In this experiment, what is the threshold potential for this motor neuron?

To contract a single muscle fibre, the same all-or-none response occurs. Then, how is it possible to lift a heavier object? When lifting a heavier weight, more muscle fibres are stimulated. To lift a smaller weight, fewer muscles fibres are stimulated.

In an experiment, the strength of a neural stimulus and the resulting muscle contraction are compared. A single motor neuron that synapses with one muscle fibre is observed in this experiment. One end of the muscle fibre is attached to a mass. If an electrical stimulus is sufficient to cause an impulse in the neuron, the muscle will contract and lift the mass. The following data were obtained from the experiment.

Analyze the data, and answer the following questions.
 Number of Trials
Strength of stimulus (mV)   Mass lifted by muscle contraction (g)
1   10   0
2    20
0
3    30   20
4    40   Not Tested


In this experiment, what is the threshold potential for this motor neuron?
What mass can be lifted at 40 mV stimulus?