How to Use Isometric and Plyometric Training to Optimise Muscle and Tendon Adaptations

The purpose of this blog post is to provide a brief summary and overview of the presentation i will deliver during the Omniathlete Virtual Summit. What i want to focus on here is the importance of musculotendinous stiffness on the performance of various sports related action. The storage and usage of elastic energy by tendon during muscle contraction⁽¹⁾ is the mechanism that enables stretch shortening cycle (SSC) to enhance force generation and movement efficiency. While a compliant tendon would allow it to be stretched to a greater magnitude, a stiff tendon would result in a greater recoil (force) when stretched to the same length as one that is less stiff⁽²⁾. 

However, in order to stretch the tendon, the muscles will have to maintain greater stiffness than the tendon during contraction. When we perform rapid SSC, there would be minimal change in fascicle length but greater lengthening of the tendon during the eccentric phase⁽³⁾. 

Hence, during the rapid eccentric phase, while the joint continues to move the muscle remains relatively isometric which allows the muscle to generate large amount of force (greater stiffness) and to stretch the tendon and store elastic energy to be used during the concentric or propulsion phase. Possessing high tendon stiffness is also beneficial to athletes’ rate of force development (RFD) as based on the laws of wave propagation, the rate of force transmission through a material is influenced by its stiffness. Tendon stiffness may influence RFD by affecting the time lag between muscle activation and muscle force production. For example, it has been reported that the electromechanical delay was inversely correlated with tendon stiffness while RFD was positively correlated to stiffness⁽⁴⁾, and tendon stiffness accounts for 35% of variability of RFD⁽⁴⁾.

I recommend the use of plyometric (PLY) and isometric (ISO) training to optimize adaptations for musculotendinous stiffness. PLY training, as we know, includes activities that involve rapid SSC of the musculotendinous system. PLY has generally been categorized into short and long response, with short response PLY being those that involve ground contact of less than 250 ms and long response being those that have ground contact of more than 250 ms. As mentioned above, muscles stiffen up and remain relatively isometric during rapid SSC so that the tendon would be stretched to a longer length to store greater amounts of elastic energy and to be used during the concentric phase. Therefore, PLY essentially trains the muscles to maintain greater levels of stiffness. Several studies have reported the effectiveness of PLY on increasing musculotendinous stiffness^(5-7). While increasing active muscle stiffness is sufficient to improve the performance of ballistic movement, imbalance in changes in tendon stiffness and active muscle stiffness after a period of only ballistic training and absence of heavy resistance training may occur⁽⁸⁾. Such imbalance may increase demand on the tendon and potentially be at greater risk for tendon injuries.  Hence, it is important to increase both muscle and tendon stiffness concurrently so as to both improve performance and reduce injury risk. A guide for plyometric training based on the National Strength and Conditioning Association’s guideline is displayed in Table 1.

Table 1. Guide to plyometric training volume prescription

Plyometric Experience	Training Volume Recommendations (Ground contacts/session)
Beginner (<3 months)	80-100
Intermediate (3-12 months)	100-120
Advanced (>12 months)	120-140

In contrast to plyometrics, isometric training is characterized by the generation of force without external movement. The effectiveness of isometrics on increasing tendon stiffness has been reported in multiple studies^(2,5,6,9,10), with some reporting isometrics resulting in greater tendon stiffness than plyometrics and heavy resistance training^(5,6,9,11). However, it is important to note that the effect of isometrics on tendon stiffness is dependent on several factors, including:

1) Duration of contraction per repetition - sustaining each contraction for longer duration may have greater effect on tendon stiffness than a higher total contraction duration per session (e.g. 4 x 20s vs 3 x 50 x 1s)

2) Intensity of contraction - sustaining an intensity of at least 70% of maximal voluntary contraction (MVC) would have a better effect than lower intensity 

3) Joint position - training at a joint position where the muscles are lengthened would result in greater tendon stiffness. For example, positioning the knee at 90^o angle (180^o = full knee extension) during an isometric leg extension exercise would result in greater tendon stiffness overtime as compared to training at knee angle of 120^o. 

You may refer to Table 2 for an idea on how I generally prescribe ISO during different phases.

I have also included Table 3 as an example on how PLY and ISO can be fitted into a training session.

Table 2. Guide to implementing isometric training for various neuromuscular adaptations

	General Strength/Hypertrophy	Maximum Strength	Power Phase
Set	3-8	1-5	1-5
Reps	-	3-5	3-5
Time under tension (s)	>10	1-10	1-3
Intensity (%MVC)	70-80	100	100
Contraction Speed	Ramp Up	Ramp Up/Rapid	Rapid
Type	Hold/Push	Push	Push/Hold (Rapid)
Rest Period	90-120s between sets	1-10s between reps (W:R = 1:1) 90-120s between sets	1-10s between reps (W:R = 1:1) 90-120s between sets
Joint Position	Long muscle length	Long muscle length/multiple position	Long muscle length/multiple position

Table 3. Combining isometric and plyometric training during general strength phase

S/N	Exercise	Week 1	Week 2	Week 3	Week 4
1	ISO Squat (@90° knee angle) (HIMA/PIMA)	3 sets 20-25s per set 70% MVC or 50-60% 1RM	3 sets 20-25s per set 70% MVC or 50-60% 1RM	3-4 sets 15-30s per set 70-80% MVC or 60-70% 1RM	3-4 sets 15-30s per set 70-80% MVC or 60-70% 1RM
2	Drop Landing or Countermovement Jump	1-3 sets 5 reps	1-3 sets 5 reps	1-3 sets 5 reps	1-3 sets 5 reps
3	ISO Split Stance Romanian Deadift (@bottom position) (HIMA/PIMA)	3 sets 20-25s per set ea 70% MVC or 50-60% 1RM	3 sets 20-25s per set ea 70% MVC or 50-60% 1RM	3-4 sets 15-30s per set ea 70-80% MVC or 60-70% 1RM	3-4 sets 15-30s per set ea 70-80% MVC or 60-70% 1RM
4	Split Stance Romanian Deadlift Medicine Ball Throw	1-3 sets 5 reps ea	1-3 sets 5 reps ea	1-3 sets 5 reps ea	1-3 sets 5 reps ea
5	ISO Bench Press (@90° elbow angle) (HIMA/PIMA)	3 sets 20-25s per set 70% MVC or 50-60% 1RM	3 sets 20-25s per set 70% MVC or 50-60% 1RM	3-4 sets 15-30s per set 70-80% MVC or 60-70% 1RM	3-4 sets 15-30s per set 70-80% MVC or 60-70% 1RM
6	Plyometric Push Up or Medicine Ball Chest Pass	1-3 sets 5 reps	1-3 sets 5 reps	1-3 sets 5 reps	1-3 sets 5 reps

References

1. Turner, AN., Jeffreys, I. The stretch-shortening cycle: Proposed mechanisms and methods for enhancement. Strength Cond J. 2010; 32(4): 87-99

2. Albracht, K., Arampatzis, A. Exercise-induced changes in triceps surae tendon stiffness and muscle strength affect running economy in humans. Eur J Appl Physiol. 2013; 113(6) :1605-15.

3. Monte, A., Maganaris, C., Baltzopoulos, V., Zamparo, P. The influence of Achilles tendon mechanical behaviour on “apparent” efficiency during running at different speeds. Eur J Appl Physiol. 2020; 120(11): 2495-505.

4. Waugh, CM., Korff, T., Fath, F., Blazevich, AJ. Rapid force production in children and adults: mechanical and neural contributions. Med Sci Sports Exerc. 2013: 45(4): 762-761.

5. Burgess, KE., Connick, MJ., Graham-Smith, P., Pearson, SJ. Plyometric vs. isometric training influences on tendon properties and muscle output. J Strength Cond Res. 2007; 21(3): 986-989.

6. Kubo, K., Ishigaki, T., Ikebukuro, T. Effects of plyometric and isometric training on muscle and tendon stiffness in vivo. Physiol Reports. 2017; 5(15): e13374.

7. Spurrs, RW., Murphy, AJ., Watsford, ML. The effect of plyometric training on distance running performance. Eur J Appl Physiol. 2003; 89(1): 1-7.

8. Epro, G., König, M., Schade, F., Karamanidis, K. Adaptations in tricep surae muscle-tendon unit mechanical properties in elite jumpers. ISBS Proceedings Archive. 2020; 38(1): 896.

9. Kubo, K., Ikebukuro, T., Yaeshima, K., Yata, H., Tsunoda, N., Kanehisa, H. Effects of static and dynamic training on the stiffness and blood volume of tendon in vivo. J Appl Physiol. 2009; 106(2): 412–417

10. Kubo, K., Kanehisa, H., Fukunaga, T. Effects of different duration isometric contractions on tendon elasticity in human quadriceps muscles. J Physiol. 2001; 536(2): 649-655

11. Lum, D., Barbosa, TM. Brief review: effects of isometric strength training on strength and dynamic performance. Int J Sports Med. 2019; 40(06): 363-375

Danny Lum is the Head of Strength and Conditioning at the High Performance Sport Institute in Singapore. Danny is in charge of the strength and conditioning program for Singapore’s national athletes from multiple sports, including diving, grappling sports, kayaking, and track cycling. He is also actively conducting research to better understand the effects of isometric training on sports performance and the post-activation performance enhancement effects of various warm-up protocols.