Characterisation of Delayed Onset of Muscle Soreness (DOMS) in the hand, wrist and forearm using a ﬁ nger dynamometer: A pilot study

Background: Experimentally-induced delayed-onset muscle soreness of large muscle groups is frequently used in as an injurious model of muscle pain. We wanted to develop an experimental model of DOMS to to mimic overuse injuries from sports where repeated ﬁ nger ﬂ exion activity is vital such as rock climbing. The aim of this pilot study was to evaluate the utility of a ‘ ﬁ nger trigger device’ to induce DOMS in the ﬁ ngers, hands, wrists and lower arms. Methods: A convenient sample of six participants completed an experiment in which they undertook ﬁ nger exercises to exhaustion after which measurements of pain, skin sensitivity to ﬁ ne touch, forearm circumference and grip strength in the hand, wrist and forearm were taken from the experimental and contralateral non- exercised (control) arms. Results: Pain intensity was greater in the experimental arm at rest and on movement when compared with the control arm up to 24 hours after exercise, although the location of pain varied between participants. Pressure pain threshold was signi ﬁ cantly lower in the experimental arm compared with the control arm immediately after exercises locations close to the medial epicondyle but not at other locations. There were no statistical signi ﬁ cant differences between affected and non-affected limbs for mechanical detection threshold, forearm circumference or grip strength. Conclusion: Repetitive ﬁ nger ﬂ exion exercises of the index ﬁ nger by pulling a trigger against a resistance can induced DOMS. We are currently undertaking a more detailed characterization of sensory and motor changes following repetitive ﬁ nger ﬂ exion activity using a larger sample.


INTRODUCTION
Delayed onset of muscle soreness (DOMS) is an acute myogenic condition classi ied as a type I muscle strain that normally occurs after strenuous eccentric and concentric exercise or after physical activity that is not accustomed for the individual [1]. Delayed onset of muscle soreness is related to microtrauma in sarcomeres and an associated inflammatory response with the involvement of neurotrophic factors manifesting as pain, stiffness, muscle tenderness, decreased strength and swelling [2,3]. Symptoms are delayed in onset with severity reaching a peak between 24 to 72 hours and disappearing by seven days after exercise. Experimentally-induced DOMS is frequently used in experimental settings as an injurious model of muscle pain [4,5]. Commonly, large muscle groups such as the hamstrings are used because protocols to induce DOMS are readily available. Disadvantages of using large muscle groups include discomfort and disability of body parts that impacts negatively on functions of daily living. We are interested in injuries associated with chronic overuse of ingers, hands, wrists and lower arms resulting from rock climbing [6][7][8]. Eccentric contractions of the wrist extensor muscles have been used in experimental studies to induce DOMS in the forearm [9,10], but to our knowledge there have been no attempts to develop a model of DOMS in smaller muscle groups of hands. The aim of this pilot study was to evaluate the utility of a ' inger trigger device' to induce DOMS in the ingers, hands, wrists and lower arms. This was achieved by characterizing sensory and motor changes for 48 hours after exercise using the inger trigger device.

Study design
A pre-post experimental design was used to evaluate the effect of inger exercises to exhaustion on pain, skin sensitivity to ine touch, forearm circumference and grip strength in the hand, wrist and forearm compared with participant's contralateral non-exercised hand, wrist and forearm.

Participants
A convenient sample of six participants was chosen for this pilot study. The study was approved by the Research Ethics Committee at the Leeds Beckett University, UK. Participants were recruited by announcing the experiment in lectures in our University. Interested volunteers received a participant information pack and were contacted again at least 48 hours later to be formally invited to take part in the study. Six, healthy, pain-free adults agreed to take part in, and completed all parts of the study (mean +SD age=38.3+8.7 years, 3 women, 3 men).

Experimental procedure
Experiments were conducted in the Pain Research Laboratory at our university and facilitated by one of our investigators (KG). During the study visit volunteers were briefed about the experiment and screened against the following exclusion criteria: a pre-existing medical condition; were currently seeking medical care; were taking medication; had experienced pain in the previous 6 months; had previously been diagnosed with a chronic pain condition; were experiencing disturbances in skin sensation such as sensitivity, numbness, or tingling; had a dermatological (skin) condition such as dermatitis, eczema or bacterial and fungal infections; were pregnant; regularly undertake vigorous exercise such as competitive sport. Eligible volunteers were formally invited to take part in the study and provided written consent.
Anthropometric data was taken and then measurements of self-reported pain, pressure pain threshold, forearm circumference and grip strength were recorded from the experimental (exercises) and control (no exercises) arms. Then each participant undertook a series of exercises with a view to inducing DOMS. A ' inger trigger device' designed by the authors was used to deliver exercises ( Figure 1). The participants squeezed the trigger of the device using their index finger in time with a metronome ( lexion=1s, extension=2s) until exhaustion they 'give-up' or for a maximum of 15 minutes. This was repeated following a 30 second rest. Measurements of self-reported pain, pressure pain threshold, forearm circumference and grip strength were taken from the experimental and control arms, immediately after exercises and then after 24 and 48 hours.

Measurements
Self-reported pain location, quality and intensity (VAS) at rest and on movement using the short form McGill questionnaire, that included a 100mm visual analogue scale (VAS) 0=no pain and 100=worst pain imaginable) to measure present pain intensity.
Pressure pain threshold using a pressure algometer probe of 1 cm 2 , rate of pressure=50 kPas -1 (SenseLab Algometer, SOMEDIC, Sweden). For discussion regarding the validity and reliability of the measurement tool refer to [11]. Mechanical detection threshold using mono ilaments (SenseLab Aesthesiometer, SOMEDIC, Sweden) applied at right angles to skin overlying the belly of flexor digitorum profundus muscle using a method of descending limits until the touch from one of the mono ilaments could not be detected. The handle of the mono ilament was held parallel to the skin and pressure applied so that the ilament buckled. The force applied by the thinnest mono ilament that the participant could detect was recorded. For discussion regarding sensory threshold testing using nylon mono ilaments refer to [12].
Forearm circumference (girth) was measured 6 cm inferior medial epicondyle with the arm held parallel to the ground with the palm facing upwards (supination) using a measuring tape.
Maximal grip strength was measured using a hand dynamometer (JAMAR ® Hydraulic Hand Dynamometer, Patterson Medical, UK) with the participant standing and the forearm parallel to the ground at 90 degrees of lexion at the elbow and the hand in a neutral position. Participants were instructed to grip and squeeze the device as fast and as hard as they could for 3-5 seconds. Three measurements were taken alternately from each hand starting with the left hand. For discussion regarding the test-retest reliability of the measurement tool refer to [13].

Data analysis
Differences between experimental and control arms were calculated for each participant at each time point. All data was normally distributed and analyzed using paired t-tests (mean+SD) with alpha set at 0.05.

Induction of DOMS
One participant did not 'give-up' (i.e. reach exhaustion) within the 15-minute timelimit for the inger exercises on run one or two. The mean+SD time to exhaustion for the other ive participants was 178.0+109.5s for run one and 87.8+39.7s for run two (n=5).

Self-reported pain
There was no pain at baseline reported by any of the six participants. All participants reported pain in the experimental arm at rest and on movement (i.e. inger lexion with and without resistance) immediately after exercises to induce DOMS and there were statistically signi icant differences between experimental and control arms (Table 1). At 24 hours three participants reported pain in the experimental arm and at 48 hours ive participants reported pain on inger lexion without resistance in the experimental arm but there were no statistically signi icant differences between experimental and control arms. All participants reported pain in the experimental arm during inger lexion with resistance at 24 and 48 hours although there were only statistically signi icant differences between experimental and control arms at the 24 hour time point. The location of pain varied between participants and included the index inder, thumb, palm, wrist, middle of forearm, upper arm (m.biceps brachii) and shoulder. Aching and heavy were the most common pain descriptors.
Pressure pain threshold was signi icantly lower in the experimental arm compared with the control arm immediately after exercises at two of the six locations: 1cm above the medial epicondyle and 1 cm under the medial epicondyle. There were no other statistically signi icant differences for locations or time points. Interestingly, many participants commented that they felt that the experimental arm was more sensitive to the pressure stimuli in the experimental arm at most locations at 24 hours and 48 hours although this was not re lected in measurements.
There were no statistical signi icant differences between affected and non-affected limbs for mechanical detection threshold, forearm circumference or grip strength.

DISCUSSION
The indings of this pilot study demonstrate the utility of a inger trigger device to induce DOMS associated with eccentric exercises of a single inger. Our protocol used two periods of inger exercise to exhaustion separated by a 30 second rest. Only one participant did not reach exhaustion within the 15-minute time limit suggesting that this protocol was viable.
DOMS was characterised as pain at rest and on movement that was distributed in the ingers, hands, wrists and lower arms at rest and on movement and persisted up to 24 hours after exercise. Pain at rest in the experimental arm had diminished within 24 hours for three participants but persisted up to 48 hours during inger movement against a resistance in all participants. The characteristics and time course of DOMS was similar to that previously reported for other body parts including arms and legs [1]. The aching nature of the pain is likely to re lect musculoskeletal trauma [2,3], although the location of pain differed substantially between participants and included ingers, palm of hand, wrist, lower and upper arm and shoulder. This might re lect variations in strategies used to pull the inger trigger that lead to recruitment of different muscle groups.
We used blunt pressure algometry to ascertain whether post-exercise pain was driven by peripheral input from deep seated strictures such as muscle and connective tissue [4]. Sensitivity to blunt pressure was pronounced immediately after inger trigger exercises 1cm above the medial epicondyle and 1 cm under the medial epicondyle but not at other locations. We were surprised that there were no signi icant reductions in pressure pain thresholds in the experimental at any other location or time point [14], although participants commented that they believed that they were more sensitive to pressure stimuli on the experimental arm. The failure to detect statistically signi icant differences may be due in part to between-participant variability in the distribution of pain increasing between-participant variance pressure pain thresholds at each location.
We measured mechanical detection threshold to ascertain whether post-exercise pain had a cutaneous component (tactile allodynia) [15]. There were no statistical signi icant differences between experimental and control arms. Thus, we have tentative evidence that DOMS associated with inger trigger exercises is driven by musculoskeletal rather than cutaneous structures.
There were no statistically signi icant changes in forearm circumference following exercise suggesting that there was no appreciable oedema. We were surprised that there were no statistically signi icant differences in grip strength between the experimental and control arms as the task involved the use of anatomical structures involved in inger trigger exercises [16]. Participants commented that they tended to guard the index inger and recruit non painful ingers during the grip strength test and this may explain our failure to detect differences between arms.
In conclusion, the indings of this study suggest that this model of DOMS has the potential to mimic overuse injuries from sports where repeated inger lexion activity is vital such as rock climbing. We plan to follow up this preliminary study with a more detailed characterization of sensory and motor changes following repetitive inger lexion activity using a larger sample.