Ok, found the book,(actually my anatomy, this is not as detailed, but I digress). Here goes the technical explanation, and this will take awhile.
First, the structure and organization of a muscle.
1 Muscles are made up of cells known as muscle fibers, which are long cylandrical shapes up to 1 foot long.(Picture a fibreoptic phone line).
2 Each cell is further made up of myofibrils. Myofybrils are composed of units called sarcomeres, which lie end to end. (Again, fibreoptic phone line)
3 Myofibrils are composed of sarcomeres, stacked end to end. Sarcomeres are the contractile units of a muscle.
4 Sarcomeres are composed of two types of myofilaments called (this is real technical here) the thick one and the thin one. The thin one is composed of actin molecules, the thick one of myosin molecules. These filaments overlap, and when the actin molecules are slid past the myosin, contraction results.
Ok, now for the actual contraction.
On each myosin filament, the are myosin heads(which look somewhat like hooks). On each actin filament, there are binding sites, each covered with troponin(negatively charged)
When an impulse is sent to contract a muscle, positive calcium ions are released, which attach to the troponin, and pull it away from the binding sites on the actin.
Once the sites are exposed, the following occurs.
1 The active myosin heads are attracted to the exposed binding sites, and cross bridging occurs.
2 As the head attaches, it bends, pulling the actin filament toward the center of the sarcomere. At this point, adenosine triphosphate(ATP) binds to the head, and returns it to its original position. The actin filament has now been slid slightly towards the center of the sarcomere.
3 The head is now ready for another “step” and it binds to the next site on the filament. Keep in mind there are many, many heads, and some are always in contact with the actin filament. If this were not the case, the filament would simply slide back to its original position.
Now picture all these heads, attaching, bending, releasing, grabbing again, attachin, etc. This is a muscle contraction. Usually, one “pull” generates a shortening of about 1% of the muscles length. Some muscles can perform 30-35% shortening, so many of these cycles are repeated.
1 Since dying cells cannot exclude calcium ions, they promote the cross bridging of dead muscle tissue. These contractions are more commonly known as rigor mortis.
2 When ATP is relatively depleted, the heads cannot detach and move on. The muscle is in a continual state of stationary contraction. Writer’s cramp is an example of this.
3 There are three types of contractions:
a)isotonic (regular contraction, the vertical lift in a bench press)
b)isometric (contraction, no movement. Push on a wall, your muscle contracts, but does not move)
c)eccentric (contraction with reverse movement. When you bring the bar back down on a bicep curl, you are not pulling it with your triceps, you are contracting your biceps in a way that you slowly let it down. A true eccentric workout is one in which you cannot physically lift the weight. Someone else helps you lift it, you let it fall as slowly as possible.
All this brings us to why eccentric contractions cause more soreness. You are correct in stating that muscles only contract one direction. The heads cannot contract , release, and move backwards. When you do an eccentric motion, the heads are attaching and trying to ratcht to the next site. However, the muscle is lengthening. You are actually tearing the heads away from the binding sites. This causes the extra soreness associated with doing an eccentric workout.
We are taught to use simple explanations so that we can pass them on to people we are doing rehab or workout programs for. Everyone knows how sore you get after the first few times you work out, and there can be several reasons for the soreness. If your job is to make sure this person continues to follow their program, you have to explain the reasons in everyday language.
Thus, the “hook, pull, and ratchet” analogy. I’ll try to recall more of the chemical reaction, it involves positive and negative ions, and the signal from the neural net that says “contract”
And yes, it is possible to get sore walking down a hill, as well as up. But who wants to climb a hill and wait two days, see how sore you are, recover, and walk down, and see how sore you are, and compare. If you are out of shape, you’re probably going to get sore.