Bertolt Meyer wears a myoelectric prosthetic arm and hand controlled by electrodes attached to his residual limb that pick up impulses generated when he consciously contracts that muscle. Those impulses are then translated into control signals for the prosthetic hand. An electronic musician, Meyer had the idea to swap out the prosthetic hand for a DIY controller for his modular synthesizers so he can play music just by thinking about it. This is the SynLimb. Meyer writes:

Together with Chrisi from KOMA Elektronik and my husband Daniel, I am in the process of building a device (the "SynLimb") that attaches to my arm prosthesis instead of the prosthetic hand. The SynLimb converts the electrode signals that my prosthesis picks up from my residual limb into control voltages (CV) for controlling my modular synthesizer. The SynLimb thus allows me to plug my prosthesis directly into my snythesizer so that I can control its parameters with the signals from my body that normally control the hand. For me, this feels like controlling the synth with my thoughts.

Researchers from the University of Chicago and Sony are developing a wearable electrical muscle stimulation system that boosts your physical reaction time without making it feel like you've lost control of your body. The latter is particularly important when considering the development of exoskeletons and other systems that bring us physically closer to machines for augmenting human capabilities. The system essentially zaps your muscles into contracting at precisely the right time while making it seem as if you're still controlling the movement. From IEEE Spectrum:

The typical reaction time for a human is about 250 milliseconds—meaning it takes you about a quarter of a second after you see something to physically react to it. But the researchers explain that "our conscious awareness of intention takes a moment to arise, around 200 ms." In other words, it takes you about 200 milliseconds for your brain to turn sensory input into a decision to do something like move a muscle, and then another 50 or so milliseconds for that muscle to actually start moving. The researchers suggest that this 50-ish millisecond gap between intention and action is a window that they can exploit to make humans react more quickly while still feeling like the action they take is under their control.

The video below shows a series of experiments that demonstrate how reflexes can be usefully accelerated without decreasing the sense of control, or agency, that the user experiences. It turns out that an EMS-driven improvement in reflexes of up to 80 milliseconds is possible while still maintaining the user's sense of agency, which is the difference between success and failure in these particular experiments.