Assumptions I am making
- I'm taking "sural sharing" as the co-contraction of the triceps surae (gastrocnemius and soleus) against the quadriceps.
- Pulling phase? Not sure which pulling phase. First pull, transition, second pull? The role of the knee flexors is different in all three.
There is a phenomenon where one muscle can indirectly act upon another joint through co-contraction between a monoarticular muscle (single joint) and an antagonist/synergist biarticular (multi joint) muscle. For example, the gluteus maximus (hip extensor) can extend the knee by co-contracting against the rectus femoris (knee extensor and hip flexor). Greg Nuckols has a write-up here on how that applies to the squat.
There are two prominent knee flexor groups involved in the weightlifting pull, the hamstrings (primary knee flexor) and the gastrocnemius (secondary, weak knee flexor). Gracilis/sartorius play a minor role and popliteus does almost nothing.
The hamstrings contract eccentrically throughout the first pull as the hip angle stays fairly constant (a few degrees of extension) and the knee extend. In the transition phase, the hamstrings extend the hip and flex the knee, with the quadriceps contracting eccentrically and getting stretched to prepare for the second pull.
Side note: this is why hamstring strains in weightlifting tend to occur at the end of the first pull and IMO should be very rare as they are almost completely avoidable. Do any hamstring exercise with a lot of hip flexion (seated leg curl, 45 deg back ext, good morning etc) once a week at a heavy controllable load (1-3 sets of 5), don't rush the first pull, problem solved. Also don't sprint to your bus stop, better to be late to training than to show up early and only be able to lift off blocks (half joke).
The gastrocnemius is a weak knee flexor and is only particularly active as a knee flexor when the leg is straight (Li et al., 2002) so it plays a relatively small role in resisting knee flexion until you get towards the end of the transition (where the hamstrings lose some leverage) and shortly before the start of the second pull.
The calves (gastrocnemius/soleus/tibialis posterior) have two roles in weightlifting IMO (underline, opinion)
#1 is balance, first and foremost. Lifters with exceptionally weak calves can tend to lean backwards because they cannot keep their balance in the right place. I never figured this was the case until I saw a powerlifter turned weightlifter with possibly the most tragic calves in existence (I can say this as he is my friend).
#2 is force/torque transfer from the hip and knee extensors into the ankle/foot joint during the second pull.
However, what you may notice in quite a number of athletes (stereotypically Asians, but also short limbed Europeans or half the Italian women's team) is a so-called "premature" raise of the heels before the second pull. Is this an error? I'm not sure, and I think as coaches we often don't like to admit that we don't know!
I don't believe we have good quality, relevant evidence (isometric mid thigh pull not relevant IMO) to suggest whether this limits vertical impulse into the bar. The biggest challenge to biomechanical reasoning is usually biomechanics research data - everything that you think makes sense is often wrong.
More than putting it in the absolutely right or wrong box, I think we should observe when it happens, ask why it happens and what the consequences are. The below is largely my observations, personal experiences and hypotheses, not to be taken as fact.
#1 First Pull
Some lifters push only from the ball of the foot with the heels completely off the ground.
This is considered an error by most coaches and it certainly makes it difficult for beginners to keep consistent balance. When the heels are off the ground in the first pull, the calves and quadriceps have to work extremely hard to produce vertical force but I think the amount of load through the adductor magnus/glutes/back is similar (if not more), so generally these lifters will have a slowed first pull. All other things being equal, this is a negative.
Why then does it happen? My best guess is that it allows the knees to temporarily remain more flexed and reduces the magnitude and rate of loading on the hamstrings.
#2 Transition
The raising of the heels in the transition allows the knee angle to get more flexion before the second pull, without losing vertical impulse in the middle of the pull. In my experience, this tends to occur with lifters who have strong calves, often (but not always) strong quads and almost always a relatively weak back.
The potential cost of this is that it may be more difficult to maintain balance and a vertical bar trajectory, but is this a question of the heel raise making balance difficult or the back weakness making balance/verticality difficult? Or both? I don't know, but I can certainly say that back weakness or fatigue makes "verticality" harder.
The other strategy to reduce the loss of vertical impulse in the transition, which is to minimise the magnitude of knee rebend during the second pull. Urik Vardanian is the eternal example of this. No contact snatches/cleans to straight legs/power/squat can be a good exercise for this, provided that it is no contact by not rebending the knees much instead of via arm curling. This will provide a relatively flat footed transition.
I don't personally believe either is inherently superior for lifting more weight. I think it depends on where someone is weak or strong, and what they can achieve consistently and successfully.
#3 Second pull/explosion
Some people argue that a flat footed explosion is ideal due to isometric pull from the power position. I think that is irrelevant, the bar is not allowed to stay still in the snatch or clean.
During the transition, both the quadriceps and triceps surae are stretched and store energy. Then in the second pull, the stretch-shortening reflex and stored energy can be used to rapidly release energy as a large force into the ground. This is very different to an isometric contraction with almost no elastic component. That elastic energy storage/release can produce a lot of force, Usain Bolt can produce peak ground reaction forces of 3932N or 400.9kg force in a sprint through one leg (Čoh et al., 2018).
This doesn't mean that you should tell people to do a calf raise, rather I think people should be told to push down through the ground and get as tall as possible.