Lost your gym membership card? No problem. A finger is all it takes to enter 24 Hour Fitness.

The athletic-club giant recently introduced a cardless check-in system in Colorado that uses fingerprint scans to identify members.

The new procedure will offer cost savings, environmental benefits and enhanced security, 24 Hour Fitness says.

It also adds to a debate on the use of biometric-identification techniques.

Critics fear the systems that use fingerprints, irises, voices and facial recognition could be susceptible to hacking and represent a Big Brother-like intrusion on personal privacy...