Abstract
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 achilies 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° 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° 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° 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. The stretch on the active GAST early in the
power phase might then be responsible for the enhanced force production in the second
quadrant of the cycle (90°-180°) and would maintain a more constant net torque despite
the declining contribution by the SOL muscles.
Key words
cycling - muscle mechanics - stretch-shortening cycle - electromyography
