In the lower leg, landing after a jump induces reflexes, the role of which is not well understood. This is even more so for reflexes following landing on inverting surfaces. The latter condition is of special interest since ankle inversion traumata are one of the most common injuries during sport. Most studies have investigated ankle inversions during a static standing condition. However, ankle injuries occur during more dynamic activities such as jumping. Therefore, the present study aimed at reproducing these situations but in a completely safe setting. EMG responses were recorded after landing on an inverting surface, which caused a mild ankle inversion of 25 deg of rotation (in a range sufficient to elicit reflexes but safe enough to exclude sprains). The results are compared with data from landing on a non‐inverting surface to understand the effect of the inversion. In general, landing on the platform resulted in short and long latency responses (SLR and LLR) in triceps surae (soleus, gastrocnemius medialis and lateralis) and peroneal muscles (long and short peroneal) but not in the tibialis anterior muscle. Landing on the inverting platform caused significant LLRs in the peroneal muscles (which underwent the largest stretch) but not in the triceps muscles. Conversely, landing on a non‐inverting platform induced larger SLRs in triceps than in the peroneal muscles. Although the peroneal LLRs thus appeared to be selectively recruited in an inverting perturbation, their role during such perturbations should be limited since the latency of these responses was about 90 ms while the inversion lasts only 42 ms. The SLRs, if present, had an onset latency of around 44 ms. In the period following the inversion, however, the responses may be important in preventing further stretch of these muscles.

Ankle inversions have been studied extensively during standing conditions. However, inversion traumas occur during more dynamic conditions, like walking. Therefore in this study sudden ankle inversions were elicited in 12 healthy subjects who stepped on a trap door while walking on a treadmill. First, 10 control trials (0 degrees of rotation) were presented. Then, 20 stimulus (25 degrees of rotation) and control trials were presented randomly. EMG recordings were made of six lower leg muscles. All muscles showed a short-latency response (SLR) of about 40 ms and a late-latency response (LLR) of about 90 ms. The peroneal muscles showed the largest amplitudes in both responses. The functionally more important, larger, and more consistent LLR response was too late to resist the induced stretch during the inversion. The functional relevance of this response must lie after the inversion. During the first trial a widespread "startle-like" coactivation of the LLR was observed. The last trials showed only a recruitment of the peroneal muscles and, to a lesser extent, the gastrocnemius lateralis, which is seen as a switch from reactive control to more proactive adaptive strategies. These proactive strategies were investigated separately by comparing trials in which inversion was expected (but did not occur) with those in which subjects knew that no inversion would occur. Anticipation of a possible inversion was expressed in decreased tibialis anterior activity before initial contact, consistent with a more cautious and stable foot placement. Furthermore, immediately after landing, the peroneal muscles were activated to counteract the upcoming stretch.

No abstract

While many studies have shown that there is a phase-dependent modulation of proprioceptive and exteroceptive reflexes during gait, little is known about such modulation for auditory reflexes. To examine how startle reactions are incorporated in an ongoing gait pattern, unexpected auditory stimuli were presented to eight healthy subjects in six phases of the step cycle during walking on a treadmill at 4 km/h. For both legs, electromyographic activity (EMG) was recorded in the biceps femoris (BF), the rectus femoris (RF), the tibialis anterior (TA), and the soleus (SO). In addition, stance and swing phases of both legs, along with knee angles of both legs and the left ankle angle, were measured. All subjects showed various response peaks. Responses with latencies of approximately 60 ms (F1), approximately 85 ms (F2), and approximately 145 ms (F3) were found. The amplitude of the reflex responses was dependent on the timing of the startle stimulus in the step cycle. Although the startle response habituated rapidly, the phase-dependent modulation pattern generally remained the same. The phase-dependent amplitude modulations were not strictly correlated with the modulation of the background activity. The TA even showed a transition from facilitatory F2 responses during stance to suppressive responses during midswing. Responses were observed in both flexors and extensors, often in coactivation, especially during stance. Furthermore the gait characteristics showed a shortening of the subsequent step cycle and a small decrease in the range of motion of ankle and knees. These results suggest that the responses are adapted to achieve extra stability dependent on the phase of the step cycle. However, even in the first trials, the changes in kinematics were small allowing a smooth progression of gait.