The present study aimed to investigate changes in muscle mechanical properties (velocity, force, and power) during the seated medicine ball throw (MBT) test under conditions of voluntary reduced effort across various loadings. Twelve males (21.0 ± 1.2 years) performed the MBT against four loadings (0.43 kg, 2 kg, 4 kg, 6 kg) and three effort levels (50%, 75%, and 100%). Results revealed a consistent decrease in absolute velocity was observed across all loading levels (p < 0.01), with the most pronounced alterations at the lowest load (0.43 kg; p < 0.001). In contrast, reductions in absolute force and power under reduced effort were more significant with higher loadings (2–6 kg; p < 0.01). The relative decreases in velocity and force were notably smaller than those in power (p < 0.001), with the relative decrease in power closely following effort instructions across all loadings. These findings highlighting muscle power as a key indicator of effort during loaded submaximal MBT. The study supports the use of loaded submaximal MBT in rehabilitation and exercise programs as an effective method to assess neuromuscular performance and manage exercise intensity for individuals unable to perform at maximum effort.