Hand Surgery, Vol. 13, No. 2 (2008) 79–83
© World Scientiﬁc Publishing Company
UTILITY OF THE POWERBALL®IN THE INVIGORATION OF THE
MUSCULATURE OF THE FOREARM∗
Sebastián Axel Balan and Marc Garcia-Elias
Received 22 August 2008; Accepted 10 October 2008
In order to ascertain the utility of a 250 Hz NSD Powerball®gyroscope in increasing the maximum grip force and muscular
endurance of the forearm, ten adults without pathology in their upper limbs exercised one forearm with the device during a period
of one month. We evaluated grip strength and forearm muscle endurance with a Jamar dynamometer both at the end of the month
as well as after a resting period of one month. There was a tendency (not statistically signiﬁcant p =0.054), for the volunteers to
increase their maximum grip strength. There was also highly signiﬁcant increase in muscle endurance (p =0.00001), a gain that
remained slightly unchanged after the rest. Because the gyroscope generates random multidirectional forces to the forearm, the
reactive muscle contraction is likely to stimulate more efﬁcient neuromuscular control of the wrist, a conclusion which our work
appears to validate. The use of Powerball®in forearm proprioception deﬁcient patients is, therefore, justiﬁed.
Keywords: Gyroscope; Grip Force; Endurance; Proprioception.
In most sports, as rehabilitation of different pathologies of the
upper limb, there is a need for incrementing force of the mus-
culature of the forearm. Several devices with ﬁxed weights have
been created for different muscular groups to work with: cuff
links and bars, bands of tension, jetty pincers. Not long ago,
an apparatus appeared in the market, developed to carry out
exercises of muscular build-up of the upper extremity, based
on the principles of gyroscope.1,2 It is a hollow sphere that
contains in the interior a rotor of 200 grammes of weight with
an eccentric mass located two centimetres away from its axis.
This internal cylinder rotates around an axis which is perpen-
dicular to the main axis. The internal rotor moves not so much
as a result of its ﬁxed weight (single weight 280 grammes)
but by the generated centrifugal force. When the internal rotor
Correspondence to: Dr. Marc Garcia-Elias, Institut Kaplan, Passeig de la Bonanova, 9, 2nd ﬂoor, 2nd door - 08022, Barcelona, Spain. Tel: (+34) 93-417-8484,
Fax: (+34) 93-211-0402, E-mail: garciaelias@institut-Kaplan.com
∗There is no interest in commercialization with the product (250 Hz NSD Powerball®) by the authors and no ﬁnancial association.
is accelerated, generates a torsion force that causes a turn in
the perpendicular plane, and because the eccentric disposition
of its mass, a rotational force to the rotor is generated up to
10,000 revolutions per minute. The gyroscope accelerates by
means of movements of wrist rotation. As the speed of the rotor
of the gyroscope increases, the centrifugal force increases and,
therefore, the necessity of muscular control becomes increas-
ingly bigger. This work was designed in order to ascertain if
this device induces signiﬁcant changes in both the maximal grip
strength and muscular endurance, understanding this last factor
is the most important parameter in most activities.
MATERIAL AND METHODS
Ten adults, ﬁve men and ﬁve women, participated in this study.
None had antecedents of traumatic lesion or pathology in both
80 S. A. Balan & M. Garcia-Elias
Fig. 1 250 Hz NSD Powerball®.
Fig. 2 Another view of the 250 Hz NSD Powerball®.
upper limbs and were not carrying out any other build-up plan
during this project. Each volunteer was given a 250 Hz gyroscope
(NSD Powerball®). The study was divided into two periods of
four weeks each. The volunteers exercised the dominant upper
extremity in two daily series of three minutes in the ﬁrst two
weeks and two daily series of ﬁve minutes in the last two weeks.
In the second period they did not carry out any muscular invig-
oration. The exercise was performed while seated, with the
elbow ﬂexed at 90◦, and leaning on a ﬁrm surface. Rotation of
the gyroscope was driven with wrist turning clockwise in case
of the right arm dominant volunteers and counter-clockwise in
case of left-handed volunteers. In all cases, the overall wrist
envelope of rotation was set around a slightly extended-ulnar
deviated position, and always trying to develop the maximum
possible speed that could be maintained and controlled com-
fortably during the whole exercise. The contralateral upper limb
did not carry out any build-up work and it was used as the con-
trol. Each person was given a chart to document the exact timing
and incidences of all their exercises.
Evaluation of Force
Both upper limbs were assessed before and after the ﬁrst period
of exercises, and again after a month of rest. The grip strength
Powerball®Utility in Invigoration of Forearm Musculature 81
Fig. 3 Powerball ready to work.
Table 1 Maximal Grip Force and Endurance Index Results.
Volunteer MaxGF (Initial) kg. MaxGF (1st Mth.) kg. MaxGF (2nd Mth.) kg. Ei (Initial) Ei (1st Mth.) Ei (2nd Mth.)
(A) 29 44 40 16 24 31
(B) 28 35 37 15 39 20
(C) 51 63 53 18 36 31
(D) 41 52 55 24 41 48
(E) 56 54 57 18 29 37
(F) 35 36 30 15 29 32
(G) 26 24 19 10 33 24
(H) 37 31 33 13 39 26
(I) 21 30 30 11 16 11
(J) 56 60 52 17 30 28
Average 38 42.9 40.6 15.7 31.6 28.8
Median 36 40 38.5 15.5 31.5 29.5
SD 12.7 13.6 13 4 7.7 9.9
MaxGF: Maximal grip force, Ei: endurance index, SD: standard deviation.
was measured with a Jamar®dynamometer3in the position two
or three depending upon the patient’s comfort. With the shoul-
der relaxed, the elbow ﬂexed at 90◦, the forearm leaning on
the examination table in neutral forearm rotation and the wrist
extended at 25.◦4To obtain maximum grip force the volunteers
were requested to compress the two bars of the dynamometer
as hard as possible, alternating both hands. The highest reading
from three attempts was used in this study.
82 S. A. Balan & M. Garcia-Elias
To evaluate muscular endurance,5,6 we established the follow-
ing assessment method. The volunteers were asked to alternate
periods of three seconds of maximal contraction with three
seconds of relaxation until the digital reading was equivalent to
40% of the maximum grip force determined earlier. The num-
ber of contractions above that level was used as an expression
of muscular endurance of each individual.
The participant volunteers were not informed of their results
until all assessments were ﬁnished. All tests were monitored by
the same investigator.
The Student’s t-test for matched samples was used to settle
down differences in the studied parameters between the two
arms, and a p-value of <0.05 was utilised as the threshold of
The average maximum grip force (MaxGF) of the dominant hand
prior to the exercise period was 38 kg [range 21–56; standard
deviation (SD): 12.7], and the average muscular Endurance
Index (EI) was 15.7 contractions (range 10–24; SD: 4).
After the ﬁrst period of one month exercising regularly with
the gyroscope, the average MaxGF was of 42.9 kg (range 30–
63; SD: 13.6). This corresponds to an increase of 15% as
compared to the initial determination, and although the gain was
not signiﬁcant, the tendency for an increase in this parameter
was clear (p =0.054). The average EI was 31.6 contractions
(range 16–41; SD: 7.7) representing an increase of 109%,
which is highly signiﬁcant (p =0.00001). After the same
period the non-dominant arm did not increase either in MaxGF
(p =0, 45) or in EI (p =0, 065).
After the second period of one month where no exercises
were carried out, the average MaxGF diminished slightly down
to 40.6 kg (range 19–57; SD: 13), although that decrease of
5.3% was not statistically signiﬁcant with regards to what was
achieved at the end of the ﬁrst period (p =0.17). Similarly, the
average EI decreased down to 28.8 contractions (range 11–48;
SD: 9.9), but that 7.7% reduction was not statistically signiﬁcant
(p =0.36). Not surprisingly, the differences between the initial
and ﬁnal recordings of both MaxGF and EI remained highly
signiﬁcant (p =0.00001).
The results of this study appear to prove the hypothesis that
regular use of a gyroscope for one month does not develop
the capacity of maximum contraction of the musculature of the
forearm but increases its endurance substantially. Indeed, the
increment of the number of contractions beyond a certain level
after a month of exercises was remarkably high. Furthermore,
it appears to remain high for an extended period of at least one
more month of not using the apparatus. As this last parameter is
one of the most trustworthy ones for the evaluation of muscular
invigoration, it is apparent that gyroscopes may have a role in our
future treatment armamentarium. Contrary to other more static
devices, the gyroscope generates forces in different directions,
in a quite random way, forcing the musculature of the forearm
to react in an unpredictable way, thus stimulating propriocep-
tion. In these regards, this device may be found particularly
useful in patients with congenital or acquired hyperlaxity having
developed wrist dysfunction secondary to poor proprioceptive
Although debatable, we believe that the muscular control that
is required to counteract the centrifugal forces generated by this
sort of apparatus is in fact an eccentric exercise, inducing active
ﬁbre elongations.7−9In other words, this sort of exercise does
not imply a reduction of the muscular ﬁbre length as when the
extrinsic activity of the muscle is propitiated.9
Needless to say, although this device may be useful for reha-
bilitation of different pathological conditions, it should be used
carefully. Although none of the volunteers experienced signiﬁ-
cant pain nor discomfort with its use, the generated force may
become quite important and, as shown in different studies,
eccentric exercises in weak or improperly trained muscula-
tures have potential for a higher pain index and damage of the
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