Which of the proprioceptors is likely to react at this point, why, and with what result?

A client is having muscular strength tested on the bench press.

For the first lift, 50 kg is put on the bar. The client will be using the pectoralis major muscle which has 500 motor units (300 slow twitch and 200 fast twitch) and the triceps muscles. The CNS stimulates 280 motor units leading to the pectoralis major muscle (180 slow twitch and 100 fast twitch). Remember, each nerve ending stimulates a separate muscle fibre.

The pectoralis major and triceps muscles shorten and the clients arm extends outward raising the bar with 50 kg on it. After a few minutes of rest, 100 kg is placed on the bar. This time the action stimulates 380 motor units (260 slow twitch and120 fast twitch). The weight is successfully lifted. After a few minutes of rest, 150 kg is placed on the bar. This time the action stimulates 460 motor units (280 slow twitch and 180 fast twitch). Which of the proprioceptors is likely to react at this point, why, and with what result?