Tendons tend to get most attention when they are injured.
This is unfortunate, as tendon stiffness is likely a key factor that determines rate of force development (RFD), which is a key component of explosive strength.
In other words, it is not just physical therapists who need to know about tendons. Strength coaches will also benefit from giving tendons some thought!
In rehabilitation programs, eccentric loading is popular for producing tendon adaptations. However, contraction type is not actually very important for producing increases in tendon stiffness.
On the other hand, the load does have an effect.
Heavier loads produce greater changes.
Load is not the only factor that is important for producing tendon adaptations.
Indeed, plyometrics can involve quite large loads (especially if high boxes are used for drop jumps), and plyometrics are often used to promote "stiffness" and "energy storage" in athletes.
But while plyometrics are a great training method, they probably don't achieve their beneficial effects by producing substantial increases in tendon stiffness.
Short duration, repeated loading types like plyometrics are not able to increase tendon stiffness by very much. On the other hand, heavy loads corresponding to around 3 seconds of time under tension are optimal.
Taking these studies together, it seems likely that conventional, heavy strength training is optimal for producing tendon adaptions, including increased tendon stiffness.
And as we saw recently, increased tendon stiffness will likely then enhance rate of force development.
On the other hand, lighter loads, ballistic strength training, and plyometrics are unlikely to achieve the same results, although they are probably essential for increasing maximum contraction velocities.