Cycling represents an activity, in which the leg extensor muscles have been thought to contract only concentrically. In the present investigation, the forces of the triceps surae muscles were recorded directly in vivo from the achilles tendon (AT) of a healthy male subject, who pedaled on the standard bicycle ergometer at varying work loads (88, 176, and 265 W). EMGs from the vastus medialis (VM), gastrocnemius (GAST), soleus (SOL), and tibialis anterior (TA) muscles were recorded telemetrically together with the AT force. Muscle length changes during the complete pedaling cycles were estimated from the film analysis. The results indicated that the peak AT forces increased from 489 N at 1 kp (90 rpm) to 661 N at 3 kp (90 rpm). This peak force, which was recorded at 115 degrees of the pedaling cycle, did not change with increase in the pedaling cadence. While SOL muscle did not demonstrate any increase in length during the power phase of the cycle, the GAST experienced an increase in length between 0 and 90 degrees of the pedaling cycle, and the total increase was 2.5% of the segment length. VM and SOL muscles demonstrated the greatest activity during the early power phase and remained almost quiescent during the latter part of the cycle. TA activation began simultaneously with the cessation of VM and SOL activity. GAST obtained its peak EMG activity at 104 degrees at a point when it was still lengthening. The results can be interpreted to indicate an existence of a stretch-shortening cycle, especially for the GAST muscles, during the active phase of the pedaling cycle.(ABSTRACT TRUNCATED AT 250 WORDS)