This text discusses '人体运动到底有没有极限？‘
弹性： 物体受外力作用发生形变、除去作用力能恢复原来形状的性质 （zdic.net)
弹性： flexibility, elasticity
However, I don't think they want to say 'the mind is flexible' or 'psychological factors are flexible'.
I think 弹性 means here 'ability to stretch, make elastic, increase', but I'm not really sure.
Psychological factors have a great ability (性） to 'stretch, increase'（弹 (our sporting ability)),
a positive outlook can unleash our body's hidden power,
and so allow the athlete in competition to display greater than normal ability.