The twin conundrum.
There’s a small issue with directly engaging the hamstrings with knee curls and adding calf raises in the same time frame of the workout session.
The twin muscles, being the calves, are composed of one single joint muscle (the soleus) and one multi-joint muscle (the gastrocnemius).
While knee curling, the hamstrings are assisted by the gastro in an “origin to insertion” movement (the heel bone moving towards the buttocks), and when moving on to calf raises – either standing or seated – both the soleus and gastro are engaged, now the gastro is engaged in an “insertion to origin” path of travel (the heel bone rises and plantarflexion occurs).
This dual contraction causes the gastro to fixate its peak contraction at either end of the tendon’s insertions, stretch to both sides and eventually causes a muscular spasm throughout the muscle.
As a general recommendation, either space the two exercises far apart from each other or do them at different days of the week, and if you’ve already got caught in it, avoid sitting for the next 20-30 minutes. Keeping the muscle stretched by standing and waiting for the lactic acid to diminish can help avert the risk of gastro cramps.
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