Tendon Adaptation: The Overlooked Half of Strength Training

You can add ten kilograms to your squat in a training block and still feel a strange lag between how strong you feel and how explosive you actually are. That lag is not always neural, and it is not always muscular. Often it is the tendon, the tissue that transmits every newton your muscle produces to the bone, quietly adapting on its own, slower timeline.

Lifters spend most of their attention on the muscle side of the muscle-tendon unit: cross-sectional area, activation, fibre type. But force only becomes movement once it crosses the tendon. A stiffer, better-conditioned tendon transmits force faster and with less energy lost to stretch, which matters directly for rate of force development, jump height, and how a heavy single actually feels off the floor. A compliant, undertrained tendon does the opposite: it soaks up the muscle's force as elastic stretch before any of it reaches the joint.

What tendon stiffness actually is, and why lifters should care

Tendon stiffness describes how much a tendon lengthens under a given load. A stiffer tendon stretches less for the same force, which shortens the time between muscle activation and force reaching the bone. This is the mechanical basis of "electromechanical delay," the gap between when a muscle switches on and when force shows up externally.

The foundational systematic review and meta-analysis on this topic, by Bohm and colleagues, pooled exercise intervention studies in healthy adults and confirmed that tendons remodel their material properties, not just their size, in response to chronic mechanical loading¹. That distinction matters. A tendon can become meaningfully stiffer without getting much bigger, because the change is happening at the level of collagen fibril packing and cross-linking, not gross cross-sectional area.

A larger and more recent meta-analysis by Lazarczuk and colleagues pooled 61 studies and 763 participants on lower-limb tendons, mostly Achilles and patellar, and found a moderate overall increase in stiffness (SMD 0.74) alongside a large increase in elastic modulus (SMD 0.82), while cross-sectional area increased only slightly (SMD 0.22)². Meta-regression identified modulus change, the material property, as the main driver of stiffness gains, not tendon size. In plain terms: your tendon is getting tougher tissue, not just more tissue.

How much resistance training actually changes tendon stiffness

The same Lazarczuk meta-analysis found a clear dose-response relationship: protocols that applied high mechanical strain to the tendon produced substantially larger gains in both modulus (SMD 0.90) and stiffness (SMD 1.04) than low-strain protocols². This is consistent with how tendon tissue is understood to respond: it needs to be loaded near its own working range, not just moved through range of motion, to trigger meaningful collagen remodelling.

A 2025 randomised trial in well-trained triathletes gives a sense of what that looks like over a single training block. Adding three heavy lower-body strength sessions per week to normal endurance training for 12 weeks increased Achilles tendon stiffness by 39.1% and patellar tendon stiffness by 15.8%, alongside an 8.9% gain in isometric knee extensor strength and a 20% gain in squat one-rep max³. The tendon changes were not small side effects, they were a substantial part of what heavy strength training was doing to these athletes' legs, even though the athletes were endurance-trained and already logging huge weekly volumes.

The adaptation gap: your muscles move faster than your tendons

The part that matters most for anyone tracking week-to-week progress is timing. Muscle strength and neural drive can shift within the first two to four weeks of a new training block, well before meaningful structural change in the muscle itself, largely through improved motor unit recruitment and coordination. Tendon remodelling runs on a longer clock, generally requiring a minimum of eight to twelve weeks of consistent, sufficiently strained loading before stiffness measurably changes²³.

A study in older women made this asynchrony explicit by tracking both muscle force and tendon stiffness across eight weeks of resistance training combined with whole-body vibration. Muscle force and tendon stiffness did not move together, they changed on different timelines within the same intervention, with the tendon lagging the muscle⁴. The practical read is that a lifter can be producing meaningfully more force than their tendon is yet built to transmit efficiently, which is one plausible mechanical explanation for why early strength gains sometimes feel less "connected" or explosive than the load on the bar would suggest, and why loading needs to be progressed rather than jumped.

Does collagen supplementation close the gap?

Because tendon is collagen-dense and comparatively slow to remodel, collagen peptide supplementation has been tested as a way to accelerate the process. The evidence is mixed and dose- and population-dependent.

A 2024 systematic review and meta-analysis pooling 19 randomised trials and 768 participants found that collagen peptide supplementation alongside training produced small but statistically significant improvements in tendon morphology (SMD 0.67) and maximal strength (SMD 0.19) compared with training alone, though the authors rated the certainty of the tendon-specific findings as very low⁵. A 2025 trial in middle-aged men found collagen plus 12 weeks of resistance training increased patellar tendon stiffness by roughly 2.7 times as much as training alone (+661 N/mm vs +247 N/mm)⁶. But a well-controlled 15-week trial in young, already well-trained men found no additional benefit of collagen peptides over placebo for any tendon outcome, with both groups showing similar stiffness gains of 17 to 21% from training alone⁷.

The most defensible reading is that collagen supplementation may help in populations whose baseline collagen synthesis is already limited, such as middle-aged and older adults, but adds little on top of a well-designed heavy resistance programme in younger trained lifters. Training strain, not supplement timing, remains the dominant lever.

Tendons age differently to muscle

Tendon adaptation is also not sex- or age-neutral. A study in older adults found the magnitude and character of training-induced tendon stiffness gains at old age differed meaningfully between men and women⁸. Comparative work in elderly women found that low-load resistance training could still improve tendon properties, though the response was smaller and slower than what younger tissue produces⁹. And a study comparing "old" with "very old" adults found that short-term resistance training did not meaningfully shift the lower baseline stiffness already present in the oldest group, suggesting a longer intervention, or earlier intervention, may be needed to meaningfully move aged tendon tissue¹⁰.

None of this means older lifters cannot build stiffer, more resilient tendons. It means the training stimulus needs to be sustained for longer, and expectations calibrated accordingly, particularly given that reduced tendon stiffness with age is one contributor to the general decline in rate of force development seen even in adults who keep their muscle mass.

What this means in practice

Rate of force development is not a pure readout of how hard a muscle contracts. It reflects the whole muscle-tendon unit, including how much of that contraction is lost to tendon stretch before force reaches the joint. A 16-week trial that tracked both properties directly found tendon stiffness gains correlated with normalised rate of torque development gains within the same lifters, evidence that this is not just a theoretical link¹¹. This is part of why early-block strength gains and explosive output do not always move together, and why RFD trends are worth reading over months rather than single sessions.

For anyone training with a Z1 sensor, the practical takeaway is patience with a specific number. If EMG activation on a lift is climbing but the associated rate of force development is flat, that is not necessarily a stall, it may be the tendon side of the equation still catching up to the muscle side, particularly in the first two to three months of a new strength block or after a long layoff. Hamstring, quadriceps, calf, and hip-extension work are exactly where this Achilles, patellar, and hamstring tendon literature applies most directly¹², so an unhurried, consistently loaded block is doing more than the bar weight alone would suggest.

Key takeaways

• Tendon stiffness rises mainly through material remodelling, not tendon size, and training near high tendon strain produces roughly double the adaptation of low-strain training.

• Meaningful tendon stiffness change generally takes eight to twelve weeks or more of consistent loading, materially slower than early neural strength gains.

• Muscle force and tendon stiffness can adapt on different timelines within the same training block, which can make early strength gains feel less explosive than expected.

• Collagen peptide supplementation shows modest, low-certainty benefit, most plausible in middle-aged and older lifters, and little added benefit over training alone in young trained lifters.

• Tendon adaptation slows and becomes more sex- and age-dependent later in life, which is one contributor to age-related declines in rate of force development.

References

1. Bohm, S., Mersmann, F., & Arampatzis, A. (2015). Human tendon adaptation in response to mechanical loading: a systematic review and meta-analysis of exercise intervention studies on healthy adults. Sports Medicine - Open, 1(1), 7.

2. Lazarczuk, S. L., Maniar, N., Opar, D. A., Duhig, S. J., Shield, A., Barrett, R. S., & Bourne, M. N. (2022). Mechanical, material and morphological adaptations of healthy lower limb tendons to mechanical loading: a systematic review and meta-analysis. Sports Medicine, 52(9), 2405-2429.

3. Jacobs, M. W., Feuerbacher, J. F., Mersmann, F., Bloch, W., Arampatzis, A., & Schumann, M. (2025). Maximal strength training improves muscle-tendon properties and increases tendon matrix remodulation in well-trained triathletes. Scientific Reports, 15(1), 27333.

4. Han, S. W., et al. (2017). Asynchronous alterations of muscle force and tendon stiffness following 8 weeks of resistance exercise with whole-body vibration in older women. Journal of Aging and Physical Activity, 25(4), 588-595.

5. Bischof, K., Moitzi, A. M., Stafilidis, S., & König, D. (2024). Impact of collagen peptide supplementation in combination with long-term physical training on strength, musculotendinous remodeling, functional recovery, and body composition in healthy adults: a systematic review with meta-analysis. Sports Medicine, 54(11), 2865-2888.

6. Nulty, C. D., Phelan, K., & Erskine, R. M. (2025). Hydrolysed collagen supplementation enhances patellar tendon adaptations to 12 weeks' resistance training in middle-aged men. European Journal of Sport Science, 25(4), e12281.

7. Balshaw, T. G., et al. (2023). The effect of specific bioactive collagen peptides on tendon remodeling during 15 wk of lower body resistance training. Medicine & Science in Sports & Exercise, 55(11), 2083-2095.

8. Onambele-Pearson, G. L., & Pearson, S. J. (2012). The magnitude and character of resistance-training-induced increase in tendon stiffness at old age is gender specific. Age, 34(3), 597-608.

9. Kubo, K., et al. (2003). Effect of low-load resistance training on the tendon properties in middle-aged and elderly women. Acta Physiologica Scandinavica, 178(1), 25-32.

10. Eriksen, C. S., et al. (2018). Lower tendon stiffness in very old compared with old individuals is unaffected by short-term resistance training of skeletal muscle. Journal of Applied Physiology, 124(6), 1471-1479.

11. Miyamoto, N., Ishihara, K., Oshima, T., Kawai, M., Oritani, Y., & Iemoto, N. (2025). Collagen peptide supplementation enhances muscle-tendon stiffness and explosive strength: a 16-wk randomized controlled trial. Medicine & Science in Sports & Exercise, 57(9), 1801-1810.

12. Lazarczuk, S. L., et al. (2024). Hamstring muscle-tendon geometric adaptations to resistance training using the hip extension and nordic hamstring exercises. Scandinavian Journal of Medicine & Science in Sports, 34(9), e14728.