In the present study, we examined the electromyogram (EMG) patterns of the
soleus and medial gastrocnemius (MG) muscles during rhythmical, two-legged
hopping to investigate the contributions of the monosynaptic short- and lon
g-latency stretch reflexes during such a natural movement in human. During
rhythmical hopping, soleus muscle is activated reflexly at near-monosynapti
c latency by stretch resulting from passive ankle flexion upon landing. Sol
eus muscle also contracts voluntarily in order to launch the body into the
next hop. This is part of the rhythmical bursts of activity producing the h
ops. Depending on the hopping interval, this phase of activation can follow
the short-latency phase or precede landing at very short hopping intervals
. In MG, there is an initial phase of activity that stiffens the muscle in
preparation for landing, and continues through the contact phase. The monos
ynaptic reflex response to landing is usually superimposed on this activity
. Depending on the hopping interval, both of these responses may be overlai
d with activity that is time-locked to the take-off into the next hop, and
serves to launch the body into the next hop. However, no evidence for a lon
g-latency stretch reflex was found. In addition, the preferred hopping freq
uency for all subjects was about 2 Hz. This frequency is associated with a
pattern of EMG activity the timing of which indicates that it balances the
requirement for a comfortable landing from a hop with the optimal muscle ac
tivation required for launching the following hop.