Why doesn't ham-curl on a Swiss ball bother after a Slight Hamstring Strain?

I’m 63. Running 30 sec hill repeats 3 days ago up a 6% grade. I’ve had great success w hill repeats - builds strength & speed, not likely to cause an injury.

On repeat #4 I feel a very slight right ham strain but I do #5 anyway (classic denial symptom) and can feel it was something but likely a niggle (another classic denial symptom). I quit the repeats and switch to 3’ BIKE hill repeats - I just do 3 of them even though they are completely (really!!) symptom and pain-free. Feels fine that afternoon, I ice in the evening.

Next day is a Rest day. I try a short jog (20 yards) that next morning - definitely a strain, but seems very slight.

Next day is an added Rest day. It seems to hurt when I am motionless and is 100% ok when I walk!

Today, I try hamstring curls and then bridges - both w a Swiss ball - ZERO pain or symptoms.

Am I healed? In my past (more severe) ham strains those ham-curls on the Swiss ball have been terrific in aiding my recovery but not this quickly. Should I return to workouts?

How can an exercise that isolates an injured muscle group not cause even discomfort in that group?

Can’t answer your question specifically as I’ve experienced the same thing. In my experience any type of concentric curl is worthless as a hamstring prevention tool for running. After many years the best thing I have found are Nordic Curls. They are an eccentric exercise which closely replicates the stress of running. They are very popular amongst Euro soccer players as a means to bulletproof the hamstrings. They are a difficult exercise at first, you will need a stretch band to assist you and don’t do too many initially.