I'd like to preface this with an acknowledgement that L. Dutch's answer is absolutely more practical. Half the reason I'm answering this is for the cringe factor.

##Why not bone?

Fibrodysplasia ossificans progressiva is a very rare genetic condition in which your connective tissue regrows as bone. Completely irreversible and untreatable ("surgery often results in explosive bone growth"), it thankfully shows up in fewer than one in a million individuals, with only about 800 cases reported to date. Genetic conditions are pretty tough to induce, so let's skip ahead down the pathway.

FOP is caused by a mutation in a kinase, which activates a particular bone morphogenic protein, which binds a receptor and recruits a bunch of other proteins to the site. Include a bunch of BMPs in the venom, and your victim will eventually die a slow, painful death.

Let's work on the "eventually"

FOP only works when the connective tissue is damaged. By inducing tetanus via either Clostridium tetani or their produced toxin, tetanospasmin, the victim will begin spasming and involuntarily contracting their muscles.

When you work out at a moderate intensity, you're sore because of micro-tears in those muscles. Your body's repair mechanisms are recruited, the cells are repaired, and your muscles become just a little bit stronger. But when spasming constantly, you'll be creating a lot of muscle tears.

Now that we're getting somewhere with the damage, let's help the repair process. By polymerizing fibrin (section 2.4), we can create a scaffold for better repair. There's plenty of fibrin present in our blood, as it's used for blood clots, so add somesome factor XIII to the venom for better fibrin cross-linking!

So...how does this revert after a couple of days? I've got nothing

The issue is that you're turning multiple tissue types (tendon, ligament, cartilage, skeletal muscle) into a single tissue type (bone). To revert the process, you'd need to trigger a signal cascade for bone resorption but somehow leave the skeleton unharmed.

I'd like to preface this with an acknowledgement that L. Dutch's answer is absolutely more practical. Half the reason I'm answering this is for the cringe factor.

Why not bone?

Fibrodysplasia ossificans progressiva is a very rare genetic condition in which your connective tissue regrows as bone. Completely irreversible and untreatable ("surgery often results in explosive bone growth"), it thankfully shows up in fewer than one in a million individuals, with only about 800 cases reported to date. Genetic conditions are pretty tough to induce, so let's skip ahead down the pathway.

FOP is caused by a mutation in a kinase, which activates a particular bone morphogenic protein, which binds a receptor and recruits a bunch of other proteins to the site. Include a bunch of BMPs in the venom, and your victim will eventually die a slow, painful death.

Let's work on the "eventually"

FOP only works when the connective tissue is damaged. By inducing tetanus via either Clostridium tetani or their produced toxin, tetanospasmin, the victim will begin spasming and involuntarily contracting their muscles.

When you work out at a moderate intensity, you're sore because of micro-tears in those muscles. Your body's repair mechanisms are recruited, the cells are repaired, and your muscles become just a little bit stronger. But when spasming constantly, you'll be creating a lot of muscle tears.

Now that we're getting somewhere with the damage, let's help the repair process. By polymerizing fibrin (section 2.4), we can create a scaffold for better repair. There's plenty of fibrin present in our blood, as it's used for blood clots, so add somesome factor XIII to the venom for better fibrin cross-linking!

So...how does this revert after a couple of days? I've got nothing

The issue is that you're turning multiple tissue types (tendon, ligament, cartilage, skeletal muscle) into a single tissue type (bone). To revert the process, you'd need to trigger a signal cascade for bone resorption but somehow leave the skeleton unharmed.

I'd like to preface this with an acknowledgement that L. Dutch's answer is absolutely more practical. Half the reason I'm answering this is for the cringe factor.

##Why not bone?

Fibrodysplasia ossificans progressiva is a very rare genetic condition in which your connective tissue regrows as bone. Completely irreversible and untreatable ("surgery often results in explosive bone growth"), it thankfully shows up in fewer than one in a million individuals, with only about 800 cases reported to date. Genetic conditions are pretty tough to induce, so let's skip ahead down the pathway.

FOP is caused by a mutation in a kinase, which activates a particular bone morphogenic protein, which binds a receptor and recruits a bunch of other proteins to the site. Include a bunch of BMPs in the venom, and your victim will eventually die a slow, painful death.

Let's work on the "eventually"

FOP only works when the connective tissue is damaged. By inducing tetanus via either Clostridium tetani or their produced toxin, tetanospasmin, the victim will begin spasming and involuntarily contracting their muscles.

When you work out at a moderate intensity, you're sore because of micro-tears in those muscles. Your body's repair mechanisms are recruited, the cells are repaired, and your muscles become just a little bit stronger. But when spasming constantly, you'll be creating a lot of muscle tears.

Now that we're getting somewhere with the damage, let's help the repair process. By polymerizing fibrin (section 2.4), we can create a scaffold for better repair. There's plenty of fibrin present in our blood, as it's used for blood clots, so add somesome factor XIII to the venom for better fibrin cross-linking!

So...how does this revert after a couple of days?

The issue is that you're turning multiple tissue types (tendon, ligament, cartilage, skeletal muscle) into a single tissue type (bone). My best guess is some sort of bone/muscle/tissue regrowth. Given this would affect most of the body, if not all, they're probably SOL.

I'd like to preface this with an acknowledgement that L. Dutch's answer is absolutely more practical. Half the reason I'm answering this is for the cringe factor.

##Why not bone?

Fibrodysplasia ossificans progressiva is a very rare genetic condition in which your connective tissue regrows as bone. Completely irreversible and untreatable ("surgery often results in explosive bone growth"), it thankfully shows up in fewer than one in a million individuals, with only about 800 cases reported to date. Genetic conditions are pretty tough to induce, so let's skip ahead down the pathway.

FOP is caused by a mutation in a kinase, which activates a particular bone morphogenic protein, which binds a receptor and recruits a bunch of other proteins to the site. Include a bunch of BMPs in the venom, and your victim will eventually die a slow, painful death.

Let's work on the "eventually"

FOP only works when the connective tissue is damaged. By inducing tetanus via either Clostridium tetani or their produced toxin, tetanospasmin, the victim will begin spasming and involuntarily contracting their muscles.

When you work out at a moderate intensity, you're sore because of micro-tears in those muscles. Your body's repair mechanisms are recruited, the cells are repaired, and your muscles become just a little bit stronger. But when spasming constantly, you'll be creating a lot of muscle tears.

Now that we're getting somewhere with the damage, let's help the repair process. By polymerizing fibrin (section 2.4), we can create a scaffold for better repair. There's plenty of fibrin present in our blood, as it's used for blood clots, so add somesome factor XIII to the venom for better fibrin cross-linking!

So...how does this revert after a couple of days? I've got nothing

The issue is that you're turning multiple tissue types (tendon, ligament, cartilage, skeletal muscle) into a single tissue type (bone). To revert the process, you'd need to trigger a signal cascade for bone resorption but somehow leave the skeleton unharmed.