Henry et al J Sport Rehab 2010

288
Journal of Sport Rehabilitation, 2010, 19, 288-300
© 2010 Human Kinetics, Inc.
Henry, Docherty, and Schrader are with the Athletic Training Research Laboratory, Indiana University,
Bloomington, IN. McLoda is with the School of Kinesiology and Recreation, Illinois State University,
Normal, IL.
The Effect of Plyometric Training
on Peroneal Latency
Benjamin Henry,Todd McLoda, Carrie L. Docherty,
and John Schrader
Context: Peroneal reaction to sudden inversion has been determined to be too slow
to overcome the joint motion.A focused plyometric training program may decrease
the muscle’s reaction time. Objective: To determine the effect of a 6-wk plyometric
training program on peroneus longus reaction time. Design: Repeated measures.
Setting: University research laboratory. Participants: 48 healthy volunteers (age
20.0 ± 1.2 y, height 176.1 ± 16.9 cm, weight 74.5 ± 27.9 kg) from a large Mid-
western university. Subjects were randomly assigned to either a training group or a
control group. Interventions: Independent variables were group at 2 levels (training
and no training) and time at 2 levels (pretest and posttest). The dependent variable
was peroneal latency measured with surface electromyography. A custom-made
trapdoor device capable of inverting the ankle to 30° was also used. Latency data
were obtained from the time the trapdoor dropped until the peroneus longus muscle
activated. Peroneal latency was measured before and after the 6-wk training period.
The no-training group was instructed to maintain current activities. The training
group performed a 6-wk plyometric protocol 3 times weekly. Data were examined
with a repeated-measures ANOVA with 1 within-subject factor (time at 2 levels)
and 1 between-subjects factor (group at 2 levels). A priori alpha level was set at
P ≤ .05. Main Outcome Measures: Pretest and posttest latency measurements
(ms) were recorded for the peroneus longus muscle. Results: The study found
no significant group-by-time interaction (F1,46
= 0.03, P = .87). In addition, there
was no difference between the pretest and posttest values (pretest = 61.76 ± 14.81
ms, posttest = 59.24 ± 12.28 ms; P = .18) and no difference between the training
and no-training groups (training group = 59.10 ± 12.18 ms, no-training group =
61.79 ± 15.18 ms; P = .43). Conclusions: Although latency measurements were
consistent with previous studies, the plyometric training program did not cause
significant change in the peroneus longus reaction time.
Keywords: muscle activation, peroneus longus, trapdoor, reaction time
Fifty percent of the general population will experience at least 1 ankle sprain in
their lifetime.1
In addition, about 20% of patients with an acute sprain will remain

Plyometric Training and Peroneal Latency 289
symptomatic because of inappropriate or delayed rehabilitation.2,3
In athletes, the
most common injury that occurs is a lateral ankle sprain, representing 15% to 20%
of all sport injuries.1
Lateral ankle sprains are most often caused by forced inver-
sion and plantar flexion.1,2
The lateral ankle musculature has a reactive component
that has the potential to help prevent inversion ankle injuries.4–7
Because inversion
ankle sprains are so prevalent, understanding the mechanics of this injury and how
the body involuntarily adapts to correct potentially damaging moments can help
professionals develop better injury-prevention strategies.
Reflexes are involuntary responses that link afferent (sensory) and efferent
(motor) signals.8
A stretch reflex occurs when a muscle is rapidly stretched.9
The
stretch reflex is made up of 3 significant activations: M1, M2, and M3.6,10
M1,
also known as the short latency or short loop reflex, occurs approximately 60 mil-
liseconds after sudden stretch. This is the first response to sudden ankle inversion,
but the reflex creates very minimal increase in muscle torque. M2, also known
as medium latency reflex, occurs after 70 to 90 milliseconds and travels along
slower conducting afferents.10,11
The afferent response is sent to the motor cortex
of the brain.10
M3, or the long latency loop, may not always occur,6
but it serves to
continue motoneuron stimulation sequentially followed by activation of voluntary
torque at approximately 150 to 170 milliseconds after the onset of unexpected
ankle stretch.10,12
For activation of each motor unit in the muscle, a certain excit-
atory input of activity is required. During a stretch reflex, stimulations by means of
M1, M2, and M3 are combined on the previous, therefore creating an increase in
motoneuron excitability.12
M3 leads to voluntary torque production, in which the
reflex is initiated by activation of Ia-afferent sensory fiber recruited in M1.6,7,13–15
It is hypothesized that the shorter the muscle reaction latency, the better the ability
to correct posture and prevent injury to the lateral ankle ligaments.16
Numerous studies have been conducted on the reaction time of the peroneal
musculature.4,5,16–26
Previous investigations have shown that the peroneal muscles
can decelerate a potentially damaging inversion moment,5,25,27
minimizing injury
to the lateral ankle structures. Eechaute et al5
recorded 2 deceleration points that
can decrease the momentum of the inversion perturbation: one between 31 and 35
milliseconds, and the other between 104 and 106 milliseconds. These deceleration
points work as a resistance to the inversion stress to correct the detected motion.
That study also showed activation of the peroneal muscle 62 to 65 milliseconds
after the sudden inversion moment. This M1 muscle reaction is the first protective
mechanism the ankle musculature creates after a sudden inversion perturbation, in
which the muscle contracts, but not to the point of joint movement. Prior studies
have shown that previous injury to the peroneal muscles can lead to an increased
incidence of lateral ankle sprains by delaying muscle activation.17,28
Conversely, reports by Isakov et al17
and Thonnard et al28
state that the reaction
time of the peroneal muscles may not be fast enough to protect the ankle from a
sudden unexpected inversion injury. This is supported by a study by Konradsen
et al,16
which showed that the initial electromyographic activity of the peroneus
longus occurred 54 milliseconds after sudden ankle inversion, but voluntary active
eversion did not occur until 176 milliseconds after sudden inversion.16
Inversion
from a standing position would put the lateral ankle structures in danger after
approximately 100 milliseconds, therefore making reflexive and voluntary motions
too slow to prevent injury to the lateral ligaments of the ankle.16
In addition, during

290 Henry et al
a dynamic task, injury may occur even faster as a result of increased forces at the
joint.16
However, the muscles are preactivated during dynamic activity so the reac-
tion to unexpected inversion may also be faster.29
Improving the reaction time of the peroneal muscles may enhance the pro-
tective reflexive mechanism, thus reducing the risk of sudden inversion ankle
injury.16
Plyometric training has been used for lower extremity muscle control and
stability.30–37
This type of training emphasizes the stretch–recoil principle but also
affects joint-position sense,31,33
balance,31
and coordination throughout functional
movement.35
Plyometric training has been shown to cause an adaptation of the
stretch reflex and improve reaction times of the lower extremity.30,34
Studies have
concluded that after training dynamic movement patterns, muscles react faster to
sudden unexpected perturbations.30,34
In addition, plyometric training has also shown
increases in the output of power and explosiveness by training functioning muscles
to maximize muscle contraction in a shorter amount of time.32
This occurs when the
stretch–recoil cycle is working optimally, decreasing the amortization phase and
rapidly transitioning from the eccentric muscle contraction to a concentric muscle
contraction.32
Plyometric training can also produce increased torque.37
Cornu et al37
concluded that a 7-week plyometric training program incorporating squat jumps,
drop jumps, and jumping courses led to significant gains in maximal isometric
torque of the ankle plantar flexors. They hypothesized that the faster and more
efficient the muscles are in the amortization phase, the better the reflex response
will be.37
In addition, by maximizing the force produced during the concentric
phase of a muscle contraction, plyometric exercises can increase joint stability.32
Plyometric training has been shown to increase lower limb stability by improving
muscle activation.32,35
Plyometric training protocols have resulted in an increase in
joint stability in the lower extremity by increasing preparatory muscle activity,35
as
well as reducing lower extremity valgus motion during landing.31
Improving reflexive muscle activation during sport-specific activities could
decrease the reaction time during sudden damaging movements. We hypothesized
that the earlier M1 reflex occurs, the sooner voluntary muscle activation can occur,
and that improving the reaction time of the peroneal muscles may reduce the risk of
ankle injuries.4,6–9
Therefore, the purpose of this study was to determine whether a
6-week functional plyometric training program can be used to reduce the reaction
time of the peroneal muscles in healthy subjects.
Methods
Research Design
The study was a pretest–posttest experimental design. The independent variables
were plyometric training group at 2 levels (training and no training) and time at 2
levels (pretest and posttest). The dependent variable was peroneus longus latency.
Subjects
Forty-eight college students from a large Midwestern university volunteered to par-
ticipate in this study (Table 1).To be eligible for this study, subjects had to participate
in physical exercise for at least 3 days a week for 30 minutes at a time. Subjects

were excluded if they had a history of an ankle sprain in the past 12 months or any
history of ankle surgery or fracture. Subjects were randomly assigned to either the
training group or the no-training group. Both groups had an equal number of male
and female subjects. Before participating in this study, all subjects read and signed
an informed-consent form approved by the university’s Institutional Review Board
for the Protection of Human Subjects, which also approved the study.
Instrumentation
Peroneal latency was measured by using the Biopac Systems MP150 with telem-
etry system TEL100M and TEL100C (Biopac Systems Inc, Goleta, CA). Surface
pregelledAg-AgCl, vinyl tape, 35-mm-diameter disposable surface electrodes were
used. Electromyography (EMG) data were collected through a remote amplifier at
a sampling rate of 1250 Hz. Each EMG channel was filtered through a band-pass
filter of 10 to 350 Hz. A custom-made trapdoor capable of inverting the ankle to
30° was also used. The angle of 30° was selected because it did not cause harm
to the soft-tissue structures of the joint but still permitted a study of the muscle
reactions during unexpected inversion. A TSD130 twin-axis goniometer (Biopac
Systems) was attached to the trapdoor to record its first movement. According to
previous studies, using EMG with a trapdoor mechanism is common practice when
assessing peroneal latency.23,38,39
Pretest Procedures
The dominant limb was used for all testing procedures. Dominant limb was deter-
mined by asking which leg the subject would prefer as the kicking limb when
kicking a soccer ball. Surface electrodes were placed on the subject according to
a study by Benesch et al.4
Peroneus longus electrodes were placed 3 cm below the
head of the fibula, and a ground electrode was placed on the lateral malleolus. The
sites were prepared with 220-grit sandpaper and shaved and cleaned with 70%
isopropyl alcohol to remove oil or lotion and reduce skin impedance.
For the peroneal latency procedures, each subject stood barefoot on the trapdoor
device. Subjects were instructed to focus on an X placed on the wall at eye level
and to support their full weight on the dominant leg. A wooden block 15 cm high
was placed beneath the contralateral limb to ensure almost full weight bearing on
the test leg (Figure 1). In this position, the contralateral limb acted primarily to
maintain equilibrium.
Subjects were positioned with their backs to the investigator to eliminate
visual cues. To remove auditory cues, they wore headphones. Once the subject
was relaxed, the trapdoor was released at a random interval. Time between drops
Table 1 Demographic Information, Mean ± SD
Group Gender Age (y) Height (cm) Weight (kg)
No training (n = 24) 12 male, 12 female 20.3 ± 1.2 177.9 ± 8.4 74.2 ± 11.9
Training (n = 24) 12 male, 12 female 19.8 ± 1.2 175.7 ± 9.2 74.8 ± 11.7

292 Henry et al
varied between 5 and 20 seconds. Each subject performed a practice trial followed
by 5 test trials.5,17,18,27
Training Procedures
The no-training group was instructed to maintain their current activity level for a
6-week period. The training group performed a 6-week plyometric training protocol
at 3 different levels. Each level was 2 weeks in length. Weeks 1 and 2 involved
plyometric jumps, weeks 3 and 4 involved 4-square drills, and weeks 5 and 6 incor-
porated both plyometric jumps and 4-square drills that were more advanced than
the previous exercises. Each training session consisted of a 5-minute cardiovascular
warm-up on a stationary bike at a moderate speed with low resistance, followed
by a 15-minute plyometric exercise protocol. Subjects were asked to perform the
plyometric exercises at maximal intensity. The plyometric training occurred 3
times per week, with no less than 24 hours between sessions. All training sessions
were supervised and timed by the primary investigator. Compliance rates for all
subjects in the training group were equal to or greater than 78%, or 14/18 sessions.
Level 1—Plyometric Jumps. During level 1, each participant went through
5 exercise jumping stations (Table 2). Each exercise was demonstrated by the
investigator. The exercises included split squats, wall touches, tuck jumps, sprint
strides, 12-in. box jumps, and 12-in. hurdles.After each set, the participant rested
for 30 seconds. These exercises were adopted from a protocol created by Chu40
and used by Rahimi and Behpur 32
and Myer et al.31
Figure 1 — Trapdoor setup showing subject after trapdoor drop.

Level 2—4 Square Drills. Level 2 required the use of jumping quadrants. The
quadrants were labeled 1 through 4 in a clockwise direction, starting with the
lower left box (Figure 2). The jumping pattern for this level is documented in
Table 3. The beginning and conclusion of each jumping pattern was controlled
Table 2 Level 1 Plyometric Jumps Protocol
Exercise Sets × Repetitions
Split squats 6 × 10
Wall touches 5 × 10
Tuck jumps 5 × 10
Sprint strides 3 × 50
12-in. box jumps 3 × 20 s
12-in. hurdles (10 hurdles) 5 × 10
Figure 2 — Jumping quadrant used in the plyometric training sessions.
Table 3 Level Two 4-Square Jump-ing Protocol, Performed With
Both Feet and on Each Individual Limb
Exercise 4-square jump
1 1, 2
2 1, 4
3 1, 3
4 2, 4
5 1, 2, 3
6 1, 2, 4
7 4, 2, 1
8 4, 3, 1

294 Henry et al
by the investigator. Participants engaged in each of the 8 jumping patterns for
20 seconds with both limbs. Then, they repeated the 8 jumping patterns on each
limb individually for 10 seconds per limb. After each exercise, the participants
rested for 30 seconds. The subjects were reminded of proper form during train-
ing protocol. Proper form was defined as (1) feet leaving the ground, not sliding
across it; (2) knees slightly bent; (3) jumping action on the balls of the feet; and
(4) eyes up, not looking at the floor.
Level 3—Advanced Drills. For level 3, a combination of advanced plyometric
drills and 4-square drills was used. These drills were more demanding than in
the previous levels. The exercises for this level are documented in Table 4. They
included lateral single-leg alternating push-offs, the 90-second drill, 18-in. box
jumps, and 4 bilateral-limb 4-square drills incorporating memory and multidirec-
tional movements. Each exercise was demonstrated by the investigator.After each
exercise, participants rested for 30 seconds. These exercises were also adopted
from a protocol created by Chu40
and used by Rahimi and Behpur32
and Myer et al.31
Posttest Procedures
The posttest peroneal latency measurements were recorded in the same method
as the pretest, 1 to 3 days after the final training session. The posttest for the no-
training group was 6 weeks after their pretest.
Data Processing
EMG data were rectified after they were collected. Latency data were processed
by MATLAB (The MathWorks, Inc, Natick, MA). Peroneus longus latency values
were calculated from 2 separate points. The first point was the trapdoor movement,
identified by the first point 3 standard deviations above a 2-second quiet segment
before trapdoor drop on the recorded goniometer channel. The second point was
peroneus longus activation, identified by the first point 10 standard deviations above
a mean 1-second quiet segment before activation. This method has been shown as
the most reliable in previous studies.39,41
The means of the 5 trials for the pretest
and posttest were used for statistical analysis.5,17,18,27
Table 4 Level 3 Advanced Drills
Exercise Sets × Repetitions
Lateral single-leg alternating push-offs 3 × 20
90-s box drill 5 × 15
18-in. box jumps 3 × 20 s
4-square—1, 2, 3, 4, 3, 2, 1 1 × 20 s
4-square—1, 3, 2, 4 1 × 20 s
4-square—2, 3, 1, 4 1 × 20 s
4-square—4, 1, 3, 2 1 × 20 s

Statistical Analysis
Data were examined with a repeated-measures analysis of variance with 1 within-
subject factor (test at 2 levels: pretest and posttest) and 1 between-subjects factor
(group at 2 levels: training and no training).The alpha level was set a priori at P ≤ .05.
Results
Means and standard deviations are listed in Table 5. We found no significant group-
by-time interaction (F1,46
= 0.03, P = .87, ηp
2
= .01, 1 – β = .53), no significant
difference between the pretest and posttest (F1,46
= 1.86, P = .18, ηp
2
=.039, 1 – β
= .27), and no significant difference between the training and no-training groups
(F1,46
= 0.64, P = .43, ηp
2
=.01, 1 – β = .12; Figure 3).
Discussion
The focus of this study was to determine whether a 6-week plyometric training
program would decrease peroneus longus muscle latency. Cornu et al37
previously
reported that plyometric training may increase stretch-reflex activity. Therefore,
we hypothesized that recreating stress at the ankle with plyometric training in
Table 5 Peroneus Longus Muscle Activation Latency for Pretest
and Posttest (N = 48), Mean ± SD
Group Pretest (ms) Posttest (ms)
No training 62.9 ± 15.7 60.7 ± 14.1
Training 60.5 ± 14.0 57.6 ± 10.2
Figure 3 — Peroneal muscle latency data (F1,46
= 0.03, P = .87, ηp
2
= .01, 1 – β = .53, P < .05).

296 Henry et al
multiple planes of motion would not only increase the stretch-reflex activity but
also subsequently decrease the time it takes for this involuntary muscle reaction to
occur. However, the primary finding of our study was that subjects who underwent
a 6-week plyometric training program had no change in peroneal latency.
Data on peroneal latency in the current study were consistent with previous
studies. We found mean latency measurements for the pretest ranging from 61 to
63 milliseconds. For the posttest, mean latency values were 61 milliseconds for
the no-training group and 58 milliseconds for the training group. Previous studies
investigating peroneal latency recorded values ranging from 59 to 79 millisec-
onds.16,17
The range of peroneal latency values in previous studies could be a result
of differences in electrode placement, type of trapdoor apparatus, or method of
calculating latency value.
We know that by using training methods, we can change the mechanics of
the lower limb,30–37,42
but there are differences in previous studies that could cause
conflicts in study comparison. One potential explanation for our lack of significant
differences after the training protocol may the subjects used in the study. We tested
asymptomatic subjects, without any acute symptoms of an inversion ankle sprain.
This may have created a ceiling effect, thereby reducing the impact the training
could have on peroneal latency. With no acute symptoms of an inversion ankle
sprain, these subjects may not have had the opportunity to improve their reaction
time.When training exercises are used as part of a functional rehabilitation protocol
after a sudden inversion injury, decreased peroneal reaction time may be present
because of the injury.38,43
It is possible that because no deficit in peroneal latency
was present in the test population, no improvement in peroneal latency could be
observed. Because neuromuscular pathways are damaged during an inversion
ankle sprain, we know that a decreased reflexive response is seen,18,24,43
providing
the basis for this type of rehabilitation exercise. In a population with chronic ankle
instability, Eils and Rosenbaum38
showed a significant decrease in peroneal muscle
reaction time after a 6-week proprioceptive rehabilitation program.
In a controlled laboratory setting, visual and vestibular cues are eliminated to
standardize the inversion perturbation. However, during dynamic situations, these
cues may help protect the ankle ligaments if they can preactivate stabilizing mus-
culature in anticipation of inversion stress at the ankle. The current study may have
recreated a scenario that would not naturally occur during dynamic activity because
the external cues were removed and the subjects stood at rest before the perturba-
tion event. This may be another reason for a lack of any significant differences.
The study by Eils and Rosenbaum38
employed ankle-disk exercises, exercise
bands, wooden inversion–eversion boards, and minitrampolines, all of which are
stationary activities. Because injuries to the lateral ankle structures rarely occur
while standing at rest, the exact mechanism of an inversion ankle sprain is difficult
to recreate in a laboratory setting. In other words, previous research has shown
improved peroneus longus reaction time from a stationary trapdoor after using sta-
tionary exercises, whereas our research investigated peroneus longus reaction time
with a stationary trapdoor after using dynamic exercises.38
In the current study, by
standardizing our testing methods to previous studies using a stationary trapdoor
device, we wanted to determine whether a multidirectional plyometric training
program designed to stress the ankle dynamically would have the same effect on
peroneal latency in a population with no reported symptoms of a lateral ankle sprain.

It is possible that plyometric training created secondary adaptations in other
structures in the lower leg. Instead of showing a peroneal reflexive muscle adap-
tation, training may have resulted in increased ankle-joint stiffness and stability
after sudden inversion caused by a cocontraction at the ankle. This would lengthen
the time from inversion stress until lateral ligament injury,16,44,45
providing more
time for the body to compensate for unexpected inversion through an enhanced
muscle-recruitment pattern or by moving the center of gravity in anticipation of
a fall. Plyometric training may also have more effect on the stability of the knee,
rather than at the ankle, as previous literature suggests.46,47
Further research may
be necessary to investigate the correlation between plyometric training and muscle
adaptation of the lower extremity.
Limitations
Despite weekly physical activity requirements to participate, physical abilities
differed greatly among the participants. Modifications to the plyometric protocol
were implemented in 9 of the 24 training subjects, because they could not complete
the activities in the protocol. Most could not flex the hip high enough to clear the
hurdles or jump on the larger plyometric box. This could be caused by many factors
including hip-flexor weakness or hamstring contractures. Modifying this exercise by
lowering the hurdle and the plyometric box to a level suited to their hip flexibility
permitted them to achieve success in the training. For 6 subjects, the hurdles were
replaced with forward jumps, emphasizing increased hip flexion, and “touch-and-
go” cues were added to make them move their feet as fast as they could and recoil
as quickly as possible for the next jump. Because of the variability in our subjects’
athletic ability, more demanding inclusion criteria based on physical ability may
be required in future studies to standardize the training.
Finally, prior investigations have shown that a 4- to 10-week training protocol
produces adaptations of lower limb musculature,37,38,43,48–53
not changes in peroneal
latency.A protocol longer than 6 weeks,49,51
that is more strenuous,38
or that includes
more sessions per week than the one used in the current study49,51
may be needed
to show significant changes in peroneal reaction time. If subjects had performed a
longer, more strenuous program that focused on recreating the reflex activation at
the ankle, we may have seen a change in the latency of the peroneal musculature.
Areas of Future Research
Plantar flexion plays a large role in detection of foot angle during inversion stresses
at the ankle. Eils and Rosenbaum38
investigated the role of plantar flexion as a
component of supination and showed that plantar flexing while inverting can help a
subject sense a possibly damaging inversion moment.The platform used in our study
held the foot at a flat, 0° position, disregarding plantar flexion, therefore limiting the
tibialis anterior recruitment. Our trapdoor device may not have shown the muscle
adaptations created because of its lack of plantar-flexion motion. Latency changes
may have been shown with a trapdoor that incorporated some degree of plantar
flexion associated with the drop.25,38
Future research may be needed to investigate
the role that plantar flexion has in peroneus longus latency measurements and to
determine whether muscle adaptations occur when the training regimen includes
exercises that recruit ankle dorsiflexors and evertors.

298 Henry et al
Under the right circumstances, plyometric training may still result in adapta-
tions to the protective reflex mechanism during a sudden inversion stress, but optimal
parameters for a plyometric training program need to be established to show these
changes in muscle adaptation because there are conflicting results for adaptation
in the reaction time of the peroneus longus. It is possible that a training program
of 8 to 10 weeks or longer may be required to develop muscle adaptations to inver-
sion stress. Simply increasing the training protocol from 15 minutes per session
to 30 minutes could also affect peroneus longus muscle activation.38
In addition,
increasing the number of training sessions per week from 3 sessions to alternate
days may enhance muscle adaptation.49,51
Because differences are shown between the static response to injury and a
dynamic response at the ankle,27
recreating an inversion moment during motion
(walking or running) may reveal changes in latency values after a plyometric train-
ing program because the training is dynamic. Further research should look at the
effect of plyometric training on the ankle musculature, tested by a dynamic-motion
trapdoor that inverts while the subject is in motion.
Finally, muscle adaptation has been shown in subjects with acute symptoms
when training is used for a rehabilitation program. Future research should look at
changes in peroneus longus latency values in subjects with chronic or functional
ankle instability to show a muscle adaptation in subjects in a neuromuscular defi-
ciency.
Conclusion
A 6-week plyometric training protocol did not affect peroneus longus reaction time
in a population without symptoms of acute ankle sprain. Further research may be
needed to investigate the effects of a longer plyometric training program or using
a population with chronic or functional ankle instability.
Acknowledgments
We would like to thank Koichi Katana, who assisted with data reduction and processing
using MatLab software.
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