2 groups in terms of their relative changes in concentric rate of force development.

• Isometric rate of force development at 150 milliseconds also increased after training, with a 14.6% increase in the plyometric group and a 28.1% increase in the isometric group, though these trends were also not significant

Takeaways

Tendon stiffness

• Both isometric and plyometric training significantly increased the stiffness of the gastrocnemius tendon.

• There was no significant difference in the relative changes in tendon stiffness between the 2 training groups.

• The 61.6% increase in gastrocnemius tendon stiffness after isometric training in this study is similar to a 58% increase found in the vastus lateralis tendon in a previous study by Kubo et al.

• The 29.3% increase in tendon stiffness for the plyometric group aligns with Kubo et al.'s findings of an 18.8% increase after dynamic resistance training.

• Kubo & colleugues suggested that tendon stiffness increases after training due to changes in tendon structure to cope with the stress of repeated loading

Rate of force development

• Plyometric training led to non-significant increases in rate of force development by 18.9% concentrically and 14.6% isometrically.

• These increases in rate of force development with plyometric training are similar to those reported by Spurrs et al. after 6 weeks of plyometric training.

• Isometric training resulted in a 28.1% increase in isometric rate of force development and a 16.7% increase in the concentric.

• The increase in isometric rate of force development from isometric training is consistent with findings from Duchateau and Hainaut, Behm and Sale, and Kubo et al.

• Some research suggests that dynamic rate of force tests are more valid for assessing rate of force development because they are more functional and better correlate with performance.

• Neural factors, such as the frequency and amount of neural firing, can influence the results

• In theory, the speed of force should improve as tendon stiffness increases, due to faster force transmission.

• This study found an association between tendon stiffness and rate of force development, although it was not statistically significant.

• Aagaard & colleagues found that during the early phase of force generation (about 40 milliseconds after onset), rate of force development is influenced more by muscle twitch properties than by maximum force capacity

Single leg jump

• Concentric jump height increased for both training groups, with a significant increase in the plyometric group (58.6%) and a near-significant increase in the isometric group (64.3%).

• Plyometric training has been consistently shown to significantly improve vertical jump performance.

Conclusion

• Both explosive isometric and plyometric training significantly increase tendon stiffness to a similar extent.

• Both training methods also show trends toward improved concentric rate of force development, which is linked to better athletic performance.

• Tendon stiffness is linked to dynamic performance, with stiffness accounting for 21% of the concentric jump height.

Top 3 resources to check out

And learn more about isometrics

1. Programming isometrics - LINK

2. Combining plyometrics & isometrics for increased peformance - LINK

3. Isometric exercises for tendinopathy- the science (VIDEO) - LINK

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