REF-1154 · Kubo et al., 2001 · The Journal of Physiology · Paper · PubMed · Free full text

Hold length matters for tendon, not for strength

12 weeks of isometric knee extension at 70% MVC with matched volume: 20-s holds and 1-s pulses built the same strength (+34% vs +32%) and quadriceps volume (+7.6% vs +7.4%), but only the 20-s holds made the tendon structures stiffer

Blimited evidenceMeasured on recreational people

Eight young men trained one leg with short isometric contractions and the other with long ones, at 70% of maximal voluntary contraction (MVC), four days a week for 12 weeks. The short protocol was 3 sets of 50 one-second contractions with 2 s of relaxation; the long protocol was 4 sets of 20-s holds with 1 min of rest. Volume per session, as integrated torque, was the same.

Strength rose by the same amount in both legs (+31.8% vs +33.9%) and so did quadriceps muscle volume (+7.4% vs +7.6%).

Tendon and aponeurosis elongation under load, measured by ultrasound during ramp contractions, did not change in the short-protocol leg; in the long-protocol leg it was shorter above 550 N, and stiffness rose only in that leg (significant group × time interaction). The authors conclude that tendon adaptation to resistance training may depend on contraction duration.

Protocol

Participants: 8 young men (age, height and mass from REF-1152's table: 22.6 ± 2.8 y, 171.5 cm, 69.2 kg).
Exercise: unilateral isometric knee extension, overcoming (pushing against a fixed dynamometer), at 70% of MVC.
Short-duration leg: 3 sets of 50 repetitions, 1 s contraction and 2 s relaxation.
Long-duration leg: 4 sets of one 20-s contraction followed by 1 min of relaxation.
Volume: the same integrated torque per session in both legs.
Frequency and length: 4 days a week for 12 weeks.
Joint angle: not stated in the abstract.
Outcomes: MVC; quadriceps muscle volume of each head; elongation of the tendon and aponeurosis (vastus lateralis) by ultrasound during ramp isometric contractions up to MVC, and stiffness from it.

Key Results

Outcome (12 weeks, n = 8)1-s pulses (3 × 50)20-s holds (4 × 1)Between protocols
MVC+31.8 ± 17.2% (significant)+33.9 ± 14.4% (significant)similar
Quadriceps muscle volume+7.4 ± 3.9%+7.6 ± 4.3%similar
Tendon-aponeurosis elongationno change at any forceshorter above 550 N—
Stiffnessno significant change (+17.5% in REF-517's table)increased (+57.3%, p < 0.05, in REF-517's table)group × time interaction significant
Tendon cross-section+1.4%, not significant (REF-517's table)+1.4%, not significant (REF-517's table)—
The percentages in brackets come from REF-517's data extraction, which combines this paper with a companion paper on the same participants; the abstract gives the direction and significance only.

Mechanisms

Measured: at equal torque-time volume and equal relative intensity, longer contractions stiffened the tendon structures and short ones did not, while muscle strength and size responded alike.
Authors' interpretation: the elasticity of tendon structures after resistance training may depend on the duration of muscle contraction.
Inference (consistent with [REF-1152]'s discussion): tendon is viscoelastic, so a sustained contraction lets the tendon creep to a larger strain than a 1-s pulse at the same force, which may cross the strain threshold seen in REF-1153.

Practical Application

Pulses or long holds for tendons? 8 men, one leg each way (12 weeks, 70% MVC, matched volume): same strength (+34% vs +32%) and muscle (+7.6% vs +7.4%), but only 20-s holds stiffened the tendon. Tiny, non-athletes, knee only.
At 70% of maximum, hold length is the variable that moved the tendon here. The calf programme tested in runners used a higher intensity (90%) with 3-s contractions (REF-1153, REF-1155, REF-1156), so a long hold is one route to tendon strain and a near-maximal effort is another.
Not from this paper; the base's practice: if an athlete uses isometrics for tendon stiffness at moderate effort (around 70%), choose holds of 15-30 s over pulses; for strength alone, either works.
Caveat from another study: 20-s calf holds at 80% MVC in highly trained runners changed neither tendon stiffness nor running economy (Fletcher 2010, PMID 20683611: stiffness 0.9 ± 25.8%, economy unchanged per its abstract and REF-801's table).

Limitations

Eight participants, no untrained control leg or group; both legs belonged to the same person, so cross-education cannot be excluded.
Read from the abstract; the joint angle, the stiffness values and the measurement reliability are not available. The percentages from REF-517's table mix this paper with a companion paper on the same men.
"Volume" was matched as integrated torque, but total contraction time differed (150 s of 1-s pulses vs 80 s of holds per session, our arithmetic from the protocol). Inference: the pulses must have averaged a lower force to match torque-time, so the two legs did not see identical mechanical exposure.
Young non-athlete men, knee extensors; transfer to the Achilles, to athletes and to performance is untested here.
Funding and conflicts of interest are not stated in the abstract.
How this entry was checked and changed
Why this level

within-subject comparison, n=8; strength, muscle volume and tendon outcomes, no performance outcome; from the abstract