You have me curious. I've done a good bit of both but I'm not terribly knowledgeable on biomechanics.
Hey man. It's not biomechanics, it's physiology.
Okay I'll try giving a coherent explanation:
Heart.
When you train your cardiovascular system, you also train your heart muscle. The heart, as with any muscle, can undergo hypertrophy, ie, get larger and stronger.
HIT: High intensity training raises blood pressure and heart rate a lot during activity (this is also true for heavy strength training but for slightly different reasons) and puts a significant demand on the heart to pump more
forcefully. When the heart rate is high, the heart has less time to fill with blood during the diastole (filling part), which happens in the right ventricle (right heart chamber). In turn it has to pump blood out of the left ventricle (left heart chamber) during the systole (pumping part) quicker and with more force. This (over)works primarily the left heart chamber in such a fashion that the
walls are thickened over time and it loses some of its elasticity. The heart becomes very effective at pumping blood out during high demand activities (which is great for performance and pushing through fatigue), but less efficient in filling and expanding with each stroke.
LISS: The blood pressure and heart rate is lower and the heart has more time during the diastole, ie, filling part. The left heart chamber also does not have to pump with a lot of force. This exposure to proper filling causes a different kind of hypertrophy of the heart than HIT. The ventricles might both become bigger, but less thick and the
elasticity is actually improved. The heart becomes more efficient at pumping more blood pr. heart beat and this puts less strain on the heart over time. However, it doesn't train the ability of having a high work rate while fatigued as much as HIT does.
Arteries.
As we already talked about a little earlier, the arteries and how they are affected by training, is important in regards to cardiovascular health.
HIT: High blood pressure once in a while is not dangerous for the arteries. We're built to be able to handle the increasing and changing demands of heart rate and pressure in accordance to a higher work rate, or distressed emotional state. Chronic high blood pressure though can be very damaging to the arteries. As I mentioned earlier, it destroys the epithel cells in the artery walls and lays the groundwork for lipids (fats) like LDL-Cholesterol to take hold and cause plaque. This raises blood pressure again, so it's like a snowball effect. HIT isn't inherently dangerous, not unless you have a serious heart condition, but if it's the only modality you use without a base, then it's not necessarily the best. On the flipside though, HIT very quickly transport glucose (blood sugar) away from the bloodstream and into the muscles, which is great if you're a diabetic and generally very healthy. High blood sugar is another thing that damages the epithel cells in the arteries.
LISS: The duration of LISS is a plus here. Increased bloodflow and oxygenation helps the arteries themselves as much as the muscles they transport oxygen to. The epithel cells really like being fed oxygen steadily over time like when doing LISS work. It's not as efficient at transporting glucose as HIT, but it's very good at using the fats from the bloodstream, like LDL cholesterol, as energy and in turn reducing the amount. As I mentioned earlier, LISS work also increases the amount of plasma (water) in the bloodstream which basicly thins the blood a little and reduces the risk of blood clots and further lowers blood pressure during the resting state.
Muscles.
The muscles also respond differently to HIT and LISS training. While it's arguable whether or not fast twitch fibers can convert to slow twitch fibers, and vice versa, they can change their characteristics to resemble each other depending on training modality. It's also important to make a distinction between ST (slow twitch) and FTa and FTx (fast twitch a or x) fibers. FTx fibers are the most explosive, but also least endurant.
HIT: Promotes FTa and FTx characteristics, which is good for performing explosively. In regards to the topic of cardiovascular health, they are less interesting.
LISS: Endurance training promotes ST characteristics. In this instance, even if you can't change FT fibers to ST fibers, you can change the level of oxydative enzymes in the fibers. Meaning, FT fibers become more endurant, and FTx perhaps changes to FTa. What's more important though, is that endurance training in muscles promotes increased capillary numbers and something called myoglobin (which can store oxygen) in muscles. All the tiny arteries (capillaries) which ultimately gives oxygen to muscles multiplies under these conditions. This is also true for the heart muscle. The heart is not oxygenated by the blood that runs through it, rather, it is oxygenated by arteries and small capillaries that run on the outside of the heart independently. Increasing the amount of capillaries here during LISS/endurance of the heart helps with the function of it as a muscle.
To summarize;
HIT is good for performance and explosiveness. It's also good for some metabolic ailments. At the same time, it's strenuous and can't replace an aerobic base.
LISS is good for cardiovascular health and recovery. This should be your baseline. At the same time, it will not train your power and high intensity capabilities.