You have a classic history of a large extruded herniated disc at L5-S1 which compressed the S1 root. This root supplies the calf muscles (gastroc-soleus group). These muscles “push the foot down” when walking and running. A weakness of these muscles will cause a type of limp and make it difficult to run, hike and climb stairs.
You underwent a microdiscectomy of L5-S1 and your pain improved but the motor weakness did not. You also have a “burning” pain residual in that leg. You notice increased weakness with prolonged walking and running.
This is unfortunately typical for a chronic radiculopathy (see website). The nerve was injured by the large disc herniation and even though the herniation was surgically removed, the nerve has yet to recover. The burning sensation is typical for a nerve injury. The reason the leg becomes weaker with activity is that only a small portion of the muscle cells are firing in that muscle group. Most of the muscle cells are not getting the signal from the brain to contract and the ones that are still connected are too few to give a normal contraction.
These working muscle cells fatigue easily as they are overloaded with work and cannot “keep up” with the load. This is why with continued exercise, the leg feels weaker.
Muscle cells that are not connected to the brain are called “deinnervated”. These cells have a number of ways to recover but this takes time. Some of the recovery methods are budding (sprouting), functional recovery , nerve regeneration and muscle hypertrophy. Budding is a phenomenon where the deinnervated muscle cells puts out a neurochemotactic factor. This is a chemical “cry for help” and any close functioning nerve will bud or sprout a branch to connect with this muscle cell. This can take 12-16 weeks.
The second recovery method is functional recovery. This is where the nerve itself that was damaged will heal which allows the signal to continue down the nerve. The functional block could be from damage to the insulation (myelin) or malfunction of the membrane of the nerve. Recovery should take place relatively quickly.
The third recovery method is by axonal regeneration. If the nerve was severed but the insulation sheath (myelin) was left intact, the nerve can grow down this pathway. The nerve grows at about one inch per month. The problem with the S1 nerve is that it is the longest nerve in the body with some examples at 22 inches long. It could take many months for the nerve to grow down to the muscle in the leg (this is assuming the insulation sheath is still intact). If it takes longer than 12-18 months to reconnect with the muscle cells, these cells will atrophy and fibrose. This means that even if the nerve grows and reconnects, the muscle cells will be useless and not be able to contract.
The last possibility for recovery is muscle hypertrophy. Arnold Schwarzenegger is what many individuals think of for muscle hypertrophy and that thought is not far off. The residual muscles can be conditioned to become stronger and last longer. Training is the key for this and this result may take three of more months of hard work to achieve success.