Table 3.

Records on muscle strength / force assessment.

Authors	Sensor used	Subjects	Muscle activity assessed	Experimental protocol	Parameters	Results	Application / Limitation / Future work
48	Accelerometer	5 men	Effect of transcranial magnetic stimulation) on the excitability of spinal motoneurons	Motor-evoked potential obtained by magnetic stimulation at 5%, 10%, 20%, 30%, 40%, 60% and 100% MVC	Motor-evoked potential area and amplitude	The peak to peak amplitude and area increases with increase in muscle contraction.	For future work, influence of motor evoked potential parameter on muscle contraction can be observed with consideration of electrical noise.
56	Accelerometer	27 (15 men, 12 women)	Muscular strength	Isometric contractions at 20%, 40%, 60%, 80% and 100% of maximal workload performed for 8 seconds.	RMS and MPF (both parameters in both directions, i.e. perpendicular to fibres and parallel to fibres)	The RMS for both directions depicted an increasing trend in both genders, while MPF for both directions showed a decreasing trend in females.	Muscle strength may be assessed for muscle under dynamic contraction in future.
55	Accelerometer	12 men	Change in the motor unit firing rate and twitch force due to muscle pain	Static isometric contraction (0%, 10%, 30%, 50% and 70% of MVC	MMG and EMG RMS, MMG and EMG MPF, MMG / EMG ratio	The MMG RMS increased after experimental muscle pain.	Results of this study are gender specific as all the subjects belong to male gender group only.
54	Accelerometer	10 men	MMG and EMG (both amplitude and frequency) of a single motor unit	Contractions at three force levels (20%, 50%, and 80% MVC)	Amplitude and mean frequency	The MMG amplitude increased from 20% to 50% MVC and remained unchanged from 50% to 80% MVC.	Subjects belong to one gender only.
49	Accelerometer	19 men	Concentric and eccentric muscle action	Unilateral forearm flexion exercise	MMG amplitude and dynamic constant external resistance (DCER)	MMG RMS as a function of the DCER showed a moderately linear relationship.	The linearity of MMG amplitude with respect to concentric DCER relationship can be investigated in future using microphone and piezoelectric MMG sensors.
53	Accelerometer	5 men	Elbow flexion force during muscle contraction	Elbow flexions from 0% to 80% MVC	RMS, zero crossing	Both temporal and spectral features establish a non-linear relationship between MMG and force using artificial neural network model.	Future work can be done on variety of muscle activities from other muscles. Other machine learning techniques like support vector machine can be used in a future study for same methodology.
52	Accelerometer	7 men	Contraction strength of muscle	Voluntary static contraction	MMG RMS and frequency variance	The MMG RMS increased slowly and the frequency variance decreased with increasing voluntary contractions.	Strength estimation algorithm can be tested for other muscles with various methodologies in future.
51	Accelerometer	27 men	Muscle output force for assessing muscle contraction properties	20%, 40%, 60%, 80%, and 100% of maximal voluntary isometric contractions (MVIC) with the elbow joint at 90ο and the palm in supine position	MMG RMS in three axes	The MMG RMS increased with increase in the muscle force in all three axes.	Muscle force gradation can be accomplished in future using a triaxial accelerometer and ultrasound for better results.
50	Accelerometer	35 (14 men, 21 women)	Effect of direct inhibitory pressure (DIP) on BB myotendinous junction	Submaximal contractions at 20%, 40%, 60% and 80% of MVC	MMG RMS, EMG RMS and electromechanical delay	After DIP, MMG RMS and EMG RMS shifted to lower values while muscle force output decreased	1) Reflex activity i.e. H-reflex and tendon reflex were not measured in this research. 2) Joints other than BB which possess less mechanical constriction may also be analysed to know the effect of DIP.